Constructing a high performance transceiver for voice communication on 14MHz

 

Abstract

A general and good practice in engineering is a steady process of improvement. This  article describes the construction of a high performance transmitter/receiver for SSB (voice) communication covering the 14MHz (20 meters) high frequency amateur radio band.

Various modules that have proven high performance, liability and ruggedness in recent constructions will be combined to form a radio with outstanding receiver performance, an ultra linear transmitter with output range 15 to 20 watts and a top audio sound quality both on transmit and receive.

DK7IH - High performance SSB 14MHz 20meters Transceiver - Front view
DK7IH – High performance SSB 14MHz 20 meters Transceiver – Front view

Key features are:

  • Dual DDS frequency generation with AD9834 (Local oscillator) and AD9951 (VFO),
  • Microcontroller (MCU): ATMega644P by ATMEL,
  • Single conversion superhet receiver with 9MHz interfrequency (IF) and preamplifier, mixer and IF  amplifier equipped with Dual-Gate-MOSFETs,
  • Audio-derived automatic gain control (AGC),
  • Transmitter with MC1496 as double sideband (DSB) modulator and NE602 as transmit mixer,
  • power transimitter with 4 stages, final stage in push-pull mode.

Another version of this radio has been built before. But this one was equipped with a variable frequency oscillator (VFO) because of nostalgia reasons. Unfortunately a VFO lacks certain features (frequency stability above all) which can be overcome by using digital frequency synthesis without losing performance. Usage of a high performance DDS systems is a prerequisite to achievement and a possible solution.

Most building blocks of that respective radio have been redesigned except the VFO section that turned out as not being able to deliver the projected frequency stability to a 100% degree. Frequency instability occurred because of the flatness of the former cabinet that brought the aluminum case too close to the VFO tuned LC circuit. Aluminum  has a huge tendency to expand under the influence of heat so the rig was very temperature sensitive. That undeniable fault lead to a complete reconstruction using the old RX and TX modules and building a new set of frequency generators.

Parts of the old cabinet were reused but because of the fact that the whole rig got increased vertical expansion, the cabinet was “stretched” with two lateral strips of Aluminum.

Also a full electronic transmit/receive switch with p-channel power MOSFETs has been designed to avoid usage of a DC switch relay and get a “smooth” switching.

Another objective of this radio was to get out the absolute best performing circuits of the recent projects and to build a real high-performance radio. Hence this transceiver is also some sort of an improvement of the “Old school SSB TRX” as well. The circuit empirically turned out to be very good for communication in the 14MHz band. Because of this frequent readers of this website might detect certain similarities. 😉

The Receiver section

The design objectives were:

  • Low noise (achieved by using Dual-Gate-MOSFETs with the receiver to a large extent)
  • High dynamic range (achieved by using a Dual-Gate-MOSFET as receive mixer)
  • High AGC range (achieved by taking RF preamp and IF amp into the AGC chain)
  • Good audio quality (achieved by using a TBA820M as integrated AF amplifier circuit and a 5 cm loudspeaker)
DK7IH - High performance Transceiver 14MHz- Receiver Frontend & Mixer
DK7IH – High performance Transceiver 14MHz- Receiver Frontend & Mixer (Full size picture)

RF preamplifier and receive mixer

The radio frequency preamplifier has been designed primarily to improve the receiver’s noise figure. Delivering additional gain only is relevant in second order.

Preselection is performed with only one tuned circuit int G1 line. The center frequency of this circuit is 14.180MHz. In the output section of the stage an another identical LC circuit has been installed. This turned out to be sufficient because there is no immediate need of higher preselection. The subsequently placed mixer, that is also equipped with a Dual-Gate-MOSFET has very good high level processing qualities. No interfrequency feedthrough could be observed with various antennas. No IMD occured even when signals were very strong. Testing out in the field with partable antenna very far from man-made noise sources the receiver was very quiet and even very weak stations could be received and read with Q5.

To get most of gain swing from AGC the preamplifier is controlled by a DC voltage between 0 and 12V supplied by the AGC control stage to be described later. This voltage is halved by a 1:1 resistor voltage divider  because maximum gain of the Dual-Gate-MOSFET occurs with about 6V DC applied to G2.

Clipping diodes that are sometimes used to prevent high voltage entering the 1st stage have not been installed because they are prone to produce unwanted IMD products if signal levels from the antenna are too high and undesired mixing takes place there.

To prevent self-oscillation in the preamplifier, the tuned circuit LC1 and LC2 are connected together in a special way. G1 is connected to the tuned section of LC1. This section has high impedance, thus it should be connected to a load which also has high impedance. The coupling section of the coil with low impedance is connected to the 50Ω antenna. There are not two tuned parts of the LC circuits together in one stage.

The output of the Dual-Gate-MOSFET (low impedance) is connected to the coupling winding, the high impedance tuned part is going the high impedance of G1 of the mixer. The impedance ratio between the two coils is 16:4 due to the winding ratio of 4:1 of the coil set.

The sensitivity and noise figure of the whole receiver is determined by these two stages. Measurements showed that the minimum discernable signal is about 0.1µV which is very good for a short wave receiver.

SSB-Filter, IF amplifier, Demodulator, AF amp and AGC

The following stages are some sort of best practice combination of circuits that have proven to perform very well in the recent projects.

DK7IH - High performance Transceiver 14MHz- Receiver - SSB filter, IF amp, ddemodulator, AF preamp, AF final amp, AGC.
DK7IH – High performance Transceiver 14MHz- Receiver – SSB filter, IF amp, ddemodulator, AF preamp, AF final amp, AGC. (Full size picture)

SSB-Filter and relay

The SSB filter is switched with a special rf relay by Teledyne® ensuring excellent isolation of relay ports with very low capacities in the unswitched signal path. Here the usage of shielded cable is mandatory for connecting the relay/filter section to the transmitter (see later text!). A clamp diode has been installed to eliminate high voltage peaks due to self-induction when the relay is switched. This will prevent the MOSFETs in the switching unit from excessive voltage and possible destruction.

IF amp

A proven and reliable circuit can be found here as well. One stage delivers IF gain of about 12dB which is sufficient because the mixer following as a demodulator (NE612) also propduces some dB of gain. Too much gain in this section only contributes to high noise in the speaker later and is not desirable.

The Dual-Gate-MOSFET in this stage is also integrated to the AGC chain. Together with the RF preamp installed in the front and also being part of AGC control end we will get some 20 to 25 dB of gain swing when AGC is fully driven. This turned out to be enough, only in some rare cases I found that the manual gain control (also included in this recevier) needs to be used in addition when AGC is not able to cope with excessive signal levels.

Compared to a MC1350 IC equipped IF amplifier I found that gain control is much smoother because the V->dB function is very much less precipitously with the Dual-Gate-MOSFET than it is with the MC1350.

Demodulator

NE612 is built-in here. The main advantage of this IC is that it requires only a few components and it has got an additional gain of about 12dB or more.

In VDD line you will find a 5.6V Zener to bring 12..14V supply voltage down to about 6V. There are also two capacitors. The 0.1uF is for bleeding off rf energy from or to the supply rail, the same is the purpose of the 10uF cap for audio frequencies or low frequency noise present on VDD line. This noise sometimes originates from the digital components in the radio and should be eliminated at all reasonable points in the circuit. Also it will help to prevent the high gain amplifier chain from self-oscillating in the audio frequency range.

Audio frequency amplifier section

Two ICs are used here. The first is an operational amplifier (uA741) with a 150kΩ resistor as part of negative feedback circuit. This value is comparatively low. If (in rare cases) higher gain should be needed it can be replaced by e. g. 330kΩ or even more.

The main audio amp is the TBA820M, an integrated audio amplifier in 8 pin DIL case. It is an interesting alternative for LM386 because tendency for self-ocillation is much lower within the TBA820M. But it requires some more components. TBA820M can be switched with the load (speaker) to VVD or GND. I use a headphone jack in the radio, that is grounded, hence I prefer the latter version.

A “good” loudspeaker with 5cm of diameter was found by ordering a larger series of different speakers from Chinese vendors via ebay. The differences in sound quality are breath-taking. So it is worthwhile spending some money and order a larger variety of speakers and install the very best one.

AGC

This is a circuit I have used many times and it has proven to work very reliable. If you wish to have different settings concerning attack and decay time then another cap can be added via a switch to GND in parallel to the 47uF cap. Another 100uF for example will give a few extra fractions of a second in attack/decay time.

A 20kΩ variable resistor is used for manual gain setting. The AGC voltage that is near to VDD (12V or more) is divided and so AGC and manual gain control can be combined. At least until the point where noch AGCing will take place because the resulting voltage is <3V.

The “AGC thres.” variable resistor shown in the schematic will determine the point where AGC becomes active. I usually set it that way that solely band noise does not affect the AGC. Stronger stations (coming with S5 or 6 with a commercial transceiver) should give first minor influence on the AGC voltage. That is the point where amp gain should start dropping gradually. Strong stations must set AGC voltage to nearly 0 V.

DK7IH - High performance SSB 14MHz 20meters Transceiver - Receiver and switchboard modules
DK7IH – High performance SSB 14MHz 20 meters Transceiver – Receiver and switchboard modules (Left: Switchboard with 2 P-channel MOSFETs. Center: receiver front end and SSB filter. Right: Demodulator and AF section plus AGC)

The Transmitter section

The transmitter generally consists of two parts:

  • The SSB generator and the TX mixer, and
  • the Power Amplifier.

The full schematic of the two parts together:

DK7IH - High performance Transceiver - Transmitter section
DK7IH – High performance Transceiver – Transmitter section (Click here for updated schematic of this assembly)

Microphone amplifier

Starting from the left we see the microphone amp. A nostalgic but still available operational amplifier integrated circuit (741) is used here. The amp has high gain (about 30dB) to make a dynamic microphone connectable. There is no DC feeding for an electret microphone. If you should wish to use one then the negative feedback resistors should be lowered to about 47kΩ and the audio level should be carefully observed to avoid excessive driving. DC must also be applied for htis type of microphone!

Double sideband generator

The MC1496 (still available as NOS in 14 pin DIL case or fresh from the market in SMD package by ON Semiconductors) offers high carrier suppression of about 50 to 60 dB. Therefore a network of 2 x 10kΩ and a 50kΩ variable resistor has been installed. The crucial point: To make full usage of this network the carrier offset must be set properly. If you should notice that there is no point within the full swing of the 50kΩ variable resistor then the carrier frequency should be readjusted.

A balanced output transformer has been installed to improve carrier suppression and to enhance output voltage.

SSB filter coupling out

The usage of shielded cable is mandatory here to avoid transfer of rf stray energy into the DSB and SSB line!

Transmit mixer

This stage also is equipped with an NE612 doubly balanced mixer due to reasons of circuit simplicity.

14MHz Band pass filter

This filter also needs observation. I use the TOKO style coil formers familiar from other projects. The winding data can be found in the schematic. The coil formers must have the ferrite caps and metal shield cans on to avoid incoupling of rf energy from the subsequent power stages. The filter should be placed away from the higher power stages to avoid self-oscillation inside the transmitter section.

RF amplifier power stages

The amplifier presented here has been tested in 2 different radios so far and has proven to be very stable, very linear and very rugged against antenna mismatch. The power levels are about 10 db gain per stage. From the second stage on the output impedance is 50Ω. This makes it easier to measure power levels with a 50Ω standard dummy load.

The 2 watt driver stage uses a PI-filter instead of a broadband transformer. This is because I intended to save some space on the veroboard and for a monoband transmitter this is a practical solution. If you should find out that there is a mismatch that results in losing gain, then the capacitors can slightly be modified because the L-network has impedance transforming capabilities. By knowing input versus output impedance and calculating a “Q”-factor subsequently L and C can be computed to get a defined step-down impedance (Link for further information). This is a useful method and, in case of low pass filter like applied here, there is also a filter for harmonics.

Driver and PA power amp are biased for AB-mode, all other stages operate in A-mode to ensure best linearity. Strategies using emitter degeneration and negative feedback are inherent in preamp and predriver stage.

All transistors apart from preamp stage require usage of heat sinks.

Impedance matching is either not done (stage 1 to 2), by transformer (stage 2 to 3) or by L-network (stage 3 to 4). Whereas from stage 3 to 4 also there is a transformer applied to split the signal symmetrically to the two bases of the final transistors.

If there should be a tendency for self-oscillation within this stage the input transformer secondary winding can be center tapped and put to GND via a 0.1 capacitor.

Power out depends on DC power voltage and is about 20 watts when run on 13.5 V DC power supply an the amplifier terminated to a 50Ω load.

Here

DK7IH - High performance Transceiver - Transmitter section - Spectrum output signal at 20 watts
DK7IH – High performance Transceiver – Transmitter section – Spectrum output signal at 20 watts
DK7IH - High performance Transceiver - Transmitter section - Spectrum carrier
DK7IH – High performance Transceiver – Transmitter section – Spectrum carrier

This is a spectroscopical analysis of the fully driven transmitter (f=14.200kHz, Pout = 20.1 watts, VDD=13.0V) and the remaining carrier.

Harmonics are filtered very effectively . This is achieved by using a push-pull final stage driven in AB mode. Some authors say this is useful to eliminate odd number harmonics. On the other hand there are two sections of low pass filtering (one between driver and PA, one following PA). The figure of the output spectrum between and 50 MHz below:

DK7IH - High performance Transceiver - Transmitter section -Showing harmonic suppression
DK7IH – High performance Transceiver – Transmitter section -Showing harmonic suppression
DK7IH - High performance SSB 14MHz 20meters Transceiver - TX modules
DK7IH – High performance SSB 14MHz 20 meters Transceiver – TX modules: Left the SSB generator, center 3 driver stages, right: final amp and antenna relay.

The Dual DDS Oscillator System

The DDS has got the following features:

  • VFO: AD9951 + amplifier,
  • LO: AD9834 + amplifier,
  • MCU: ATMega644P,
  • LCD: NOKIA 5110,
  • Tuning: Optical rotary encoder by Bourns,
  • User interface: 4 keys to control the digital settings,
  • Analog inputs: User keys (ADC0), VDD (ADC1), S-Value (ADC2), TX PWR (ADC3), PA Temp. (ADC4).

The schematic:

DK7IH - High performance Transceiver - Dual DDS AD9951 and AD9834
DK7IH – High performance Transceiver – Dual DDS AD9951 and AD9834 (Full size image)

The control lines for DDS1 (AD9951) and DDS2 (Ad9834) are as follows:

//DDS1
//IO_UD: PB0 (1) (green) 
//SDIO (DATA): PB1 (2) (white)
//SCLK PB2 (4) (blue)
//RESET PB3 (violet)

//DDS2
//FSYNC: PC0 (1) (green) 
//SDIO (DATA): PC1 (2) (white)
//SCLK PC2 (4) (blue)
//RESET PC3 (pink)

The colors are the colors used for the cables in my radio.

The LCD is connected likewise:

//LCD
//RES: PD4
//DC: PD5
//DIN: PD6
//SCLK: PD7

The NOKIA5110 LCD has been designed for VDD=3.3V. Please use 10kΩ resistors in the control lines which are not in the schematic! 3.3V are derived more or less closely by switching 2 Si-Diodes in series which results in a voltage drop of about 1.4V. Hence the LCD gets 3.6V DC from the 5V supply chain which is no problem for the module. One big advantage of the Nokia LCD should not be forgotten: It is very quiet and does not produce any discernable digital noise. Thus it is my favourite meanwhile for receivers on the RF bands.

For both DDS modules coupling out the rf is done with symmetrical circuits using trifilar broadband transformers. 10 turns on a FT37-43 core are a good choice. This will enhance gain and reduce spurs.

DDS2 is clocked to 110MHz, but keep in mind, that AD9834 is specified for 75 MHz max. clock rate only. I found out that modules from the “grey market” sometimes fail and produce lousy signals when overclocked. You can see that on a scope when extra peaks appear or with the spectrum analyzer when spurious signal are frequent. I recommend buying with Mouser or anther trusty vendor for example or reduce clock rate in case of problems in signal quality.

Power consumption is not excessive because both DDS modules are for low power application, unlike the AD9850 or AD9835, that draw much higher current. Power rate is 300mA when in receive mode with LCD backlight on.

The C-code for the software has about 2600 lines source code and can be downloaded here.

DK7IH - High performance SSB 14MHz 20meters Transceiver - Dual DDS modules
DK7IH – High performance SSB 14MHz 20 meters Transceiver – Dual DDS modules (Left: AD9951, center ATMega644P, right: AD9834)

Rear view:

A standard CB DC supply cable is used here. Unfortunatel the plugs equipped with a cable and fuse holder are widely availabe but the sockets have to be stripped from old CB trasnceivers.

DK7IH_14MHz_20DK7IH - High performance SSB 14MHz 20meters Transceiver - Rear view
DK7IH_14MHz_20DK7IH – High performance SSB 14MHz 20 meters Transceiver – Rear view

On the air the transceiver performs great. Audio is clear and powerful what the QSO partners often tell me. The receiver is fun to listen to, sounding soft AND precise. Maybe I will do a YouTube video the next weeks to prove it! 😉

General construction

All my rigs are for portable, hiking, bicycle trips and travel to foreign countries. I use Aluminum as a basis for the hardware to keep the radio lightweight. With this radio a ground plane made of 0.8mm Aluminum sheet metal has been used that one has been enforced with a lateral additional ground plane carrying the DDS system (see pictures in this article, please). Thus the base frame is pretty rigid and not prone for bending.

Front an rear panel are made from 0,.8mm Al sheets (rear) and 0.5 mm Al sheet (front).

The various subassemblies (DDS, receiver, transmitter) are split into different modules and are seperatelay fixed with bolts and washers mounted to special spacer bolts for screws of 2mm diameter. This ensures better grounding instead of using larger veroboards. Connections are made from flexible stranded hook-up wire and shielded cable for rf and audio signals.

On the undersides of the single boards copper foil is used for lines with GND portential.

Vy 73 and thanks for watching! Peter (DK7IH)

 

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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)

 

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)

 

 

My “Vintage Style QRP Transceiver” on YouTube

This transceiver was built to celebrate my 30 years anniversary in homebrewing QRP transceivers. The 14MHz rig was designed building an SSB transceiver like I did in the 80s. The only exception was, that this one uses SMD technology whereas in 1987 I used thru-hole components. But the rest is original 80s-style:

  • VFO instead of DDS
  • Standard S-Meter for TX and RX instead of OLED
  • Dynamic mic instead of electret mike

Have fun watching!

73 de Peter (DK7IH)