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)

 

 

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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

A compact project: The “Micro42” – Another “shirt pocket” SSB transceiver.

Having deferred the work on the “micro multibander” for some time I finished another small QRP rig (this one for 7MHz) that is suitable for my summer excursions by bike or hiking the local mountains here in the State of Rhineland-Palatinate or the Black Forest that is not that far away on the other side of the Rhine valley.

Besides, this transceiver to be discussed here is some sort of a “remake” of a 20 meter rig I built 3 years before. And this time, the transceiver really fits into a shirt pocket without having to wear “XXXXL”- clothing. ;-):

The Micro42 - A really shirt pocket sized QRP SSB transceiver
The Micro42 – A really shirt pocket sized QRP SSB transceiver (this is my work shirt, so don’t mind the stains! 😉 )

General circuit description (instead of presenting a block diagram)

The rig uses two mixers NE602 plus one filter as central elements. The signal way is reversed when switching from receive to trasmit mode. This is done by 2 relays and is a well known technique for simple QRP rigs. You will find lots of equivalent ideas on the internet (Example 1, Example 2).

But not to ignore the shortcomings of these designs: They are somehow inferior to my requirements, particularly concercing receiver performance. I prefer to have higher signal gain and an AGC circuit. AGC for me is a must. But these designs can be expanded easily, so I added an AGC controlled interfrequency amplifier with dual gate MOSFET BF998 into the receiver’s signal path enhancing performance significantly.

Frequency layout

The frequency generation of the superhet transceiver scheme is simple: Again I use one interfrequency (i. e. 9MHz). The VFO is DDS based on AD9835 operating below the desired radio frequency, which means that it is set to the range of about 2 MHz. Due to this low frequency you could replace the DDS by a VFO if you don’t like the relatively complex work with the software programming and microcontroller stuff). A 2MHz VFO can also be made very stable, so this is an alternative not to be ignoered.

Due to the fact that the schematic is not very difficult to analyze you are kindly requested to refer to it for further talking:

Schematic - The Micro42 - A really shirt pocket sized QRP SSB transceiver
Schematic – The Micro42 – A really shirt pocket sized QRP SSB transceiver. Click for full size picture

Circuit description

In the center of the schematic you can see the main elements of the circuit: One SSB filter (9MHz), correctly terminated by 2 resistors of 1k each (to ensure proper filter response curve) and two relays with a double set of switches. These relays reverse the way the signal travels through the filter. The advantage of this: You can use the integrated oscillator of the NE612 controlled by a crystal and a tuning capacitor to set the carrier frequency correctly for the lower sideband because the mixer is used as SSB generator and as product detector in common.

A word on chosing the proper relays: An intense examination of the relays’ data sheet is essential. I built a prototype of this transceiver on a breadboard prior to soldering the components to a veroboard. I found that some SMD relays have signifikant coupling capacities between the unused relay contacts (in the range of some Picofarads). So stray coupling was a severe problem. Later I used some second-hand Teledyne RF relays that I had purchased via ebay two years ago (price originally 50€!) for 1€ each. These relays are absolutely superb!

The receiver

Before we go: In the circuit scheme above I missed out the antenna switch relay because I think every homebrewer knows what to do in this case. 😉 So the receiver’s signal path starts with a band filter for 7MHz consisting of to tuned LC circuits.  The coupling is relatively loose. As coils I use the well known coil formers in TOKO style with 5.5mm outside measure.

Coil data for the 7MHz band pass filter (BPF) is 39 turns primary and 9 turns secondary of 0.1 mm enameled wire. The respective capacitor is 33pF. This is a high L to C ratio which gives you excellent LC quality factor. This is mandatory especially when working on the 40 meter band, because of the strong broadcasters starting from 7.200 kHz intermodulation might be a problem when the receiver is connected to a high gain antenna and broadcasters’ signals might overload the first mixer (remember that NE612 has a relatively low IM3!). If you still should have problems coping with too strong out-of-band signals you can reduce the coupler from 4.7pF down to 2.7pF.

In practical terms I could not detect any unwanted signal products even when using an antenna with high rf output voltage. One reasons for this is, that there is no rf preamplifier for the receiver. This avoids overloading the first mixer generally.

The NE612 has two mixer inputs and two outputs. This makes it very suitable for this sort of radio. In receive mode pin 2 of the right NE612 is used as signal input. VFO signal is fed into pin 6. The resulting mixer products are taken out from pin 4. Next the 9MHz filter follows from right to left.

The 9MHz IF signal then is fed into an IF amplifier. This one is equipped with a dual gate MOSFET (BF998), gain is about 15dB when full AGC voltage is applied wich leads to about 6V by the 1:1 volatge divider in the applied to gate 2 of the MOSFET.

The left NE612 is the product detector. I use the internal oscillator with a 9MHz crsytal and a tuning capacitor here. This saves building an extra oscillator and simplifies the rig again.

One AF low pass filter made of 1k resistor, 100uF rf choke and a 0.1 uF capacitor eliminates high frequency remainders generated by the mixing process.

The audio stages are also made simple: One preamplifier (using bipolar transistor in grounded emmitter circuit) and a final stage with LM386 transform the signal to a level that is sufficient to be fed into a small 8 ohm loudspeaker or a set of standrd MP3-player headphones. Because the rig is very small and there was definetely no space for a loudspeaker I use headphones instead.

Keep an eye on the power supply switching of the two audio stages. The problem was to eliminate the switching click and pops to a minimum and to avoid acoustic feedback when unsing a loudspeaker. So the audio preamp is only connected to DC on receive. When switching to transmit the charged capacitors avoid instant cut off supplying some Milliseconds DC to the amp until significantly discharged. The main amplfier on the other hand is connected to permanent DC supply. So it won’t pop when switching from tx to rx an vice versa but can cause feedback. To avoid feedback a transistor is used to cut the speaker/earphone from the power amplifier.

AGC

AGC is audio derived. A two stage amplifier provides a DC voltage analog to the audio input sginal strength. First amplifier stage is a common emitter bipolar transistor supplying sufficient audio voltage. This voltage is rectified by a two diode circuit letting only the positive halfways pass. You can use silicon diodes (1N1418) oder Schottky diodes here. An electrolytic capacitor (100uF/6V) provides the time constant respectively DC decay once the signal has disappeared. Output of the DC stage is split. The collector is connected to 12V via a 4.7k resistors causing a voltage drop when the transitor’s conductivity increases. The emitter is fed to the ADC of the microcontroller (pin ADC1) causing a proportional voltage to the voltage of the applied audio signal so that on the OLED an S-meter can be displayed.

The transmitter

An electret microphone picks the operator’s voice. The signal output level of these microphones is high enough to drive the left NE612 (which serves as balanced modulator in this case) directly. Signal input for the mixer should be 200mV RMS according to data sheet. An electret produces about 0.5 to 1 V pp if spoken with a decent voice in the distance of some centimeters. So you have more than enough audio signal power for the modulator.

BTW: Carrier suppression of the modulator is excellent. I achieved 56dB without doing anything else!

The resulting DSB signal then is fed into the SSB filter, the SSB signal subsequently is directly sent into the right NE612. A band pass filter for 7 MHz eliminates the unwanted mixer products. You should have 400 to 500 mV pp of rf signal here when the transmitter input is fully driven. I recommend a two-tone test generator to check out the linearity of this and the remaining amplifier stages!

Next parts of the transmitter are a band pass filter (same coils and capacitors like th rx bandpass filter), a preamplifier and a driver. The later should put out about 150 mW into a 50 ohm load. They are made more linear by emitter degeneration (4.7 and 2.2 ohm resistors for predriver and driver) and negative feedback. This helps to ensure that transmitter performance is fine when IMD3 products are concerned even if the main IMD3 problems usually occur in the final stage.

To transfer the rf power into the final stage proper impedance matching is mandatory. Input impedance of the final stage is fairly low (<10ohms), therefore a broadband (down)transformer is used. Data is: Core T37-43, primary 12 turns, secondary 4 turns of 0.4 mm enamled wire.

Last stage is a single ended linear amplifier in AB mode equipped with a 2SC1969 rf power transistor by eleflow.com.

BIAS circuit: The combination of the 1k resistor, a silicon diode (1N4002 or equ.) and a capacitor sets up the correct bias. Bias is fed into the cold end of the input transformer. Quiescant current should be around 40mA. A good thermal contact between the diode and the transistor is recommended. As the transistor gets warmer the diode will increase its conductivity so reducing bias current. This will prevent thermal runaway effectively!

To avoid bulky output transformers the PI-filter (7MHz LPF) is part of the tank circuit of the final amplifier transistor. For this power level this is an acceptable and practical solution because the output impedance of the stage is nearly equivalent to 50 Ohms. A certain mismatch is not a severe problem. DC to the final transistor is applied via an rf choke, for exact data please refer to the schematic!

T2 helps to suppress unwanted signals that I encountered when taking the transmitter from the dummy load test environment to a real antenna. I observed unwanted parasetic oscillation in the range of about 1MHz. T2 has a low reactance for this frequency range thus eliminating the oscillations in a reilable way by short circuiting them towards ground.

Powered with 12.5V DC the transmitter will put out slightly more than 5 watts PEP.

DDS VFO

AD9835 is a simple but well performing 10-bit DDS chip made by Analog Devices (AD). It is controlled via 3 SPI lines transmitting the frequency data. Maximum output frequency is around 16MHz when the chip is clocked with its maximum clock rate of 50 MHz. Oscillator output voltage is some hundred millivolts peak-to-peak, so you can connect the output directly to pin 6 of the NE612 mixer.

Control signals come from an Arduino Pro Mini board. The microcontroller in this module is, if you are an Arduino user, preinstalled with a bootloader program. I overwrote this small portion of code and use the ATMega168, which is the core of the Arduino, in “native” mode. My software is written in C and transferred via “AVR dude” software using the ISP lines MOSI, MISO, SCK and RESET. These lines are not in the schematic, please refor to ATmega168 data sheet. Alternatively you can use, like shown in the schematic, an ATmega168 controller. So you have to de neccessary wiring on your own.

You will find the source code here. I packed it into an Open Document Text File because of problems I encountered when I tried to store the code into this Blogtext. If you need a compiled HEX-file, please feel free to email me!

Display is a very small OLED with 64 by 32 pixels. The OLED is, to my point of view, a little bit noisy. To suppress any rf traveling on VDD line I use an 82 ohm resistor and a set of bypass capacitors of 100uF and 0.1uF capacity closely connected  to the OLED VDD pin to GND.

A low pass filter by the output of the DDS ensures spectral purity and avoids clock oscillator feed through. Remember that if you need another output frequency other than 2 MHz you should redesign the low pass filter.

Frequency control

Tuning is done by a rotary encoder connected to PD5 and PD6 of the microcontroller. I use the pull up resistors internal to the microcontroller, so you won’t see any other things than the mere encoder.

Tunings steps are selected by pushing the encoder knob or another suitable push button. This button is connected to ADC0 in the ATMega168 via a 3.9k resistor. The resulting ADC voltage might be problem because of a certain variation in the values of the pull up resistors that form the second resistor of the voltage divider.  There is an outcommented section in the code that will show you the exact ADC value that has to be typed into the code so that key recognition works exactly.

The button once pushed will increase the tuning step by a certain amount of Hz. Steps are 10, 50, 100 (standard step), 250, 500, 1000 and 5000 Hz in and endlessly revolving chain.  The step will be reset to 100Hz (standard tuning step) by leaving the tuning knob idle for 2 seconds. That’s all with the controls. Very simple, but sufficient.

Practical aspects

The transceiver is constructed on a double sided veroboard with 6 by 8 centimeters area. Components are through hole and SMD where available. The Arduino is mounted to the front panel (another Veroboard carrying the controls etc.) as well as the OLED is. The veroboard is inserted into an aluminium frame connected to the front panel with 4 lateral M2 screws:

Mounting frame - The Micro42 - A really shirt pocket sized QRP SSB transceiver
Mounting frame – The Micro42 – A really shirt pocket sized QRP SSB transceiver

Design hints:

Wiring can be made by using the colored lines stripped from old parallel printer cables. These cables have a diameter of precisely 1mm an fit through the holes of the veroboard excactly.

If you connect any external components that are not on the same veroboard use standard 2.54 mm (0.1″) male and female board connectors! This will make it much easier to dismantle and reassemble the rig in case troubleshooting is neccessary.

Use M2 srews instaed of M3 when building very small rigs like this one!

The reverse side of the main arrangement:

Reverse side of mounting frame - The Micro42 - A really shirt pocket sized QRP SSB transceiverord-and-front-assembled-in-frame-reverse
Reverse side of mounting frame – The Micro42 – A really shirt pocket sized QRP SSB transceiver

Two brass made bends (from the local hardware store and each cut to a length of 8 centimeters) hold the PCB inside the mounting frame. A winding has been cut into the brass to fix the bends with screws in M2.

Final assembly

Together with 2 halves of a bent aluminium cabinet covered with “DC-fix” (a German manufacturer of self-adhesive PVC coating) the final rig looks like that:

The Micro42 - A really pocket sized SSB QRP transceiver for 7MHz
The Micro42 – A really pocket sized SSB QRP transceiver for 7MHz

So, that’s the end of the story so far. Now it’s time for going outdoor and test the rig in field use. 😉

73 and thanks for watching!

Peter (DK7IH)