A “lean design” SSB-Transceiver for 14 MHz

Preface

Currently I am revising older projects that are in my radio shelf, some of them not finished yet, postponed to a later date, some without a cabinet, some with severe problems with performance and so on. All the stuff that needs a “second chance” ;-). This project is one of this collection. The transmitter did not work correctly (severe parasetic oscillations occurred when the section was driven to power levels >1 watt).

By careful testing and examining  I found the reason: The grounding of the rf power amplifier stage was defective due to a connection that had not been soldered properly. After having cured that I found the output was 5 to 6 watts PEP output (very clean). Then, having the project on “GO!”,  I finished the design. Thus I got a nice little “vintage style” SSB QRP trsanceiver as a travel or hiking companion:

DK7IH - Simple SSB Transceiver for 14MHz (VFO controlled, 5W PEP)
DK7IH – Simple SSB Transceiver for 14MHz (VFO controlled, 5W PEP)

Basic concept

Frequent readers on my blog know that one thing I really enjoy is building radios based on a minimalist concept. The fewer components you need for a working transceiver, the better it is. At least in my point of view. Here is another one of these “very lean design” transceivers.

The radio originally was designed as a study for my “Old School Transceiver“. After having not built a “real” analog VFO for a number of years I wanted to find out if I still can set up a construction that is really stable concerning frequency. And because it is not very challenging to just watch the result on a frequency counter, a full transceiver had to be built along with the VFO. The VFO was OK, (see later text!) the power transmitter, as mentioned before, was not. Until I had revised it.

The design is another remake of the „Kajman Transceiver“ by  SQ7JHM. A design I absolutely love because of its simplicity. The radio basically has been designed for 80 meters (even when lot of websites quote it as a 20m rig) so it shows some weaknesses when adapted to 14MHz without any changes. Thus some improvements had to be made.

Improving performance of the SQ7JHM basic design

Some changes that were top of the agenda to meet my requirements:

  1. The receiver needed a preamplifier for bands where atmospheric noise is not that strong. A dual-gate MOSFET equipped radio frequency preamplifier improves noise figure significantly and can be put into the AGC chain to give more dynamic range and a more pleasant listening experience.
  2. An AGC (automatic gain control) is a good idea if you want to use the receiver in a more comfortable way without the need to lower the volume when strong stations appear. In addition the S-meter reading can be derived from the output of the AGC DC amplifier stage.
  3. A little bit more rf output power can be achieved by using a push-pull amplifier. Linearity also improves to a certain degree when using this design because AB mode combined with separated amplification of the half waves plus suppression of even-order harmonics.
  4. To enhance receiver gain a single stage interfrequency amplifier has been added that is only in use when on receive. It is also connected to the AGC chain.
  5. And, last, a microphone amplifier allows you to talk in a moderate way into the microphone which is good for me because I often have my QSOs when the rest of the family is asleep and not keen on listening to my strange “This is DK7IH/QRP, do you copy?” messages.

The schematic of my enhanced design:

DK7IH - Simple SSB Transceiver for 14MHz (VFO controlled, 5W PEP)
DK7IH – Simple SSB Transceiver for 14MHz (VFO controlled, 5W PEP) – REV1 – TNX to Paul, VK3HN for error report! (Full size picture)

Fascination originates from the fact that you only need a handful of components (OK your hand should not have the size of that of a new born baby!) to set up a working short wave SSB transceiver.

The VFO

Some thoughts on frequency stability

Careful design is the key for stable operation. This means component selection as well as setting it up on the veroboard.

The basic problem for every conventional free running VFO is temperature and its influence on the size of components. Due to the theory of thermodynamics all materials change their mechanical dimensions with temperature. This is caused by the kinetic energy of the molecules forming the crystals of a solid body. Thermal energy leads to enhanced oscillation of the molecules and therefore the need of larger spaces of each in individual molecule in a crystal. Because we have capacitors in a tuned circuit this will affect the values of all caps (wanted and unwanted ones) to a certain degree.

Something that helps the builder is called “temperature coefficient”. This means that electronic components increase OR decrease their respective value when they get warmer. The first is called “positive temperature coefficient”, the opposite is called “negative temperature coefficient”. So, you might guess, the fine art of radio building involves the knowledge of the characteristic behavior of components when heated.

I quote my findings about temperature behavior listed in the article referred to on the beginning of this text:

Capacitors:

  • Ceramic capacitors: —
  • Polystyrene capacitor: –
  • NP0 (C0G) capacitor: no measurable effect

Inductors:

  • Air coil on polystyrene coil former: +++
  • Coil wound on T50-6 yellow toroid: +

The more “+” or “-” signs, the more steep the function of T->dC or T->dL is. So you can see: The best choice are polystyrene capacitors combined with coil on a yellow toroid. This combination is likely to outbalance temperature effects. If extra capacity is needed, NP0 caps are recommended.

The circuit

From the existing principles of building a free running radio frequency oscillator I prefer the Hartley circuit. It uses a tapped coil (tap about 1/5 from the “lower” end) and saves capacitors compared to the Colpitts design. The tap achieves in-phase feedback. The lower you put the tap to the end the lower the amount of fed back energy will be. This leads to more frequency stability because the circuit does not heat up by excessive internal radio frequency. But be sure that oscillation is always strong enough and does not stop. The Hartley circuit is more simple and caps always inherit the risk of thermal problems when poorly selected.

The tuning is done with a Vernier drive and a homemade variable capacitor. For this a foil variable cap of an old AM radio has been dismantled an reassembled with air as dielectric. Lots of experiments were necessary to get the “frequency swing” correct and the basic capacitance to the right area.

Other measures that support frequency stability are :

  • Low DC power into the oscillator stage (avoids heating the device up by DC current),
  • Stabilizing voltage for the VFO stage by 2 consecutive steps,
  • Using a FET instead of bipolar transistor (no PN boundary layers in a FET),
  • Very loose coupling between oscillator and buffer stage reduce fed back of impedance changes by the output,
  • Low impedance output with emitter follower,
  • Avoid metal sheets (spec. Aluminum) close to the tuning elements! Aluminum sheet metal changes its size largely with even low temperature differences.

Practical results

This oscillator is stable. It needs 5 to 10 minutes to settle which is in the normal range of what can be expected. I then can have it tuned to one frequency and there is a maximum change in frequency < 50Hz for hours. And, to compare with synthesizer  technology: NO birdies at all. Really not. I love it! 😉

The mixers and filter section

NE602 and its derivatives have been used in legions of amateur transceivers. Basically designed for cell phones and small cordless phones radio amateurs quickly have found out that this mixer IC can be the universal mixer in lots of possible amateur radio designs. The main weakness is its low IMD3. But for a 14MHz rig the risk of appearance of strong out-of-band signals is not that likely. Besides, the selectivity of the receiver’s input section supports this. Strong in-band signals did not appear so far due to low band conditions. We’ll have to see how the receiver performs here.

On the other hand NE602 gives a good sensitivity which makes it ideal for radios on the higher bands where signal levels are not so high.

The NE602 has a balanced input AND a balanced output. This allows the designer to get two different signal sources to the input then subsequently mixed with the oscillator signal. As well the two outputs can be used to send the mixed signal to different paths.

This is what is the basic idea behind the design described here.

The mixer that is used together with the microphone to produce the DSB signal by mixing the audio signal with the local oscillator (LO) also serves as the product detector on receive by mixing the interfrequency with the LO. Correct signal path is set with the two relays depending on the fact you are either on transmit or receive mode.

The same principle is for the other mixer. It is transmit mixer or receive mixer, depending on the position of the relays.

The relays connect the SSB filter either to the input or the output of a distinct mixer. A graphical presentation should make it clear:

DK7IH - Simple SSB Transceiver for 14MHz (VFO controlled, 5W PEP) - Signal path display
DK7IH – Simple SSB Transceiver for 14MHz (VFO controlled, 5W PEP) – Signal path display

RX amp and interfrequency amplifier

These 2 stages are more or less the same. They provide 2 to 12 dB of gain depending on the AGC voltage applied to gate 2 of the dual gate MOSFET. In this version of the radio a potentiometer of 20kΩ is used to have the possibility to lower the DC voltage manually, by doing this an MGC (manual gain control) is achieved in a simple way.

Audio amplifier

A bipolar transistor and the inevitable LM386 amplify the filtered audio signal from the product detector to a volume that can be discerned even in a louder environment. The audio low pass filter prior to the AF preamp should be selected due to the users individual preferences concerning tone pitch of the audio signal.

RF power amplifier

This is more or less my standard power amplifier for small QRP rigs. I put stress on linear amplification, so I use emitter degeneration and negative feedback in collector circuit to get best IMD3 results. Even if the circuit could deliver one or two more watts I let the output power level at about 5 watts pep.

Here ist the result of a dual tone modulation:

DK7IH microsize QRP SSB transceiver ("Micro24") for 14 MHz - Output signal modulated with 2-tone signal
DK7IH microsize QRP SSB transceiver (“Micro24”) for 14 MHz – Output signal modulated with 2-tone signal

Voltage division is 10 volts per cm, so this is 45Vpp which equals to about 5 watts max. peak output. Quite OK for QRP. And here is the spectrum of a 2-tone-modulated signal:

micro24_dk7ih_output_spektrum_dual_tone

Practical setup

The whole transceiver is built on a 12×8 cm Veroboard (4.7″ x  3.1″). There is only one layer. The cabinet is 4 cm high (1.55″), 14 cm long (5.5″) and 9 cm wide (3.5″).

DK7IH - Simple SSB Transceiver for 14MHz (VFO controlled, 5W PEP) - Inside view
DK7IH – Simple SSB Transceiver for 14MHz (VFO controlled, 5W PEP) – Inside view

Left the vernier drive with the homemade capacitor attached. Left of the 9MHz filter you can see the LO, more far left the S-meter (from an old CB radio) hiding the audio amps. The 2 mixer ICs and the relays are sited around the SSB-filter. On the right side the power amp partly hidden by the DC switching board.

Well, that’s the story how a nearly failed project was saved from the scrapyard and came to life by carefully searching the faulty element in the circuit.

Vy 73 de Peter (DK7IH)

 

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Revision of the “Cigarette Pack”- 14MHz SSB QRP Micro-Transceiver

DK7IH microsize QRP SSB  transceiver ("Micro24") for 14 MHz
DK7IH microsize QRP SSB transceiver (“Micro24”) for 14 MHz – Fits into one hand
DK7IH microsize QRP SSB transceiver ("Micro24") for 14 MHz
DK7IH microsize QRP SSB transceiver (“Micro24”) for 14 MHz

This article describes the “Cigarette Pack” SSB QRP transceiver” for 14MHz that I first had mentioned some months before. Recently, when taking it from the shelf, the transceiver dropped to the floor and was severely damaged. This lead to serious defects in the front panel area, the main frame, the cabinet and so on. The interior parts were, luckily, not affected by the crash. So, I had to revise the whole radio, make a new front panel and cabinet, ply the frame straightly (as far as possible) and so on. This is the full description of the rig now to complete the files here. The good news: The radio is fine again and fully operational!  And the even better news: I still have not started smoking!  smile1

During reconstruction the transceiver has been extended for about 5 mm so that overall length now is 100mm (3.9 inch). This was done because I intended to build in a loudspeaker. The other dimensions remain unchanged: Width is 52mm (2 inch.), height is 30mm (1.2inch). OK it is slightly longer now than a standard pack of cancer sticks, but who cares? Total cabinet volume is 150cm³.

Basic concept

The transceiver is based on the “Micro 23” rig, that I have described here. Some simplifications of that already simplified radio have been made. Here is the full schematic of this even smaller transceiver:

DK7IH microsize QRP SSB  transceiver ("Micro24") for 14 MHz - Schematic
DK7IH microsize QRP SSB transceiver (“Micro24”) for 14 MHz – Schematic in full size

Very simple rigs like this one always use parts of the circuit for receive and transmit purpose. Here these parts are the 2 mixers (NE602), the SSB-filter  and the interfrequency amplifier.

Signal flow schematic

The NE602 has a balanced output. With mixer 1 only one of them is used. If higher gain is desired, a broadband (or even better a tuned LC circuit) transformer could be used to connect pin 4 and 5 (the mixer outputs) in push-pull mode. I did not do that to save the transformer.

The signal flow can be derived from the design:

DK7IH microsize QRP SSB transceiver ("Micro24") for 14 MHz - Signal flow on receive and transmit
DK7IH microsize QRP SSB transceiver (“Micro24”) for 14 MHz – Signal flow on receive and transmit

Receive mode signal flow

From the antenna relay (not drawn) the rf energy runs through a 2 pole LC filter for 14 MHz. The coils are wound small TOKO coil formers, all respective data is given in the schematic. Coupling is loose via a 3.3pF cap.

NExt stage is an rf preamp for 14MHz with a broadband output. The acitve element here is a dual-gate MOSFET.

After having left this stage the 14MHz signal travels through another 470pF capacitor. This one has high resistance for audio frequency and low for rf frequencies due to the equation: XC =1/(2*PI*f*C). The signal is then fed, together with the audio signal from the microphone (when on transmit), into mixer 1 input on pin 1.  The 1k resistor prevents the rf energy from flowing into the microphone circuit. The two signals are separated from each other by simply exploiting reactance and resistance in a rather clever way 😉.

When receiving the Si5351A clock chip is programmed in a way that the VFO signal (23 MHz) is present on output CLK0.  It is fed into mixer 1 via a small cap to prevent overloading of the mixer. The Si5351A breakout board delivers about 3 Vpp. clock signal, so this must be reduced to about 200mVpp. A 5.6pF capacitor is OK here.

The resulting signal is sent to the SSB filter (a 9MXF24D) that is terminated with 1kOhm and 20pF in parallel. The wanted SSB signal is present at the output of the filter.

Next stage is the interfrequency amplifier, equipped with a dual-gate-MOSFET semiconductor. This one is connected to the AGC chain, on receive a variable voltage is applied to gate 2 (range 0 to 6 V), on transmit the AGC is fully powered to ensure maximum gain.

Next is mixer 2 which is the product detector when receiving. The signal (9MHz +/- sideband shift) is applied to pin 6. Due to the fact that this mixer also serves as transmit mixer, the two signals are taken from the two mixer outputs on pin 4 (serving as audio output) and pin 5 (serving as rf output for transmitting).

Two audio amplifiers (preamplifier and power stage) give a sufficient signal level for an 8 ohm loudspeaker or a headphone.

For the loudspeaker I tried out the tiny ones for smartphones with good success. Only the volume was a little bit low. Then I found another speaker in an old toy of my daughter that turned out to be very much OK for this transceiver. Its diameter is about 3 cm (1.2 inch) and just fits in the housing.

Transmit signal flow

The microphone in this radio is an electret one. The advantage is that these microphones have an internal preamplifier equipped with a field-effect-transistor. The output voltage is fairly high, about 1Vpp. when normally speaking into it. Therefore an audio preamp is obsolete. The microphone signal is directly fed into pin 1 of the first mixer. On transmit the Si5351 signal generator is switched that the 9MHz (+/- sideband shift) signal is fed into pin 6. The SSB filter eliminates the unwanted sideband, the interfrequency amplifier lifts the SSB signal to an appropriate level. The TX mixer is fed with the 23MHz signal resulting in a 14 and 37 MHz signal. The TX band pass filter cleans the signal from the unwanted 37MHz component resulting from the mixer process.

RF power amplifier

The power amplifier is a 3 stage circuit. Stage 1 (preamplifier) brings the signal to about 10 mW. This is coupled into the driver stage via a cap of 0.1uF without any further impedance matching.

The subsequent driver stage shifts the signal level to about 200mW. Linear amplification is ensured her (as well as in the previous stage) by negative feedback in the collector circuit and emitter degeneration with a non-bypassed resistor to GND. An output transformer (winding rate 4:1, impedance rate thus 16:1) lowers the impedance of some 100 ohms to a few 10 ohms present on the input of the final amplifier stage.

The final amplifier brings up a signal level of 3 to 4 Watts PEP. This stage is in AB mode, the appropriate bias is achieved by the 1k resistor going to +12V TX and the current to GND via the silicon diode. This diode must be thermally connectod to the final transistor to stabilize the bias.When the transistor heats up, the silicon diode increases the current through it thus decreasing bias to the transistor.

The 68 ohm resistors serves 2 purposes: First it prevents the input signal from being shorted by the bypass caps in the bias circuit and it stabilizes the rf behavior  of the stage by limiting the gain because certain amounts of the input power are led to GND. This prevents self-oscillation.

DC ad the collector is fed through a radio frequency choke to hinder rf from flowing into the DC line. Radio frequency is directly fed into the low-pass-filter. The output impedance of this stage is roughly 50 Ohms, so the filter can be a 50 ohm circuit with a cutoff frequency slightly above 14MHz.

The VFO section

The Si5351A clock chip used here has three frequency outputs that can be set individually. Only CLK0 and CLK1 are used in this radio. The Si5351A chip is programmed by software in the following manner:

  • Receive: CLK0 is the VFO, CLK1 is the BFO.
  • Transmit: CLK0 is the BFO, CLK1 is the VFO.

The microcontroller reads the tx/rx status and switches the frequencies respectively.

Construction

The radio is a full SMD design on a 0.1″ pitch double sided Veroboard:

DK7IH microsize QRP SSB  transceiver ("Micro24") for 14 MHz - Inside
DK7IH microsize QRP SSB transceiver (“Micro24”) for 14 MHz – Inside

The control panel on the left with tuning knob and volume set. The 64×32 pixel OLED between these controls. Following the microcontroller behind the fron panel (here covered). The controller is an ATmega168 on an Arduino Pro mini board.

The isolated board left of the SSB is the AGC section. The receiver and transmitter shared parts follow, the TX band pass filter is in the foreground. The power transmitter is on the right behind the shield. The shield is necessary to avoid unwanted oscillations when rf is coming back from the power transmitter to the band pass filter prior to the tx section.

On the right there is the SMA socket for connecting the antenna plus a 3 pin header for connecting a headphone. When there is no headphone in use a jumper connects the internal speaker to the speaker line. VDD is applied via a standard DC connector.

The underside of the board has only some SMD components and the wiring on it:

DK7IH microsize QRP SSB transceiver ("Micro24") for 14 MHz - Underside
DK7IH microsize QRP SSB transceiver (“Micro24”) for 14 MHz – Underside

“On the air”

My longest distance achieved with this transceiver (after rebuilding it) has been R2DLS near Moscow who gave me a “59”-report. smile1 The antenna in use is, as always, a Deltaloop.

73 and thanks for reading this article!

Peter (DK7IH)

 

 

SSB Transceiver, 7MHz, 50 Watts, with Dual-DDS-System

DK7IH QRO SSB transceiver for 7MHz/40m
DK7IH QRO SSB transceiver for 7MHz/40m

In this paper we will discuss a single sideband amateur radio transmitter/receiver for the 40 meter band that has been designed to ensure  good performance characteristics with reasonable number of parts (no “overkill” in component use), particularly concerning the receiver. Circuit simplicity and over-average performance were to be combined.

The background: Some years ago I had built the ancestor of this transceiver and afterwards posted an incomplete series of articles (starting here). The transmitter was considered to be quite OK (I could even work a station from South Korea when operating as GJ/DK7IH some years ago) but the receiver was weak.

The shortcomings originated from the rf preamplifier I used together with the 1st mixer, an NE602. The latter had severe problems to cope with the high signal levels on the 40 meter band from out-of-band broadcast stations transmitting on the 41m band (f>7200kHz) or from very strong amateur stations transmitting in-band. This is caused by the technical specs of this Gilbert cell mixer. NE602 has been designed for mobile phone applications and not for shortwave radios. Its IMD 3 is only -15dBm whereas it is able to detect weak signals (-119dBm with an S/N ratio of 12 dB) according to datasheet. Due to this NE602 was excluded from being used at least in the receiver.

Another point was that the rig was too small and too densely packed to be called “service friendly”. Thus I dismantled the radio some times afterwards and had in mind rebuilding it with another receiver and a little bit more space inside.

The Basics

The project has had to meet certain requirements that I would like to point out first:

Frequency generation: Dual-DDS-System: AD9835 as local oscillator and AD9834 as VFO. ATMega644A as MCU (Download source code here)

Receiver: Single conversion superhet, 9 MHz interfrequency with commercial filter (supplied by http://box73.de) shared by transmitter and receiver and relay switched, “NE 602-free zone” ;-), 4 dual gate MOSFETs in rf preamp, rx mixer, if amplifier and product detector, audio stages with BC547 as preamp and LM386 as main audio amplifier.

Edit: I found that there was strong signal of self-reception around 7.100kHz which was not a spurious signal from one of the DDS. It has been a mixing product of one or two oscillators together with a signal from the microcontroller. So I changed the interfrequency to 10.7MHz which cured the problem. I tried to calculate the issue but was not succcesful because I do not know all the frequencies in the microcontroller. I think it is most probable that it is a harmonic of the PWM signal I use for controlling the LED front lights.

Transmitter: 4 stages, 3 of them in push-pull mode, Siemens made mixer IC S042P (really old fashioned, but still available) as DSB generator and TX mixer, rf amplifiers (2N2219A) after filter and tx mixer.

Design: Really “cool” with blue backlight. Sandwich built, not the size of a “micro transceiver”, but handy for travelling.

The Block Diagram

The diagram can be derived from the old project, it is nearly the same:

DK7IH QRO SSB transceiver for 7MHz/40m - Block diagram
DK7IH QRO SSB transceiver for 7MHz/40m – Block diagram

The basic outline of the radio is standard and should not be further discussed.

Dual DDS (VFO and Local Oscillator (LO))

This time I wanted to use 2 digital oscillators. The reason was just to have fun. 😉 Here is the schematic:

DK7IH QRO SSB transceiver for 7MHz/40m - Dual DDS (VFO and LO)
DK7IH QRO SSB transceiver for 7MHz/40m – Dual DDS (VFO and LO) – (Full sized image)

Microcontroller (MCU)

The source code has got about 2200 lines. With the GNU C compiler this leads to a HEX-file of about 43kB. Because of this the controller had to have a little bit of more memory. A “644” is a good choice here. It is clocked internally to 8 MHz clock rate. Radio and user data (user operated keys, S-Meter, TX PWR meter, temperature sensors attached to final transistors) is lead to the analog-digital-converter (ADC) of the MCU. Rotary encoder (optical) is fed into digital inputs. Integration of an RTC is projected but not done yet.

DDS1 (VFO)

Here an AD9834 is used. It is overclocked with 110MHz clock rate. For my receiver with a DDS chip purchased from Mouser this works without any abnormality. With a a chip from the “free market” (ebay) I found that there were strange clicks in the signal. So, I do not really recommend overclocking under any circumstance and/or not to such a high degree.

This DDS is is not terminated with a low pass filter. Due to the high clock rate there is no clock oscillator feedthrough which is supported by the  design of the following amplifier having an audio frequency transistor in the last stage (BC547 and later BCY59) that limits high frequency components due to its early gain decay in the frequency spectrum. The two stage amplifier has been designed for excellent linearity to prevent impurities in output spectrum.

mini43-qro-7mhz-dk7ih_12

The first peak showing the 16MHz signal and the next peak is the first harmonic about 30dB below. Other peaks are from local sources (PC, Printer).

The sine wave also looks quite OK:

mini43-qro-7mhz-dk7ih_13

DDS2 (LO)

This one contains an AD9835 synthesizer clocked to 50 MHz. An LPF here is mandatory. A simple but linear amplifier brings the signal up to 3Vpp which is OK for driving the dual gate MOSFET in the receiver. For the transmitter mixers this amount of voltage is too high, small capacitors reduce the voltage to an acceptable value.

LCD

From another project that I once had built and that is not more in use, a dive computer, I had a 4 lines/20 characters text display that is fairly large. This was to be designated as the LCD for this transceiver.

The Receiver

Building a receiver for the 7MHz amateur band is challenging. On one hand the circuit should be very sensitive for weak signal reception, particularly during day when the band conditions are low due to solar radiation and density of the D-layer. This means the receiver should have a higher gain whereas noise figure does not play a predominant role due to band characteristics with high atmospheric noise on 7MHz.

Next request is high dynamic range to eliminate the spurious signals that occur when front end stages are loaded with high input signal levels.

And last but not least AGC control range should be as wide as possible to cope with weak and very strong signals without the request to intervene by adapting manual gain control. For this a preamp also benefits.

Active mixers like the NE602 show low performance under these conditions. Some high-current mixers like the SL6440 exist, but there are alternatives. On one hand the classical diode ring mixer might come into perspective, otherwise Dual-Gate MOSFETs are well known as having a fairly good ability to cope with high signal levels and so don’t tend to  deteriorating the receiver’s performance severely. Besides they offer some gain and low noise figure (which has not been the main objective in this case) and the circuit is very compact and therefore it was the best choice for a receiver that had been intended to be constructed onto a board of 6 x 8 centimeters.

After these thoughts the following circuit turned out to be the right onset for a receiver inside the projected rig.

DK7IH QRO SSB transceiver for 7MHz/40m - The Receiver
DK7IH QRO SSB transceiver for 7MHz/40m- The Receiver (full sized image)

Circuit explanation (Receiver)

Front end

On the left we start with a 2 pole LC band pass filter for 7 MHz. The coils are wound on TOKO style coil formers (5.5mm size), winding data and parallel capacitors are given in the drawing. The coupling capacitor (2.7pF) between the two LC circuits is very small for such a low frequency. This makes the filter response curve sharper but leads to a slight weakening of the signal coming through the filter. But as the whole receiver has plenty of gain and a very good noise figure, this is the reason why  some weakening of the input signal is acceptable.

Preamplifier

Next is the preamplifier for the received band. It is connected to the AGC chain. You can expect some 25 to 30dB  gain swing by driving up gate 2 of the dual gate MOSFET from 0 V to 6V. A 1:1 voltage divider decrease the 0..12V AGC voltage to 0..6 V where th3N205 MOSFET is close to amplify with maximum gain. Exceeding 6 to 7 volts does not result in significant more gain swing, so I usually drive the MOSFET from 0 to 6.5 volts UG2 (with 13 Volts of supplied voltage.

3n205-ug2-gain-figure
UG2->Gain-Function 3N205 (Source: Datasheet)

The coupling when going from the preamplifier to the receiver mixer is in broadband style. The 3N205 has a very high gain and tends to self-oscillate. A second LC circuit makes the device more prone to going self-resonant and hence produce unwanted signals.

RX mixer

This mixer is very simple and needs only a few components. Both signals are fed into the gates of the dual gate MOSFET. Rf goes to gate 1 whereas gate 2 (the AGC input) is fed with the oscillator signal). Gate voltage depends on the voltage drop at the source resistor and therefore is stabilized. The oscillator signal should be in the range of 2 to 3 volts rf (pp) for a dual gate MOSFET. Lower values will deteriorate the performance of the mixer, e. g. its dynamic range. This signal switches the semiconductor and a superposition of the two signals occurs thus leading to the production of sum and difference of the original frequencies. These signals are fed into…

The SSB filter

which is a commercial one (Supplier box.73.de). The reason why I don’t ladder filters anymore is that I found it extremely difficult (not to say impossible) to get a symmetric filter response curve thus making the lower and upper sideband of the receiver sounding different even when the carrier frequency has been adjusted very thoroughly.

The filter is used for the SSB transmitter as well. To ensure maximum signal separation between the two branches (tx and rx) and between filter input and output I again us a high quality rf relay made by Teledyne. When choosing a relay intercontact capacitance  is crucial. It should (if possible) be < 1 pF.

Don’t forget a clamp diode to VDD over the relay coil to eliminate high voltage voltage peaks generated by self inductance when the coil is switched off. Voltages up to 100 Volts can occur. This might damage the transmit-receive section of this transceiver that is equipped with semiconductors only and does not use a relay.

IF amplifier

This circuit is the same like that of the rf preamp. It also is part of the AGC chain, thus delivering another 25 to 30 dBs of gain swing so that overall gain swing is around 50 to 60dB. In practical research over a long period of observation I found that with an antenna delivering high signal voltage (Delta loop) it was not possible to overdrive the receiver  to a level where signal distortion was audible.

A tuned circuit is also placed here to increase gain. Tuned amplifiers usually have higher gain than broadband ones. It is highly recommended to ground the metal cans of the coil to prevent any self-oscillation. But as I found out, this amplifier is not very prone to go to self-oscillation state.

Product detector

Here again a dual gate MOSFET is used. The circuit is nearly the same like the RX mixer except from the output section. We can see a low pass filter here, consisting of 2 Cs (0.1uF) and a resistor (1k). You can use a radio frequency choke instead, 1mH is recommended.

Audio amplifier

This section consists of two parts, a preamp (with bipolar BC547) and a final amplifier (LM386 IC). It is well-known that this IC tends to oscillate. One measure to prevent this is to keep leads short, switch a low-pass filter (capacitor 100uF and R=33Ω) into the VDD line and to reduce the gain capacitor between pins 1 and 8 to a degree where self-oscillations terminate.

A switching transistor cuts off the audio line by short circuiting it when on transmit. This eliminates any noise when switching. The rx/tx switch now is 100% “click free”. A very pleasant way of operation. 😉

AGC

This is another re-use of a circuit I have frequently used before. It is desired to reduce its output voltage down to 0 volts when a more or less strong af signals appear at the input. The agc voltage is derived from the audio signal of the receiver. Some say that this is not the best choice because you need more time (an af cycle last much longer as an rf cycle) for the waveform to generate the regulating DC voltage.

Nonetheless I have never observed popping or unpleasant noise from incoming very strong signals. The agc response rate is so fast that you won’t notice that it just has regulated even when a strong signal comes in. Only with very, very strong signals a slight “plopp” sound is observable but it is not unpleasant.

A second capacitor can be switched in parallel to the 33uF one. This can either be done by a transistor switch (like shown in the schematic) that in this case is controlled by an output PIN of the MCU. An alternative that I found later is to use the MCU pin directly to switch the cap. When not using the additional cap you must switch the pin as an input so that there is no positive voltage from the pin to the circuit. When you intend to ground the transistor (agc in “slow” position) then the pin mus be set as output by defining the DDR-register respectively AND the pin must be set to 0. So you can get rid of the switching transistor.

Another possibility would be to derive the agc from the interfrequency signal. The problem that occurs in this case is that you have to decouple the local oscillator (bfo) very carefully from the place where agc circuit is placed. Otherwise you are at risk to detect the bfo signal by the agc which leads to reduced response range in the agc. In addition this receiver uses a higher rf voltage level for the mixers (2 to 3 Vpp each). By this the amount of stray energy is higher inside the circuit and thus this rf energy might be detected very early by the agc.

In the emitter line there is a resistor (68Ω) which produces a voltage drop when the transistor is driven. This is fed into the ADC of the microcontroller driving the S-meter display part.

The Transmitter

First the circuit:

DK7IH QRO SSB transceiver for 7MHz/40m - The Transmitter
DK7IH QRO SSB transceiver for 7MHz/40m – The Transmitter (Full sized image)

Microphone amplifier

This amplifier is a simple common-emitter circuit with the directly grounded emitter of the BC547 transistor. This circuit is linear only for low input voltages but suitable for the connected dynamic microphone since this does not produce more than some millivolts of audio energy. Bias comes from the 390kΩ resistor. At the input you find a 2.2nF capacitor from base to GND which helps to prevent coupling in rf energy from the transmitter to the audio stage and thus leading to an impure signal.

The DSB generator + amplifier

The amplified microphone signal is used to produce a double-sideband signal. The ic I use here is an antique but still available part by German manufacturer Siemens, the S042P. It includes a so-called “Gilbert-cell” mixer and an oscillator but the latter is not used here (Datasheet Application note (in German)).

The S042P mixer needs some more components compared to the well-known NE602 integrated circuit but fewer ones than the MC1496. It is designed for 12V usage, thus no voltage regulation is required.The ic can be applied in balanced mode or non-symmetrical. To save components I use the unbalanced circuit alternative. A slight loss in output power is acceptable in this case, there are amplifiers post each mixer in this transmitter.

Ic gain is about 16.5 dB, DC current is about 3 mA.

A crucial point is the signal level of the local oscillator. S042P needs only some hundred  millivolts of oscillator voltage. To prevent overdriving I experimented with different values of the coupling capacitor. 5.6pF seemed best because the LO produces some volts peak-to-peak.

Following there is an amplifier that is a standard circuit and has been tuned for maximum linearity in order to reduce distortion to a minimum (which is also true for the following stages). You can see the well understood 2 master ways of achieving max. linearity in an amplifier stage:

  • Negative feedback between collector and base (i)
  • Emmitter degeneration (II)

Explanation:

i) The first measure goes along with the 2.7kΩ resistor between collector and base of the transistor. This resistor provides positive dc bias voltage to the base and leads 90° out-of-phase ac voltage to the transistor’s input. This reduces gain and therefore distortion. But due to the fact that the whole transmitter strip has plenty of gain, this loss in gain is not a serious problem.

ii) The 10Ω resistor in the emmitter line is not bypassed by a capacitor. This stabilizes the circuit. When the current through transistor increases the emmitter voltage will rise (according to Ohm’s law) and the voltage between collector and emmitter drops. This reduces voltage difference between base and emmitter and hence also reduces gain.

The coupling to the next stage is done by a capacitor of 0.1uF. This causes some impedance mismatch. But that is as well not a big problem because the gain reduction here helps to prevent the whole transmitter from unwanted oscillations by diminishing overall gain.

TX mixer

Here the second S042P is used. The 9 MHz SSB signal is coupled to pin 13 of the ic, a DC connection is established to pin 11. These two pins represent the base connectors for the two current control transistors and should be bridged by a DC resistor in this circuit.

The 150Ω resistor from pin 10 and pin 12 to GND defines the gain of the mixer. Here you can use down to 150Ω but should have a resistor towards VDD to limit current and avoid excessive heating of the device. In this case another 150Ω is used.

VFO signal is coupled symmetrically to pins 7 and 8 via a small trifilar toroid. See schematic for details and please note that center tap is not used here. This is in contrast to the output transformer where the tap is used to feed supply voltage into the mixer.

Another 7 MHz band pass filter terminates the mixer, data for coils and capacitors is in the schematic.

Power amplifier

This amplifier has got 4 stages and except from the first one all are in push-pull mode. The power distribution for these 4 stages is as follows:

Stage Power
Preamp 5mW
Predriver 200mW
Driver 2.5 W
Final amp 50W

Preamplifier

The first of the 4 power stages is the same as the post dsb generator amplifier so there is not more to add concerning this stage. Rf energy is taken out via a transformer with a primary and a tapped secondary winding. This is to provide the balanced structure necessary for the following push-pull stage.

Prediver

This is the first push-pull stage. Its bias is derived from a voltage divider connected to the tap of the input transformer.

Please note: In contrary to the schematic I have installed 2 devices of the 2SC1973 type because the signal turned out to be much purer with these ones on the spectrum analyzer.

A tapped output transformer feeds the amplified rf energy to next board. Output impedance is 50Ω. The coupling to next stage then is done via a shielded cable of (nearly) the same impedance.

Driver stage

This one has an input transformer also center tapped. The tap goes to a bias network consisting of a current limiting resistor (1kΩ), two diodes forming the lower part of a voltage divider and some capacitors as part of a low pass filter to avoid coupling in of radio frequency (rf) energy. The two diodes must be thermally connected to the cases of the transistors. In case these heat up, the diode increases its conductivity thus reducing its resistance. The bias voltage drops and heating is stopped. So, thermal runaway is prevented.

For these two stages (predriver and driver) DC is fed through low pass filter (RFC and 2 caps 0.1uF) to prevent coupling of rf energy via the VDD line.

Final stage

This stage receives input from a balanced structure without a center fed transformer. Instead bias current is linked in via a network of radio frequency chokes and two resistors of 5.1Ω each.

Bias is provided by a current regulating transistor and should be set to about 100mA.

The MRF455 transistors are mounted directly to the aluminium structure of the sheet metal carrying the whole transceiver boards. When mounting them to the Veroboard I did not solder them directly. I used 1.6mm screws and washers to press the brass connectors to the copper strips of the amplifier board:

DK7IH QRO SSB transceiver for 7MHz/40m - Power amplifier underside
DK7IH QRO SSB transceiver for 7MHz/40m – Power amplifier underside

With this I could have been able to remove the precious transistors without having to unsolder them when the device might have turned out to be a failure. But it was not, thank God!

The output transformer is the one I have used in my old 14MHz PA and the ancestor of this radio. It is from an old ATLAS 215 transceiver and I hope that this will be the final place for the transformer.

Two temperature sensors (KTY-81-210) have been installed to measure the temperature of each transistor. They are connected to the microcontroller via voltage dividers (see schematic, please!)

Low Pass Filter and Power Measurement Unit

For the low pass filter I use 2 toroids T50-2. These might appear small but from one source (that I have forgotten) I remember to have found that for 50 watts of power this core is still suffice. Metal powder cores can stand much more power compared with same sized ferrite toroids.

The power measurement unit consists of a network that starts with a resistor of 12kΩ to ensure a significant voltage drop in signal level, then two rectifier diodes (1N1418 or equivalent) follow, some low pass filtering eliminating the last rf energy and the resulting direct current voltage is fed to a variable resistor to set an adequate voltage level for the ADC in the microcontroller.

The rf output made out of a two-tone audio signal measured at the antenna connector:

DK7IH QRO SSB transceiver for 7MHz/40m - Two tone signal, power about 57 watts, close to overdrive
DK7IH QRO SSB transceiver for 7MHz/40m – Two tone signal, power about 57 watts, close to overdrive

The spectroscopical analysis shows the signal on the f -> V figure:

DK7IH QRO SSB transceiver for 7MHz/40m - Output spectrom with max. Pout (>50W PEP)
DK7IH QRO SSB transceiver for 7MHz/40m – Output spectrum with max. Pout (>50W PEP)

RX/TX-switching

A very simple circuit. Two PNP power transistors are used but they don’t have that much to do. They are only designed for switching the low-power parts of the radio. The high current to the drivers and final amplifiers is permanently present in the collector lines but the bias lines are tx/rx-switched and go to 0V during receive periods. This reduces requirements for the power rating of the switch board.

DK7IH QRO SSB transceiver for 7MHz/40m - RX/TX switch board.
DK7IH QRO SSB transceiver for 7MHz/40m – RX/TX switch board.

When pushing the PTT the base of the lower transistor is pulled to GND. So it becomes conductive and TX DC is applied. Via the diode the upper transistor loses its negative voltage and becomes non-conductive.

Construction

The Backlight

One interesting thing was the blue backlight to illuminate the front panel controls. It is made using SMD LEDs that are soldered to small pieces of Veroboard and fixed with 2-component glue to transparent light-scattering plastic bought from a local supplier for architects and designers. This material is used for making models of houses and stuff like that. As light distributor this material is excellent. The LEDs are powered by a linear transistor connected to the pulse width modulation (PWM) output of the microcontroller so that light intensity is adjustable.

Hint: When programming the PWM functions it might occur that PWM frequency is audible in the receiver. If something like that occurs another frequency can be selected without changing the performance as soon as it is high enough that human eyes aren’t able to recognize a flickering.

DK7IH QRO SSB transceiver for 7MHz/40m
DK7IH QRO SSB transceiver for 7MHz/40m

The covers used for the labels and the LCD shield are made from 2mm acrylic and fixed with screws of 1.6 respective 2mm diameter.

The two push-buttons right in top position consist of two bars of acrylic (4.2mm diameter) and are having mechanical contact to small spring-loaded switches behind the front panel:

mini43-qro-7mhz-dk7ih_16

Directly under these acrylic bars there are two LEDs shining into these rods and because of total reflection inside the tubing the optic conductor is sending the light to the front side when the LEDs are powered on. That is how it looks at night:

mini43-qro-7mhz-dk7ih_19

 

General setup

This is a sandwich construction again. On the first side there is the DDS  board (left), the receiver (center) TX mixer and preamplifier (right) and the SSB generator (back). Also there is a 5 lead connector holding the 5 ISP lines (MOSI, MISO, CLK, RESET and GND). This makes firmware updates easy because you don’t have to open the case when you want to update software.

DK7IH QRO SSB transceiver for 7MHz/40m - DDS, RX, TX mixer and SSB generator
DK7IH QRO SSB transceiver for 7MHz/40m – DDS, RX, TX mixer and SSB generator

The other side holds the TX low pass filter plus power measurement unit (left), the power amplifier (center) and the predriver and driver (right). In the back you can see the rx/tx switch board:

DK7IH QRO SSB transceiver for 7MHz/40m - TX LPF, PA, Drivers, RX/TX switch board.
DK7IH QRO SSB transceiver for 7MHz/40m – TX LPF, PA, Drivers, RX/TX switch board.

“On the air”

Again big fun this transceiver! During the ARRL DX contest last weekend I could work some statesiders. With Delta Loop and 50 watts, fairly OK. Working Europe all day is no problem with 50 watts.

During the first QSOs I had reports that the audio sounded clear but somehow “narrow”. I had used an electret mike that time and could not use a dynamic one because the preamplifier following the microphone did not have enough gain. Then, to solve this problem, I decided to do a full reconstruction of the SSB generator board. The one then had used had an AN612 mixer integrated circuit (from an old CB radio). This one was dismantled and replaced by the S042P board. The change took me 3 hours to develop and solder but it paid. I use a Motorola dynamic microphone now that has a very rich and clean sound. I monitored it on a web based SDR receiver, made a recording and found it to be OK.

OK, dear fellow hams, that’s the story so far, some supplements will sure be made, so stay tuned!

Thanks for reading and vy 73 de

Peter (DK7IH)

Decoding NMEA data from a GPS module with a microcontroller

I found one of those widely distributed GPS receiver modules in my collection of electronic material. I had bought it some years ago for a school project I did with students to launch a balloon into the stratosphere (Link to text in German). Here a video of the flight phase when maximum altitude was reached:

The GPS module that was used then has still been sleeping in my junk box. When looking through the C code I had written then I found that this could be optimized. I connected the GPS receiver to an AVR ATmega32 microcontroller, added a simple 2-lines-16-characters lcd and started rewriting the software for communicating with the module.

The Hardware

OK, an ATmgea32 is somehow “overkill” for this simple circuit, but I had already installed it on my breadboard. The data lines of the ATmega32 controller that are in use for this project are pointed out here:

ATMega32_GPS-Connections

The GPS module is connected to the controller via the serial RS232 interface (TxD, RxD, PIN 15 and 14 of the micro). Note that RX of the GPS module must be connected to TxD with the controller and vice versa.

The rest of the layout is also very simple.

Microcontroller interface connections for this project

The lcd is driven in 4-bit-mode, thus only four display data lines are needed. Plus the 2 controls RS and E:

ATMega32 => LCD

  • PD4 => LCD PIN D4
  • PD5 => LCD PIN D5
  • PD6 => LCD PIN D6
  • PD7 => LCD PIN D7
  • PC2 => LCD RS
  • PC3 => LCD E

ATMega32 => GPS Module

  • PD0 (RxD) => GPS TX
  • PD1 (TxD) => GPS RX

Decoding GPS data

GPS data from the module comes coded as a so-called “NMEA”-string. “NMEA” stands for the American National Marine Electronics Association . The data is encrypted using a standardized ASCII format that appears in various patterns and is transferred by the GPS module regularly in very short intervals (about 1 second or less). The communication settings for the RS232 interface are fixed and can not be altered:

9600 baud, 8 data bits, 1 stop bit, no parity

The ATMega’s UART (universal asynchronous receiver and transmitter)  must be set with an initialization routine to the required parameters:

//Init UART
void uart_init()
{
    /* 9600 Baud */
    UBRRL = 51; UBRRH = 0;

    /* RX and TX on */
    UCSRB = (1<<RXEN)|(1<<TXEN);

    /* 8 databits, 1 stopbit, No parity */
    UCSRC = (1<<URSEL)|(1<<UCSZ1)|(1<<UCSZ0);
}

NMEA strings (example)

There are various types of data strings transferred by the module, each serving an individual purpose. They are all coded with a leading identifier marked by a $-sign and an identifier code. We will discuss the absolute basic string that delivers essential GPS information, the so-called “GPRMC”-string, where the letters “RMC” represent “recommended minimum sentence”.

This sequence contains all the necessary data for a basic GPS operation and therefore is ideal for experiments with the GPS module. Example for such a string:

$GPRMC,191410,A,4735.5634,N,00739.3538,E,0.0,0.0,181102,0.4,E*19

The various data fields are separated by a comma, please note that the data fields may have various width so just counting bytes is not reliable if you want to “land” in a specific field. Instead the comma separators must be counted.

What do the separate data fields mean?

The first entry (“191410”) after the “$GPRMC” intro is the UTC time stamp, telling when the recent GPS data set was processed, i. e. the current time in HHMMSS- format. There might be even 10th of seconds then the format is HHMMSSss.

Next is a letter (“A” for correct data, “V” (void) if the receiver was not able to get correct data), that indicates if the data is valid and can be used or not. You can use that to discard the data set when the receiver is not able to detect a valid GPS signal.

Following a number (here “4735.5634”) together with the subsequent letter (N or S) indicates the latitude of the current position. This is a 100fold decimal value formatted  by a decimal separator. Latitude can be in the range from 0 to 90 degrees North or South. Here it is about 47 degrees in the northern hemisphere.

Subsequently the same coding is used for transferring longitude data. Longitude is in the range from 0 to 180 degrees East or West with the Prime Meridian in Greenwich as separator.

Next two values (0.0 and 0.0) indicate ground speed in knots and bearing in degrees.

“181102” is the date stamp (18th Nov 2002).

“0.4” and “E” indicate a magnetic declination (degrees and direction) that might occur.

“19” is the  XOR generated checksum of the string. I found that data transfer is very reliable so I don’t calculate this.

Calculating QTH locator

For a radio amateur it is clear that GPS data must be converted to the Maidenhead system to determine the current QTH locator (JN49AB in my case). Thus I have programmed a short function that can convert decimal data for latitude and longitude to the Maidenhead system:

//Calc QTH locator from decimal value for latitude and
//longitude. o1 and o2 are signifier for 'E or 'W' (longitude) resp.
//'N' or 'S' for latitude
//example for calling: calc_maidenhead(8.002, 'E', 49.1002, 'N', buf);
void calc_maidenhead(double longitude, char o1, double latitude, char o2, char *buffer)
{

    //Longitude
    int deg_lo, lo1, lo2, lo3;
    double deg10_lo;

    //Lattitude
    int deg_la, la1, la2, la3;
    double deg10_la;

    deg_lo = longitude; //Truncate values for longitude and latitude
    deg_la = latitude;

    deg10_lo = (longitude - deg_lo) * 12; //Calculate fraction behind decimal separator
    deg10_la = (latitude - deg_la) * 24;

    //Longitude
    if(o1 == 'E')
    {
        lo1 = 180 + deg_lo;
    }
    else
    {
        lo1 = 180 - deg_lo;
    }
    lo2 = lo1 / 20;
    lo3 = lo1 - lo2 * 20;

    //Lattitude
    if(o2 == 'N')
    {
        la1 = 90 + deg_la;
    }
    else
    {
        la1 = 90 - deg_la;
    }

    la2 = la1 / 10;
    la3 = la1 - la2 * 10;

    *(buffer + 0) = lo2 + 65;
    *(buffer + 1) = la2 + 65;
    *(buffer + 2) = lo3 / 2 + '0';
    *(buffer + 3) = la3 + '0';
    *(buffer + 4) = (int) deg10_lo + 'A';
    *(buffer + 5) = (int) deg10_la + 'A';
}

Parameters handed over to the function are

  • Longitude in decimals (double),
  • ‘E’ or ‘W’ for orientation,
  • Latitude in decimals (double),
  • ‘N’ or ‘S’ for orientation,
  • a 6 byte buffer (+ 1 byte for “\0” termination) referenced by address for the locator data.

The program rolls through the data permanently giving relevant information on the display with changes every second. Due to the fact that I haven’t implemented error handling there might be more or less useles indications when data from the GPS receiver is corrupted or invalid. But mostly the routine works out very fine without any complaint. You can use the ‘A’ or ‘V’ indicator transferred to change the display routine if unvalid data is detected.

By the end of this article you can get the full  C-code for the project. Thanks for reading!

73 de Peter (DK7IH)

/*****************************************************************/
/*                    GPS with ATMega32                          */
/*  ************************************************************ */
/*  Mikrocontroller:  ATMEL AVR ATmega32, 8 MHz                  */
/*                                                               */
/*  Compiler:         GCC (GNU AVR C-Compiler)                   */
/*  Autor:            Peter Rachow  2018                         */
/*  Fuses: -U lfuse:w:0xe4:m -U hfuse:w:0xd9:m                   */
/*****************************************************************/
// This code reads data from a GPS module via RS232 interface and
// decodes the basic NMEA string of $GPRMC type, the so-called
// "RMC" (Recommended Minimum Specific GNSS Data) string.
// Displayed are Date, Time, QTH-Locator (Maidenhead), Lattitude and
// Longitude and Ground speed
/*   PORTS */
// O U T P U T
// LCD
// RS      = PC2
// E       = PC3
// D4...D7 = PD4..PD7

// PD0  TX at GPS module
// PD1  RX at GPS module

#define F_CPU 8000000

#include
#include
#include
#include 

#include
#include
#include
#include
#include 

int main(void);

/***************/
/* LCD-Display */
/***************/
#define LCD_INST 0x00
#define LCD_DATA 0x01

void lcd_write(char, unsigned char);
void lcd_write(char, unsigned char);
void set_rs(char);
void set_e(char);
void lcd_init(void);
void lcd_cls(void);
void lcd_line_cls(int);
void lcd_putchar(int, int, unsigned char);
void lcd_putstring(int, int, char*);
void lcd_putnumber(int, int, long, int);
void lcd_display_test(void);
void setcustomcharacters(void);

/*******************/
/* Stringhandling */
/*******************/
int strlen(char *s);
int instring(char*, char*);
int strcomp(char*, char*);
void int2asc(long, int, char*, int);

/*******************/
/*   U  A  R  T    */
/*******************/
void uart_init(void);
void uart_putc(char);
void uart_send_string(char*);
void init_rx_buffer(void);
char make_crc(int, int);

/**********************/
/* V A R I A B L E S  */
/**********************/
//USART defines &amp; variables
#define RX_BUF_SIZE 64
char rx_buf[RX_BUF_SIZE];
int rx_buf_cnt = 0;

/*****************************/
/* Result string formatting  */
/*****************************/
void get_time(char*, char*);
int get_receiver_status(char*);
void get_latitude(char*, char*);
void get_longitude(char*, char*);
void get_latitude_ns(char*, char*);
void get_longitude_ew(char*, char*);
double get_gps_coordinate_decimal(char*, int, char*);
void get_ground_speed(char*, char*);
void get_ground_speed(char*, char*);
void get_date(char*, char*);
void calc_maidenhead(double, char, double, char, char*);

/**************************************/
/*            L  C  D                 */
/**************************************/
/* Send one byte to LCD */
void lcd_write(char lcdmode, unsigned char value)
{
    int x = 16, t1;

    set_e(0); 

    if(!lcdmode)
	{
        set_rs(0);
	}
    else
	{
        set_rs(1);
	}	

    _delay_ms(1);

    set_e(1);

    /* Hi */
	for(t1 = 0; t1 &lt; 4; t1++)
	{
	    if(value &amp; x)
	    {
	       PORTD |= x;
	    }
        else
	    {
           PORTD &amp;= ~(x);
	    }  

		x *= 2;
	}
	set_e(0);

	x = 16;

	set_e(1);
	/* Lo */
	for(t1 = 0; t1 &lt; 4; t1++)
	{
	    if((value &amp; 0x0F) * 16 &amp; x)
	    {
	       PORTD |= x;
	    }
        else
	    {
           PORTD &amp;= ~(x);
	    }  

		x *= 2;
	}

    set_e(0);
}

/* RS */
void set_rs(char status)
{
    if(status)
	{
        PORTC |= (1 &lt;&lt; PC2);
	}
    else
	{
	    PORTC &amp;= ~(1 &lt;&lt; PC2);
	}
}

/* E */
void set_e(char status)
{
    if(status)
	{
        PORTC |= (1 &lt;&lt; PC3);
	}
    else
	{
	    PORTC &amp;= ~(1 &lt;&lt; PC3);
	}
}

void lcd_putchar(int row, int col, unsigned char ch)
{
    lcd_write(LCD_INST, col + 128 + row * 0x40);
    lcd_write(LCD_DATA, ch);
}

void lcd_putstring(int row, int col, char *s)
{
    unsigned char t1;

    for(t1 = col; *(s); t1++)
	{
        lcd_putchar(row, t1, *(s++));
	}
}

void lcd_putnumber(int y, int x, long number, int dec)
{
	char *buf;

	buf = malloc(32);
	int2asc(number, dec, buf, 31);
	lcd_putstring(y, x, buf);
	free(buf);
}	

void lcd_cls(void)
{
    lcd_write(LCD_INST, 1);
}

void lcd_init(void)
{

    lcd_write(LCD_INST, 40);

    //Matrix 5*7
    lcd_write(LCD_INST, 8);

    /* Display on, Cursor off, Blink off */
    /* Entrymode !cursoincrease + !displayshifted */
    lcd_write(LCD_INST, 12);

	//4-Bit-Mode
    lcd_write(LCD_INST, 2);	

	lcd_cls();
}

void lcd_line_cls(int ln)
{
    int t1;

	for(t1 = 0; t1 &lt; 15; t1++)
	{
	    lcd_putchar(1, t1, 32);
	}
}	

//Define own chars
void setcustomcharacters(void)
{
    int i1;
    unsigned char adr=64;

    unsigned char customchar[]={ 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,     // \0
		                         0x04, 0x0A, 0x04, 0x00, 0x00, 0x00, 0x00, 0x00};    //°-sign
    lcd_write(LCD_INST, 0);
    lcd_write(LCD_DATA, 0);

    //Send data to CGRAM in lcd
    for (i1 = 0; i1 &lt; 16; i1++)
    {
        lcd_write(LCD_INST, adr++);
        lcd_write(LCD_DATA, customchar[i1]);
    }
}

/***********************/
//
// STRING-FUNCTIONS
//
/**********************/
//Convert int number to string
void int2asc(long num, int dec, char *buf, int buflen)
{
    int i, c, xp = 0, neg = 0;
    long n, dd = 1E09;

    //Write 0 to buffer in case value == 0
    if(!num)
    {
		buf[0] = &#039;0&#039;;
		buf[1] = 0;
		return;
	}	

    if(num &lt; 0)
    {
     	neg = 1;
	    n = num * -1;
    }
    else
    {
	    n = num;
    }

    //Fill buffer with \0
    for(i = 0; i &lt; 12; i++)
    {
	    *(buf + i) = 0;
    }

    c = 9; //Max. number of displayable digits
    while(dd)
    {
	    i = n / dd;
	    n = n - i * dd;

	    *(buf + 9 - c + xp) = i + 48;
	    dd /= 10;
	    if(c == dec &amp;&amp; dec)
	    {
	        *(buf + 9 - c + ++xp) = &#039;,&#039;;
	    }
	    c--;
    }

    //Search for 1st char different from &#039;0&#039;
    i = 0;
    while(*(buf + i) == 48)
    {
	    *(buf + i++) = 32;
    }

    //Add minus-sign if neccessary
    if(neg)
    {
	    *(buf + --i) = &#039;-&#039;;
    }

    //Eleminate leading spaces
    c = 0;
    while(*(buf + i))
    {
	    *(buf + c++) = *(buf + i++);
    }
    *(buf + c) = 0;
}

//Compare 2 strings
int strcomp(char *s1, char *s2)
{
    int t1;
    for(t1 = 0; t1 &lt; strlen(s2); t1++)
    {
        if(*(s1 + t1) != *(s2 + t1))
	    {
	       return 0;
		}
    }

    return 1;
}

//Get length of string
int strlen(char *s)
{
   int t1 = 0;

   while(*(s + t1++));

   return (t1 - 1);
}

//Find s2 in s1 if present, hand back position in string if yes
int instring(char *s1, char *s2)
{
   int t1, t2, ok;

   for(t1 = 0; *(s1 + t1) ; t1++)
   {
	    ok = 1;
	    for(t2 = 0; t2 &lt; strlen(s2); t2++)
	    {

	        if(*(s1 + t1 + t2) != *(s2 + t2))
	        {
	            ok = 0;
	        }
	    }

		if(ok)
	    {
	        return t1;
	    }
    }

    return 0;
}

//************/
//    UART
//************/
//Init UART
void uart_init()
{
    /* 9600 Baud */
    UBRRL = 51;
    UBRRH = 0;

    /* RX and TX on */
    UCSRB = (1&lt;&lt;RXEN)|(1&lt;&lt;TXEN);    

    /* 8 Datenbits, 1 Stopbit, keine Paritaet */
    UCSRC = (1&lt;&lt;URSEL)|(1&lt;&lt;UCSZ1)|(1&lt;&lt;UCSZ0);
}

//Send 1 char to UART
void uart_putc(char tx_char)
{
    while(!(UCSRA &amp; (1&lt;&lt;UDRE)));

    UDR = tx_char;
}

//Send one string to UART
void uart_send_string(char *s)
{
    int t1 = 0;
	while(*(s + t1))
	{
	    uart_putc(*(s + t1++));
	}
	uart_putc(13);
	uart_putc(10);

}

uint8_t uart_getc(void)
{
    while(!(UCSRA &amp; (1&lt;&lt;RXC)));   // warten bis Zeichen verfuegbar
    return UDR;                   // Zeichen aus UDR an Aufrufer zurueckgeben
}

//Init RX buffer
void init_rx_buffer(void)
{
	int t1;

    for(t1 = 0; t1 &lt; RX_BUF_SIZE - 1; t1++)
    {
		rx_buf[t1] = 0;
	}
	rx_buf_cnt = 0;
}		

/* CRC-calculation */
char make_crc(int buflen, int addchar)
{
    int t1, x = 0;

    for(t1 = 0; t1 &lt; buflen; t1++) /* Puffer bis dato */
	{
        x = x ^ rx_buf[t1];
    }
	x = x ^ addchar;                /* Sendebyte */

    return x;
}

///////////////////// END UART ///////////////

//////////////////////////////
//                          //
// NMEA decode functions    //
//                          //
//////////////////////////////

//Format date string
void get_date(char *buffer2, char *buffer3)
{

    int t1, t2 = 0;
    int p1, p2;

    //Zero buffer3
    for(t1 = 0; t1 &lt; RX_BUF_SIZE; t1++)
    {
		*(buffer3 + t1) = 0;
	}

	//Find 9th datafield
	t1 = 0;
	t2 = 0;
	while(t2 != 9 &amp;&amp; t1 &lt; RX_BUF_SIZE)
	{
		if(*(buffer2 + t1) == &#039;,&#039;)
		{
			t2++;
		}
		t1++;
	}
	p1 = t1;

	//Find end of datafield
	t1 = 0;
	t2 = 0;
	while(t2 != 10 &amp;&amp; t1 &lt; RX_BUF_SIZE)
	{
		if(*(buffer2 + t1) == &#039;,&#039;)
		{
			t2++;
		}
		t1++;
	}
	p2 = t1;

	//Copy relevant prt of string to buffer3
	t2 = 0;
    for(t1 = p1; t1 &lt; p2 - 1; t1++)
    {
		if((t1 - p1) == 2 || (t1 - p1) == 4)
		{
			*(buffer3 + t2++) = &#039;.&#039;;
		}
		*(buffer3 + t2++) = *(buffer2 + t1);
	}
}	

//Format time string
void get_time(char *buffer2, char *buffer3)
{

    int t1, t2 = 0;
    int p1, p2;

    //Zero buffer3
    for(t1 = 0; t1 &lt; RX_BUF_SIZE; t1++)
    {
		*(buffer3 + t1) = 0;
	}

	//Find first datafield
	t1 = 0;
	while(*(buffer2 + t1) != &#039;,&#039; &amp;&amp; t1 &lt; RX_BUF_SIZE)
	{
		t1++;
	}
	p1 = t1 + 1;

	//Find dot in timestring
	t1 = p1 + 1;
	while(*(buffer2 + t1) != &#039;.&#039; &amp;&amp; t1 &lt; RX_BUF_SIZE)
	{
		t1++;
	}
	p2 = t1;

	//Copy relevant prt of string to buffer3
    for(t1 = p1; t1 &lt; p2; t1++)
    {
		if(t1 == 9 || t1 == 11)
		{
			*(buffer3 + t2++) = &#039;:&#039;;
		}
		*(buffer3 + t2++) = *(buffer2 + t1);
	}
}	

//Format receiver status string
int get_receiver_status(char *buffer)
{
	int t1, t2 = 0;
    int p1;

	//Find second datafield
	t1 = 0;
	while(t2 != 2 &amp;&amp; t1 &lt; RX_BUF_SIZE)
	{
		if(*(buffer + t1) == &#039;,&#039;)
		{
			t2++;
		}
		t1++;
	}
	p1 = t1;

	if(*(buffer + p1) == &#039;A&#039;)
    {
		return 0;
	}
	else
	{
		return 1;
	}
}	

//Format latitude to degrees
void get_latitude(char *buffer2, char *buffer3)
{

    int t1, t2;
    int p1, p2;

    //Zero buffer3
    for(t1 = 0; t1 &lt; RX_BUF_SIZE; t1++)
    {
		*(buffer3 + t1) = 0;
	}

	//Find 3rd datafield
	t1 = 0;
	t2 = 0;
	while(t2 != 3 &amp;&amp; t1 &lt; RX_BUF_SIZE)
	{
		if(*(buffer2 + t1) == &#039;,&#039;)
		{
			t2++;
		}
		t1++;
	}
	p1 = t1;

	//Find end of datafield
	t1 = 0;
	t2 = 0;
	while(t2 != 4 &amp;&amp; t1 &lt; RX_BUF_SIZE)
	{
		if(*(buffer2 + t1) == &#039;,&#039;)
		{
			t2++;
		}
		t1++;
	}
	p2 = t1;

	//No data available
	if(p2 - p1 == 1)
	{
		return;
	}	

	//Copy relevant prt of string to buffer3
	t2 = 0;
    for(t1 = p1; t1 &lt; p2 - 1; t1++)
    {
		if(t1 == p1 + 2)
		{
			*(buffer3 + t2++) = 1; //°
		}	

		*(buffer3 + t2++) = *(buffer2 + t1);
	}
}	

//Format latitude orientation
void get_latitude_ns(char *buffer2, char *buffer3)
{

    int t1, t2;
    int p1, p2;

    //Zero buffer3
    for(t1 = 0; t1 &lt; RX_BUF_SIZE; t1++)
    {
		*(buffer3 + t1) = 0;
	}

	//Find 4th datafield
	t1 = 0;
	t2 = 0;
	while(t2 != 4 &amp;&amp; t1 &lt; RX_BUF_SIZE)
	{
		if(*(buffer2 + t1) == &#039;,&#039;)
		{
			t2++;
		}
		t1++;
	}
	p1 = t1;

	//Find end of datafield
	t1 = 0;
	t2 = 0;
	while(t2 != 5 &amp;&amp; t1 &lt; RX_BUF_SIZE)
	{
		if(*(buffer2 + t1) == &#039;,&#039;)
		{
			t2++;
		}
		t1++;
	}
	p2 = t1;

	//No data available
	if(p2 - p1 == 1)
	{
		return;
	}	

	//Copy relevant prt of string to buffer3
	t2 = 0;
    for(t1 = p1; t1 &lt; p2 - 1; t1++)
    {
		*(buffer3 + t2++) = *(buffer2 + t1);
	}	

}	

//Format longitude
void get_longitude(char *buffer2, char *buffer3)
{

    int t1, t2;
    int p1, p2;

    //Zero buffer3
    for(t1 = 0; t1 &lt; RX_BUF_SIZE; t1++)
    {
		*(buffer3 + t1) = 0;
	}

	//Find 5th datafield
	t1 = 0;
	t2 = 0;
	while(t2 != 5 &amp;&amp; t1 &lt; RX_BUF_SIZE)
	{
		if(*(buffer2 + t1) == &#039;,&#039;)
		{
			t2++;
		}
		t1++;
	}
	p1 = t1;

	//Find end of datafield
	t1 = 0;
	t2 = 0;
	while(t2 != 6 &amp;&amp; t1 &lt; RX_BUF_SIZE)
	{
		if(*(buffer2 + t1) == &#039;,&#039;)
		{
			t2++;
		}
		t1++;
	}
	p2 = t1;

	//No data available
	if(p2 - p1 == 1)
	{
		return;
	}	

	//Copy relevant prt of string to buffer3
	t2 = 0;
    for(t1 = p1; t1 &lt; p2 - 1; t1++)
    {
		if(t1 == p1 + 3)
		{
			*(buffer3 + t2++) = 1; //°
		}	

		*(buffer3 + t2++) = *(buffer2 + t1);
	}	

}	

//Format longitude orientation
void get_longitude_ew(char *buffer2, char *buffer3)
{

    int t1, t2;
    int p1, p2;

    //Zero buffer3
    for(t1 = 0; t1 &lt; RX_BUF_SIZE; t1++)
    {
		*(buffer3 + t1) = 0;
	}

	//Find 5th datafield
	t1 = 0;
	t2 = 0;
	while(t2 != 6 &amp;&amp; t1 &lt; RX_BUF_SIZE)
	{
		if(*(buffer2 + t1) == &#039;,&#039;)
		{
			t2++;
		}
		t1++;
	}
	p1 = t1;

	//Find end of datafield
	t1 = 0;
	t2 = 0;
	while(t2 != 7 &amp;&amp; t1 &lt; RX_BUF_SIZE)
	{
		if(*(buffer2 + t1) == &#039;,&#039;)
		{
			t2++;
		}
		t1++;
	}
	p2 = t1;

	//No data available
	if(p2 - p1 == 1)
	{
		return;
	}	

	//Copy relevant prt of string to buffer3
	t2 = 0;
    for(t1 = p1; t1  lattiude
double get_gps_coordinate_decimal(char *buf, int ctype, char *retbuf)
{
	int t1, t2;
	int sp; //Position of 1st comma of relevant data field
	double rval = 0;
	double x;

	char *l_str = malloc(20);

	if(!ctype)
	{
		sp = 5;  //Longitude
	}
	else
	{
		sp = 3; //Lattitude
	}	

	//Init temporary string
	for(t1 = 0; t1 &lt; 20; t1++)
	{
		*(l_str + t1) = 0;
	}

	//Search start position of value (3rd or 5th &#039;,&#039;)
	t1 = 0; t2 = 0;
	while(buf[t1] != 0 &amp;&amp; t2 != sp)
	{
		if(buf[t1] == &#039;,&#039;)
		{
			t2++;
		}
		t1++;
	}
	sp = t1;	

	//Load relevant part of original string to new bufferstring
	t2 = 0;
	for(t1 = sp; *(buf + t1) != &#039;,&#039;; t1++)
	{
		*(l_str + t2++) = buf[t1];
	}

    //Check multiplier
	if(!ctype)
	{
		x = 100;
	}
	else
	{
		x = 10;
	}

	//Convert string to FP number
	for(t1 = 0; l_str[t1] != 0; t1++)
	{
		if(l_str[t1] != &#039;.&#039;)
		{
		    rval += (double)(l_str[t1] - 48) * x;
		    x /= 10;
		}
	}

	free(l_str);

	//Get orientation indicator (N/S or W/E)
	if(!ctype)
	{
		sp = 6; //Longitude
	}
	else
	{
		sp = 4; //Lattitude
	}	

	//Search start position (4th or 6th &#039;,&#039;)
	//where letter is expected
	t1 = 0; t2 = 0;
	while(buf[t1] != 0 &amp;&amp; t2 != sp)
	{
		if(buf[t1] == &#039;,&#039;)
		{
			t2++;
		}
		t1++;
	}
	sp = t1;	

	retbuf[0] = buf[sp];

	return rval;
}	

//Ground speed
void get_ground_speed(char *buffer2, char *buffer3)
{

    int t1, t2;
    int p1, p2;

    //Zero buffer3
    for(t1 = 0; t1 &lt; RX_BUF_SIZE; t1++)
    {
		*(buffer3 + t1) = 0;
	}

	//Find 5th datafield
	t1 = 0;
	t2 = 0;
	while(t2 != 7 &amp;&amp; t1 &lt; RX_BUF_SIZE)
	{
		if(*(buffer2 + t1) == &#039;,&#039;)
		{
			t2++;
		}
		t1++;
	}
	p1 = t1;

	//Find end of datafield
	t1 = 0;
	t2 = 0;
	while(t2 != 8 &amp;&amp; t1 &lt; RX_BUF_SIZE)
	{
		if(*(buffer2 + t1) == &#039;,&#039;)
		{
			t2++;
		}
		t1++;
	}
	p2 = t1;

	//No data available
	if(p2 - p1 == 1)
	{
		return;
	}	

	//Copy relevant prt of string to buffer3
	t2 = 0;
    for(t1 = p1; t1 = RX_BUF_SIZE)
        {
			/*
			for(t1 = 0; t1 = 0; t1--)
		    {
			    //Scan for specific identifier of NMEA message RMC Example &quot;$GPRMC&quot;
		        valid = 1;
		        for(t2 = 0; t2 &lt; 5; t2++)
		        {
				    if(rx_buf[t1 + t2] != msg_code[t2])
				    {
					    valid = 0;
				    }
			    }		

		        if(valid) //Infotype OK, data found
		        {
					//Copy rx-buffer to buffer2

					//Init buffer2
					for(t2 = 0; t2 &lt; RX_BUF_SIZE; t2++)
					{
						*(buf2 + t2) = 0;
					}	

					//Copy relevant part of rx-buffer to buffer2
					t3 = 0;
					for(t2 = t1; t2 &lt; rx_buf_cnt &amp;&amp; rx_buf[t2] != &#039;*&#039;; t2++)
			        {
					    *(buf2 + t3) = rx_buf[t2];
					    t3++;
					}   

					lcd_cls();

					//Display information
					lcd_putstring(0, 0, &quot;  GPS Receiver&quot;);
                    lcd_putstring(1, 0, &quot;   DK7IH 2018&quot;);
                    _delay_ms(dispdelay);
					lcd_cls();

					//Message string
					lcd_putstring(0, 2, &quot;Message type&quot;);
					lcd_putstring(1, 4, msg_code);
					_delay_ms(dispdelay);
					lcd_cls();

					//QTH locator (Maidenhead)
					lcd_putstring(0, 2, &quot;QTH Locator&quot;);
					rbuf0 = malloc(4);
					rbuf1 = malloc(4);
					for(t1 = 0; t1 &lt; 4; t1++)
					{
						*(rbuf0 + t1) = 0;
						*(rbuf1 + t1) = 0;
					}	

					lon = get_gps_coordinate_decimal(buf2, 0, rbuf0); //Calc longitude
					lat = get_gps_coordinate_decimal(buf2, 1, rbuf1); //Calc latitude

					rstr = malloc(7);
					for(t1 = 0; t1 <span data-mce-type="bookmark" id="mce_SELREST_start" data-mce-style="overflow:hidden;line-height:0" style="overflow:hidden;line-height:0;"></span>&lt; 7; t1++)
					{
						*(rstr + t1) = 0;
					}
					calc_maidenhead(lon, rbuf0[0], lat, rbuf1[0], rstr);
					//calc_maidenhead(8.3362, &#039;E&#039;, 49.0145, &#039;N&#039;, rstr); //Test for &quot;JN49EA&quot;
					lcd_putstring(1, 4, rstr);
					_delay_ms(dispdelay);
				    free(rbuf0);
   			        free(rbuf1);
   			        free(rstr);
					lcd_cls(); 

					//DATE
					lcd_putstring(0, 6, &quot;Date&quot;);
					get_date(buf2, buf3);
					lcd_putstring(1, 4, buf3);
					_delay_ms(dispdelay);
					lcd_cls();

					//TIME
					lcd_putstring(0, 3, &quot;Time (UTC)&quot;);
					get_time(buf2, buf3);
					lcd_putstring(1, 4, buf3);
					_delay_ms(dispdelay);
					lcd_cls();

					//Receiver status
					lcd_putstring(0, 0, &quot;Receiver status&quot;);
					if(get_receiver_status(buf2) == 0)
					{
						lcd_putstring(1, 7, &quot;OK&quot;);
					}
					else
					{
						lcd_putstring(1, 7, &quot;ERR&quot;);
					}
					_delay_ms(dispdelay);
					lcd_cls();

					//Lattitude
					lcd_putstring(0, 4, &quot;Lattitude&quot;);

					get_latitude(buf2, buf3);
					lcd_putstring(1, 2, buf3);
					get_latitude_ns(buf2, buf3); //N or S
					lcd_putstring(1, 14, buf3);
					_delay_ms(dispdelay);
					lcd_cls();

					//Longitude
					lcd_putstring(0, 4, &quot;Longitude&quot;);
					get_longitude(buf2, buf3);
					lcd_putstring(1, 2, buf3);
					lcd_putstring(0, 4, &quot;Longitude&quot;); //E or W
					get_longitude_ew(buf2, buf3);
					lcd_putstring(1, 15, buf3);
					_delay_ms(dispdelay);
					lcd_cls();

                    //Ground speed
					lcd_putstring(0, 2, &quot;Ground speed&quot;);
					get_ground_speed(buf2, buf3);
					lcd_putstring(1, 5, buf3);
					lcd_putstring(1, 12, &quot;[kn]&quot;);
					_delay_ms(dispdelay);
					lcd_cls();
		        }
		    }
	        rx_buf_cnt = 0;
	    }
	    else
	    {
		    rx_buf[rx_buf_cnt++] = ch;
	    }
    }
	return 0;
}

Interfacing an analog temperature sensor to an AVR microcontroller

For a transceiver I wanted to design a temperature control for the final amplifier transistors. I have some KTY81-210 temperature sensors here in my junk box. So, one of these should be used. The transceiver again holds digital equipment (AD9951 as VFO and AD9834 as LO) driven by an AVR ATMega32 microcontroller. One of the ADC inputs of this device was designated to process the data delivered by the temp sensor.

Some theory in advance: How does a temperature sensor work?

The KTY81-210 is a “PTC” element. This to say that electrical resistance increases with temperature.

PTCs are mostly made from barium titanate (BaTiO3). This material has an interesting feature: Below the so called “Curie temperature” the material holds an NTC characteristic. Above this point the material changes its behavior and increases resistance sharply and resistance from now on increases with temperature.

The T->R function shows a relatively linear approach with some limitations:

kty81-210

For the accuracy in a simple temperature measurement for PA transistors this should be sufficient.

Connecting the PTC to the microcontroller

A simple voltage divider to GND is the best idea to get a voltage depending on the conductivity of the sensor element.

PTC-ADC-connect

One thing should be kept in mind: When current flows through the sensor, the sensor will heat up. If the current is too high this will corrupt your temperature reading. Thus I recommend having Rv > 5 kΩ.

Setting up the ADC (analog digital converter)

AVR microcontrollers have a 10-bit analog digital converter (ADC) operating on the successive approximation principle. The resolution due to the number of bits used is limited to 1024 discrete values (ADC value in the range 0..1023). They can be operated with a reference voltage of either 5V or 2.56V determined by software setup.

Before you can take a measurement the ADC must be set up. This can be done in the function that is used to read an ADC value or at the beginning of the program before enterin the main() loop. I here use the complete setup in the called function (code for ATmega32):

//Read ADC value
int get_adc(int adc_channel)
{
    int adc_val = 0;

    //ADC config and ADC init
    ADCSRA = (1<<ADEN) | (1<<ADPS2) | (1<<ADPS1); //Activate ADC, Prescaler=64

    ADMUX = (1<<REFS0) + adc_channel; //Read ADC channel with Vref=VCC

    _delay_ms(3);

    ADCSRA |= (1<<ADSC);
    while(ADCSRA & (1<<ADSC));
    _delay_ms(3);

    ADCSRA |= (1<<ADSC);
    while(ADCSRA & (1<<ADSC));

    adc_val = ADCL;
    adc_val += ADCH * 256; 

    ADCSRA &= ~(1<<ADEN); //Deactivate ADC

    return adc_val;
}

With this the ADC references an input voltage set to 5V.

Calculating the ADC value

The ADC value depend on the input voltage: 0 represents 0 volts, 1023 represents 5V. Any value in between is calculated by:

Vx / 5V = ADC / 1024

where Vx is the applied voltage to the ADC input channel, ADC is the figure between 0 and 1023 representing the voltage.

So, Vx equals to

Vx = ADC * 5V / 1024

Calculating the temperature from the ADC voltage

The voltage divider equation lets us deduce the value of RPTC:

5V / UADC = (Rv + RPTC) / RPTC.

which equals to

5V / UADC = Rv / RPTC + 1

5V / UADC – 1 = Rv / RPTC

(5V / UADC – 1) / Rv = 1 / RPTC

Rv / (5V / UADC – 1) = RPTC (where UADC != 0)

Once having calculated RPTC we can calculate the temperature. Therefore we assume a linear dependency of the resistor to temperature:

R = T * m + R0.

(m is the slope factor (gradient), T0 is the constant term, in this case the PTC’s resistance at 0°C).

Changing the equation so that T is the result, we get:

(R – R0) / m = T.

R0 from the datasheet is 1630Ω typical. The slope factor (gradient) can also be deduced from the data sheet. It is

m = Δy/Δx.

So we can take two points out of the function (e . g. 0°C and 100°C) and calculate

m = Δy/Δx = 100K / (3392Ω-1630Ω) =100K / 1762Ω = 0.0567K/Ω

That means that the PTC element equals a change in resistance by 1Ω when the temperature changes by 0.0567K.

Packing all this into a C-function for the microcontroller we get:

//Measure temperature of final amplifier
//Sensor = KTY81-210
int get_temp(void)
{
    double temp;
    double rt, rv = 5100, ut;
    double r0 = 1630; //Resistance of temperature sensor at 0°C
    double m = 17.62; //slope of temperature sensor in Ohms/K

    //Calculate voltage from ADC value
    ut = (double)get_adc(4) * 5 / 1024;

    //Calculate thermal resistor value from ADC voltage ut
    rt = rv / (5 / ut - 1);

    //Calculate temperature from rt
    temp = 10 * ((rt - r0) / m);

    return (int)(temp);
}

Rv of the voltage divider in this case is set to 5100Ω. For other values please change value in the definitions section.

get_adc(4) is the function to get an ADC reading (see above!), in this case the voltage divider is connected to ADC4 input.

By the way: The functions returns a 10 fold value because in the reading I add a decimal to get one tenth resolution.

73 de Peter

(DK7IH)

 

 

Going back in time: “Old school” transceiver for 20 meters with 20 watts of output

Hi again! This project directly “beams” you back to the “Good ol’ 80s” when there was no stuff like “DDS, “OLED” or even “SDR” or other modern technology we today use to build our radios.

I designed this transceiver using the “old school” techniques because in a German QRP forum on the internet some hams originated a “Back to the roots”-movement which I thought was a great idea. So I too went back in time 3 decades and constructed a radio like I did it in the eighties at the beginning of my “homebrew career”. That meant: No digital stuff, just a simple VFO but (and that is new) higher rf output power because condx are fairly low on the hf bands currently.

I later presented this radio at an annual German convention of homebrewing hams called the “Black Forest Meeting” named by the place where it is held the beginning of October each year.

To give you an impression, that’s how the radio looks from the outside. Pretty “old school”, isn’t it?

“Old school” SSB transceiver for 14MHz by DK7IH (2018)

The main design objectives were very simple:

  • Compact in size (even without using SMD components),
  • Analog VFO with vernier drive (1:10 gear) and variable capacitor,
  • No digital stuff (=> no digital noise!),
  • RF Output in the range from 15 to 20 watts pep in SSB,
  • Single conversion superhet (9MHz interfrequency)
  • No “save as many components as possible”-design.

First the block diagram giving you the basic structure of this radio::

“Old school” SSB transceiver for 14MHz by DK7IH (2018) – basic outline

The VFO

I decided to use an analog VFO in this project due to three reasons:

  1. It’s really old style,
  2. it is much less prone to produce any unwanted “birdies”, and
  3. phase noise performance usually is better than most of the digital ways to generate a signal.

For the VFO I chose the Hartley design characterized by a tapped coil. This type uses less critical components than a comparable Colpitts circuit thus reducing number of parts (2 caps in this case that are avoided) which might lead to unwanted frequency changes (drift).

How to build a VFO that is really stable

Lots of pages have been written about this topic. This another one. First, be aware of the fact that it is not possible to build a VFO that has the same frequency stability like a modern digital system. This is because these systems are all crystal controlled. But it is possible to achieve a drift of some dozen Hertz within an hour or so which is absolutely sufficient for having even a longer QSO.

The main problem is based on physics, or thermodynamics to say more exactly. All material expands when heated and contracts when ambient temperature decreases. OK, some exceptions exist, water below 4°C is the commonly known example of them.

Avoiding thermal runaway

Heat is the problem in such a circuit. It comes from the interior of the components when current flows through them and from the outside, for example when the transceiver is exposed to sunlight or placed near another source of thermal energy. Also heating of the final rf amplifier stages may contribute to heating the cabinet inside. The electronic parts forming the central strucure of the tuned circuit exert the main influence connected to thermal runaway of the frequency that is generated.

The general approach is: When we can’t avoid physical effects we must choose components that change their values in such a way to compensate the changing of the values of the other parts. That means we have to look carefully on temperature coefficients of the varoius components we intend to build into our VFO.

Choosing the “right” components for your VFO

Choosing advantageous components is crucial for frequency stability. So I did some brief research to find out more about temperature coefficients of coils of various types and available capacitors. Here are some of the outcomes.

Explanation of syntax: If a relation is negative, a minus sign (“-“) is given. In this case the value (C or L) decreases when temperaure increases. A plus sign (“+”) indicates a positive coefficient. When the relation of value change by temperature change is weak (that means no intense changing of the value when heated), there is only one “-“-sign. The more “-“-signs you have, the higher this respective ratio is. The same applies for “+”-signs to indicate a positive relation.

Capacitors:

  • Ceramic capacitors: —
  • Polystyrene capacitor: –
  • NP0 (C0G) capacitor: no measurable effect

Inductors:

  • Air coil on polystyrene coil former: +++
  • Coil wound on T50-6 yellow toroid: +

Based on this short survey, the best combination would be NP0- and Polystyrene caps together with an inductor wound on an T50-6 (yellow) core. Hopefully their temperature behavoiur will compensate more or less and lead to best stability. Hint: On the photos appearing later in this text you will see an air coil wound on a TOKO style coil former that has been used because it does not need so much space.

The VFO circuit

I finally chose the Hartley circuit for my VFO. There it is:

“Old school” SSB transceiver for 14MHz by DK7IH (2018) – VFO

VFO Circuit explanation

Starting from the left you can see the tapped coil (here 60 turns tapped at 10 turns from the bottom end) on a 5.5 mm TOKO style coil former without any core. In parallel there are various capacitors (polystyrene and NP0 mixed) to build up the total capcity. It is common use to spread the total capacity needed to various single capacitors because it has turned out that the effects of temperature change are less significant if you use more (and therefore smaller) single capacitors.

A 100k resistor is used to pull the gate to ground and therefore provides a correct bias at the FET’s gate. The 1N914 diode is a so called “clamp” diode that has been installed to stabilize (and therefore reduce) the rf voltage in order to avoid excessive rf voltage coming to the FET’S gate which would lead to distortion. This diode has a negative side effect, but that an be accepeted for a VFO in the rf bands: It slightly increases phase noise because it works as a regulator. With some the designs you can see this diode in reverse position, don’t worry, the regulating effect takes place either.

To ensure the oscillator to produce radio waves, in-phase feedback between gate and source is generated via the tap you can see with the coil. A tap of about 1/6 of the whole number of windings provides enough feedback voltage to let the oscillator start by its inherent thermal noise and generate clear sine waves afterwards. Putting the tap too close to the “hot end” will cause distortion because the amount of energy coupled back to the gate will be too high. Also instabilites are probable because of excessive drive power to the gate of the FET.

On top of the tuned circuit there is a varactor diode that is used to be controlled by a positive voltage to form a RIT (receiver independend tuning) control circuit. It is very loosely coupled to the tuned circuit to minimze temperature effects and because only 1 or 2 kHz “swing” is needed. The generation of the RIT voltage will be described later in this text.

The main tuning capacitor

An air capacitor is mandatory here! You can either use a ready made one from the surplus market. But to keep it as small as possible I built my own by dismanteling one of old variable capacitors formerly used for homemade AM radios. Use a small drill to remove the rivets, dismantle the capacitor completely and rebuild it again as an air capacitor (get rid of the plastic dielectric interlayers!) by using M2-screws and nuts. Youl will have evenings of endless fun with this game! 😉

Buffering and amplifying the Signal

The second stage with another FET is very loosely coupled to the source of the first FET. This is made to minimize effects of load changes to the frequency. This stage is a so called “source follower” giving a very low impedance signal to the final stage that is responsible for the amplification of the signal to a level of 2 to 3 volts pp which you will need for the rx mixer that has been designed as a dual-gate-MOSFET mixer (see receiver chapter later!).

DC voltage in the VFO

Voltage stabilization is crucial for best performance of this critical part of the radio. Supply voltage changes always lead to frequency changes. So a two-level buffering is common use here. The first (and most critical) stage is buffered twice (10V voltage regulator integrated circuit 78L10 and subsequently by a 6.2V zener diode) whereas the buffer and the amplifier stages are supplied with 10V regulated DC voltage only.

Ambient thermal isolation

To avoid the VFO being affected by interior thermal convection (flow of warm air inside the cabinet) it is recommended to shield the VFO from the rest of the transceiver. I do not recommend using metal sheets as walls here because these form other unwanted capacities that will lead to thermal effects on the generated frequency. Metal also is a good conductor for thermal energy, so you might run counter to your goals. My thermal insulation therefore is made of simple cardboard.

The Local Oscillator (LO)

This oscillator is much more uncritical than the VFO because it is crystal controlled. The purpose of the LO is to supply a carrier signal for the SSB modulator. Due to the fact that there are two sidebands we theoretically can use this LO must be switched to either one of two possible frequencies. In case of an interfrequency of 9MHz (9000kHz) these are: 9001.5 kHz for the first sideband and 8998.5 kHz for the second sideband. Please note that I did not write “USB” or “LSB” because the frequencies forming each sideband might be changed because of the frequency plan of the transceiver where by mixing with the VFO frequency the sideband might be inverted depending on if you add or substract the VFO frequency from the 9MHz-SSB signal.

There are several possibilities to produce these two frequencies:

  1. Using two different oscillators each equipped with a single crystal,
  2. switching two crystals with one osillator,
  3. using a variabale capacitor or a coil to “pull” one crystal to the desiered frequency.

Discussion:

  1. This method means high effort but surely is the most exact one because there are no influences of the unneeded choice to the other crystal currently on duty.
  2. Is the worst idea because the unswitched crystal is highly prone to influence the freqeuncy of the switched one because they are linked to parasetic capacities within the wiring, the switch and so on. Forget this one espacially when using the internal oscillator of a NE602/SA612!
  3. This to my point of view is the best compromise between circuit simplicity and function. You can see this way of sideband switching in my transceiver.

This is my local oscillator:

“Old school” SSB transceiver for 14MHz by DK7IH (2018) – Local Oscillator (LO)

It is a simple Colpitts circuit where in-phase feedback and feedback voltage control are achieved  by a series of two identical capacitors. A simple switch,  a capacitor (90pF max.) and a coil (4 to 8uH max.) that are either connected to the base of the transistor via the 9MHz crystal determine the sidband freqeuncy of the oscillator. Signal is taken out via the collector.

The Receiver

The receiver will be presented step-by-step starting with the front end stage:

The RF Preamp

This stage is connected to the antenna relay. It provides an amount of basic amplification for the antenna signals. But that is not the main purpose. Noise figure improves significantly if you use a stage with low inherent noise. Thus a dual-.gate MOSFET is installed here. This semiconductor is also used to control stage gain because gate 2 of the MOSFET is connected to the AGC chain of the transceiver. About 12 dB gain swing are possible here. Stage gain is about 15dB.

“Old school” SSB transceiver for 14MHz by DK7IH (2018) – RX PREAMP

Note the position of the primary and secondary windings of the input and the output coil. To avoid self-oscillation the output (drain) of the MOSFET is connected to the untuned part of the LC circuit. Coils must be shielded and should be equipped with ferrite heads (in the photo the piece in left bottom corner).

The Receiver’s Mixer

In this stage also a dual-gate MOSFET is used. This type of mixers provides good capabilities to cope with high signal levels without producing unwanted signals (high IMD3), gives some dB of gain and is low-noise also.

One “disadvantage”, if you want to say so, is that it needs a little bit of higher VFO drive (about 2 to 3 volts pp). Gate 2 bias is generated via the voltage drop on the source line. The tuned circuit in drain line is adjusted to the desired interfrequency. See the schematic for the exact winding data and parallel capacitor.

“Old school” SSB transceiver for 14MHz by DK7IH (2018) – RX MIXER

The SSB Filter section

Transmitter and receiver share the same SSB filter in this transceiver. So some sort of switching is recommended even if circuits exist that go without one. I used a high quality relay made by Teledyne that I bought in a 10-piece bundle for low price (1€ each!) via a well-known internet marketplace. Caution: Some SMD-relay I tested prior to building this rig were disastrous concerning signal isolation between terminals. To avoid any disappointment or frustration  I recommend testing a relay before you finally install it.

“Old school” SSB transceiver for 14MHz by DK7IH (2018) – SSB FILTER

All connections to the rest of the circuit must be made with shielded cable. I found an interesting alternative: I sometimes design my own very thin shielded “cable” with brass tubing (1 mm inside diameter) where I put insulated cable inside. The brass tube is connected to GND on the Veroboard. You can not bend these tubes but longer lines can be interrupted for a short piece so that the “bend” can be made by putting two parts of tubing in 90° degree angle for example.

The IF Amplifier

This one might look familiar to you. It is a simple “remake” of the front-end stage. The one remarkable thing is the secondary of the output transformer. This coil has 4 windings (prim. 16 turns). The secondary is center tapped (2 + 2 turns). This is because the product detector (SSB demodulator) has a symmetric input. Very important in this stage is the 100uF capacitor in VDD line. This cap prevents the stage from AF resonating and self-oscillations on the VDD line and makes the receiver much more “quiet”.

“Old school” SSB transceiver for 14MHz by DK7IH (2018) – IF AMP

The SSB demodulator

This stage is probably the most “old-school” part of the whole transceiver. It uses an old CA3028A differential amplifier as mixer circuit:

“Old school” SSB transceiver for 14MHz by DK7IH (2018) – PROD DET

You won’t be able to buy large amounts of this IC anymore. And if you get one, the prices are close to or beyond a rip-off. But there is an alternative. You can build your own “IC”. Watch this page where all the information you need is provided!

Signal input goes to the paired transistors forming the amplifier stages. LO is fed into the line transistor that is used to set the current of the differential amplifier thus providing a switching and therefore superposition of the two signals.

The output circuit is made of an audio transformer formerly used in the final audio amp  of old AM radios (coil resistance is about 300Ω each side). The 2.2nF capacitors eliminate remainders of the rf signals and “ground” the terminals of the AF transformer.

Audio amplifier

This final receiver part consists of two stages: An audio preamplifier with a bipolar transistor and a final amp with a TBA820M integrated circuit.

“Old school” SSB transceiver for 14MHz by DK7IH (2018) – AUDIO AMPS

The two caps 0.22 and 0.1uF determine how the higher frequency components in the audio signal are cut off. The higher the total value the more the higher frequency components of the audio signal will be limited due to the equation XC=1/(2*PI*f*C).

Tr1, which is a universal purpose NPN transistor, provides high gain. Thus a 10k resistor is installed to form a voltage divider with the audio gain potentiometer.

In the final stage I use a TBA820M ic (8-pin DIL version). This one is more linear than the well-known LM386 that you usually can find in this place and it is not so prone to self-oscillation. The cap aside the 100uF in the top left corner of the schematic is not marked, its value is 0.1uF.

Loudspeaker impedance is 8Ω.

The AGC

Automatic gain control makes listening to signals much more comfortable. AGC voltage is audio derived, like in my other transceivers. The circuit also is nearly the same:

“Old school” SSB transceiver for 14MHz by DK7IH (2018) –

Due to the very high gain of the product detector this stage is directly connected to this circuit and not to the AF preamp. A potentiometer is used to set the threshold of the AGC onset.

Next stage is a simple audio preamplifier followed by a “Greinacher Circuit” serving as voltage doubler and providing DC voltage proportional to the audio signal level. A dc amplifier with another NPN transistor lets its collector voltage drop as soon as it is fed with significant dc input voltage. Thus this voltage decreases and so it can be used to control gate 2 of the MOSFETs in the various receiver stages that are equipped with tetrodes.

The S-Meter is connected to the emmitter of the final transistor. If conductivity in the transistor rises, the emmitter becomes more positive and the S-Meter needle is deflected proportionally. The 220Ω potentiometer in the emmitter line must be set in accordance to the respective S-Meter you are using. One shortcoming should be mentioned: If you have a not so sensitive meter then the value of the pot can be set to nearly 100Ω or above. This will prevent the collector from dropping to nearly 0V in case there is a strong signal and hence reduce the maximum dB you can get from the AGC chain.

The Transmitter

The transmitter section is designed for an output level of about 20 watts and uses 4 stages all equipped with bipolar transistors. The last stage is a push-pull stage, the 3 low-power stages are single ended. I prefer push-pull for the last stage (if possible)  because this circuit inherently does not create even harmonics thus simplifying output filtering.

The first parts of the transmitter to be shown here are the microphone amplifier, the SSB-generator and the TX mixer:

“Old school” SSB transceiver for 14MHz by DK7IH (2018) -MIC AMP

The mic amp is simple but provides enough gain and good linearity for using an old-style dynamic microphone. It works in common emmitter mode and has gain of about 15 to 20 dB.

“Old school” SSB transceiver for 14MHz by DK7IH (2018) – SSB GEN

The audio signal amplified by the microphone amplifier is fed into PIN1 (Input 1) of an NE602/SA612 mixer IC which is the simpliest way to generate a DSB signal with a Gilbert cell. LO input is fed to PIN7 and should be in the range of 200 to 300mVpp. Thus a 12pF cap has been installed to limit LO voltage going to input at PIN7.

Carrier suppression is around 45dB when LO offset frequency is correctly set for each of the two sidebands and LO voltage is not much higher than the 300mVpp mentioned before.

The DSB signal produced by this mixer goes on to the SSB filter relay and filter that has been described before. The use of shielded cable is mandatory, too.

The TX Mixer

“Old school” SSB transceiver for 14MHz by DK7IH (2018) – TX MIXER

You won’t be able to recognize many differences if you compare this TX mixer to the DSB generator. In fact, there are none.

The 14MHz Band Pass Filter

Next is the band pass filter that consists of 2 coupled tuned LC circuits for 14MHz. They are also wound on TOKO style coil formers. Data can be found in the schematic underneath.

It is important to also install the ferrite heads that are provided with most of the coil formers and to use the shield “metal cans” that are also standard for these coils. This is to prevent stray coupling of rf energy into the first stage of the power amplifier strip and therefore preventing self-oscillation of the transmitter strip.

For proper adjustment set the transmit frequency to about half of the frequency swing ((i. e. to about 14.200 kHz) and tune for max. output.

“Old school” SSB transceiver for 14MHz by DK7IH (2018) -BPF

If you modulate with a two-tone signal to the mic amp you should see about 500mVpp by the output of the BPF when the chain is fully driven.

The Preamplifier

We start with the low power end of the power transmitter section. A bipolar rf type transistor is the center part of this stage.

“Old school” SSB transceiver for 14MHz by DK7IH (2018) – TX PREAMP

This one is a standard circuit and has been “trimmed” for maximum linearity in order to reduce distortion to a minimum (which is also true for the following stages). You can see the well understood 2 master ways of achieving max. linearity in an amplifier stage:

  • Negative feedback between collector and base (i)
  • Emmitter degeneration (II)

Explanation:

i) The first measure goes along with the 2.7kΩ resistor between collector and base of the transistor. This resistor provides positive dc bias voltage to the base and leads 90° out-of-phase ac voltage to the transistor’s input. This reduces gain and therefore distortion. But due to the fact that the whole transmitter strip has plenty of gain, this loss in gain is not a serious problem.

ii) The 10Ω resistor in the emmitter line is not bypassed by a capacitor. This stabilizes the circuit. When the current through transistor increases the emmitter voltage will rise (according to Ohm’s law) and the voltage between collector and emmitter drops. This reduces voltage difference between base and emmitter and hence also reduces gain.

The coupling to the next stage is done by a capacitor of 0.1uF. This causes some impedance mismatch. But that is as well not a big problem because the gain reduction here helps to prevent the whole transmitter from unwanted oscillations by diminishing overall gain.

The Predriver

This stage is somehow a copy of the stage before but allows more current to flow through the stage. It is also operated in class “A” mode and uses the same methods to maximize linearity like the preamp stage.

“Old school” SSB transceiver for 14MHz by DK7IH (2018) – TX PREDRIVER

You can use a 2N3866 transistor here which is available. But any other rf power transistor for driver stages (2SC1973 etc.) will also do the job well. A heatsink is recommended even if stage current ist not that high. T1 should be a toroid, a “pig-nose” core in this place to my experience is not the best choice. The 10uH RFCs are ready made ones but you can also wind 20 turns of 0.4mm enameled wire to a FT37-43 toroid core.

RF output of this stage could be measured as 100mW into a 50Ω load.

The Main Driver

This stage has an old 2SC2078 CB transistor and is operated in class “AB” mode. An alternative could be a 2N3553 that is available on ebay for example. A heatsink is neccessary for whatever type you use.

“Old school” SSB transceiver for 14MHz by DK7IH (2018) – TX DRIVER

Correctly set bias for “AB” operation is ensured by the 1kΩ resistor from VDD to the bias circuit. The 1kΩ resistor limits the current whereas the diode works as a stabilizing element (thermistor). It must be connected directly to the case of the transistor ensuring good thermal contact. If the temperature of the devices rises the resistance of the diode will decrease. Hence current through the diode increases thus reducing the part of the current that can pass through the base-emmiter line of the 2SC2078. Quiescent current is stabilized and thermal runaway is prevented.

The rf output is uncommonly terminated with a low-pass-filter. This is because I first intended to build the transceiver for an output level of about 4 watts. But then I had the idea that the space still available on the veroboard could be used by another amplifier definitely leaving the QRP power level. So I left the circuit how it first was and just added the final amplifier stage.

Output of this driver stage now ist set to 1 watt into a 50Ω resistor.

The Final RF Amplifier

Now let’s go for the power machine in this transceiver:

“Old school” SSB transceiver for 14MHz by DK7IH (2018) – TX POWER AMP

2 rf power transistors 2SC1969 by Eleflow provide up to 20 watts of rf power. Bias for such a high power stage can not be set by a simple resistor. Here a line transistor (BD137) serves as current control. Diodes D1 and D2 (1N4002 or equ.) follow the same purpose like the single one in the stage described before. They must be mounted with excellent thermal contact to each of the 2 power devices which ensures secured protection against thermal runaway. The transistors also must be connected to a large heatsink. I use Aluminium metal strips (2mm thickness) to connect them to the back wall of the cabinet.

RF is fed into the power transistors via a network of 8.2Ω  resistors and two 22uH rf chokes that seperate the rf line from the dc bias line letting only dc pass. This method makes construction of the input transformer easier. Winding ratio is 4 turns primary, 2 turns secondary. This is because the input impedance of the stage ist fairly low (aorund some ohms).

The output transformer is a homemade “pig-nose” of 6 toroids FT50-43, where 3 toroids are stacked (using 2-component glue) and 2 of these stacks are glued in parallel (see picture at the end of this text for details!). Winding ratio is  1 + 1 (primary center tapped) to 4 on secondary.

Quiescent current of this stage should be set to about 100mA.

A low-pass-filter terminates this stage and is connected to the antenna relay.

In addition you find a section to measure rf power. This is again the so called “Greinacher-Circuit” which doubles the voltage and serves as a charge pump. The dc output of this circuit directly leads to the S-Meter indicating output power of the transmitter.

Performance

First the spectrum of the signal with the transmitter fully driven to 20 watts output power with a two-tone-signal:

IMD3 is about 28dB below signal peak which I think is acceptable.

Amplitude diagram is as follows:

2-Tone-Signal Amplitude
2-Tone-Signal Amplitude

Max. radio frequency voltage is 90.4Vpp which calculates to about 20 watts of rf power (P=(Vpp/(2*SQR(2)))²/50Ω).

Power switch board and RIT voltage

A 12V relay with two pairs of contact sets is the heart of this unit. DC power is lead to TX, RX and permanent supply via the respective power lines.

RIT voltage generation is a little bit more complicated. When the RIT switch is in “OFF” position, RIT voltage always is taken from the fixed voltage divider that is formed of the two 4.7k resistors either when on receive or transmit mode.

If RIT is “ON” then there are two possibilities: When on receive mode, RIT voltage is gained from the 10k lin. potentiometer in the front panel. When on transmit mode RIT again is taken from the fixed voltage divider.

There is also a false polarity protection diode. This can be any silicon type with max. current >= 5 A.

"Old school" SSB transceiver for 14MHz by DK7IH (2018) - TR SWITCH RIT
“Old school” SSB transceiver for 14MHz by DK7IH (2018) – TR SWITCH RIT

Mechanics

The construction is sandwich style made of 2 layers:

“Old school” SSB transceiver for 14MHz by DK7IH (2018) -Inside View – VFO, LO, RX, SSB generator, TX mixer etc.
“Old school” SSB transceiver for 14MHz by DK7IH (2018) -Inside View TX and switchboard

OK, that’s the story. Thanks for joining me on the trip to the past! 😉

73 de Peter.

Shrinking a QRP transceiver to (nearly) the size of a pack of cigarettes

Hint: This article is only a preview to this miniature sideband transceiver. The full description can be found here:

https://radiotransmitter.wordpress.com/2019/04/22/revision-of-the-cigarette-pack-transceiver/


The challenge started some weeks ago, when John, ZL2TCA, commented to this blog

you next challange is to build a rig into a cigerette packet size case.

My problem: I don’t smoke, have never smoked and probably never will. 😉 But I have a new transceiver for 20 meters, that might come close to the dimensions of a pack of “cancer sticks”.

DK7IH pocket sized qrp transceiver 20-4 a
DK7IH pocket sized QRP transceiver 20-4

The transceiver is nearly the same circuit as applied with the “Micro 20-III” but uses a single ended final amplifier instead of a push-pull circuit. I hope to find time the next days to publish an article on this rig featuring full description of the radio. Currently I’m in the IOTA contest and working stations from all over Europe.

73 de Peter