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

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

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! šŸ˜‰ )

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 50ā‚¬!) for 1ā‚¬ 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. šŸ˜‰ So the receiver’s signal path starts with a band filter for 7MHz consisting of to tuned LC circuits.Ā  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 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.


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 connectedĀ  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.Ā  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.Ā  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. šŸ˜‰

73 and thanks for watching!

Peter (DK7IH)


A Micro Mulibander – Step by step (Part I)

This project tries a new personal approach in designing a very small (i. e. a micro) QRP radio. And also new is the way I want to report about it. The blog entries will be published more or less simultanously to the building progress of the respective modules.

1 A brief project description

The main objective is to set up a SSB QRP transceiver for 6 HF bands (similar to my 5-bander introduced in 2015) now starting with 160m, then 80m, 40m, 20m, 15m and 10m at last.

Another idea I have in mind is to build the rig from separate modules for each single stage so that each main circuit (mainly receiver and transmitter section) is constructed with the needed stages on verobaords that are mechanically identical.

The idea behind that is that a board which does not show top performance in function (or even completely fails) can be changed quickly and an improved version can be installed easily without the need to throw the whole receiver (for example) into the junk box labelled with “failed projects”.

Band switching will be done by small relays again (I purchased 60 SMD relays for 40ā‚¬ some months ago). The band filter section will be shared by tx and rx section this time. This saves space and reduces effort.

The transmitter will be a 4 stage unit again (more or less the same like in my 5 bander). Output power projected is 5 to 10 watts on all the bands.

The receiver is designed once again as a single conversion superhet because experiments with double conversion were not successful due to a large number of spurs audible in the receiver.

The first mixer is set to be an SBL-3 diode ring mixer. This will give the receiver a very good handling of strong signals, I hope. IF amplifier will be a two staged one with dual gate mosfets controlled by an audio derived AGV voltage. The rest? The usual suspects, I would say. Wait and see!

1.1 The VFO module

I have become quite familiar with SiLab’s Si5351 oscillator module the recent months. I first used it in my “Micro 20-3” trx which was a success. The module is very small, completely ready for use (I’m still using a breakout board made by Adafruit) and able to handle 5V. It provides 3 oscillators that can be programmed independently to put out something lieke a square wave ranging from 8kHz to 160MHz. I have developed a software that avoids any tuning noise, so, this oscillator (which is a clock oscillator by intention) can be used as a VFO for an amateur project.

To keep the effort simple, I reused the 1306 oled module that you can see in lots of my previous projects. Both boards (Si5351 and 1306 oled) are controlled by IĀ²C-bus which allowes me to use a relatively simple micro controller. In this case again I have the Arduino Pro Mini containing an ATMega168 controller (16 MHz) on board. If it should turn out that I might need more memory, the same board here is on stock carrying an ATmega328 controller. Let’s see how this will work out.

This is the circuit of the complete VFO module:

Si5351 VFO for Micro Multiband QRP SSB TRX (C) DK7IH 2018
Si5351 VFO for Micro Multiband QRP SSB TRX (C) DK7IH 2018

The module will be placed behind the front panel.

Tuning will be done by a Bourns optical encoder that turns very smoothely. I purchased some for under 5 Euros each from pollin.de. An unbeatable price! Unfortunatley they have been quickly sold out.

The core of this module is the Arduino Pro Mini microcontroller centered on the diagram. It is connected to the Si5351 breakout board and the 1306 oled display by IĀ²C bus.

Si5351: Output 0 is used as VFO terminal and output 1 carries the LO signal with 9Mhz. To avoid digital noise spreading on the +5V line a 100ĀµF capacitor should be switched close to the VDD terminal. Proper and short grounding also is recommended to avoid spurs.

OLED1306: Also a 100ĀµF electrolytic capacitor has been added including a 10ĀµH rf choke forming a low pass filter together. I found that these oleds a very prone to distribute digital noise via VVD line, so this measure contributes much to keep your receiver clean from any unwanted signal spektrum generated by the oled.

Keep in mind to tie SCK and SDA lines to +5V via two resistors of 4.7kOhms each!

Band switching: It is software controlled. To save output ports I did not connect the 6 relay drivers for the 6 bands directly to the ports of the microcontroller. I’m using an open collector BCD to DEC driver (74LS145) instead. Ports PB0, PB1 and PB2 are forming a 3-bit pattern that is switched to 6 output lines (output 0 to output 5) of the BCD2DEC driver IC. 74LS145 is capable of handling switch voltages up to 15V thus working with 12V coil relays is easy.

User control interface: This rig has 4 different switches that will be explained later from the functional point of view. The operator can set nearly all functions of the transceiver by using these push buttons and the main tuning wheel. The buttonsĀ  switch to GND by 4 different resistors and are read by PC0 port of the micro. PC0 equivalents toĀ  channel 0 of the integrated analog-to-digital converter (ADC) inside the ATMega168. This also saves controller ports to a large extent (using 3 instead of 6 ports!). So, all in all, I think I can dare controlling a multibander by a relatively small microcontroller.

(To be continued!)

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)

A very compact SSB transceiver for 40 Meters with 50 watts of output power (Product detector, AF, AGC)

The demodulator section of the transceiver’s receiver starts with the product detector, which is made of another SA602.Ā To get more audio volume a preamplifier has been added before the LM386 follows.

Homemade SSB amateur radio transceiver 40 meters (SSB demodulator, AF, AGC section)
Homemade SSB amateur radio transceiver 40 meters (SSB demodulator, AF, AGC section)

The AGC section hast got 2 crucial components: OneĀ resistor (this case 100k) and an electrolytic capacitor (in this case 100uF): They determine the time ramp for the AGC regulation curve. This meansĀ they define the response and decay time for the AGCĀ and thusĀ should be made easily changableĀ for example by putting them into socket strips.

Hint: In certain casesĀ it can be useful to add a potentiometer toĀ give you control on the audio input of the AGC preamplifier.

Thanks for watching!

73 de Peter

A very compact SSB transceiver for 40 Meters with 50 watts of output power (IF amplifier)

The if amplifier has been slightly revised. I added a preamplifier after the 1st mixer to enhance overall gain. Due to the fact that the veroboard is crowded with the components that had already been installed, the preamplifier has been worked out in SMD technology using the reminaing space on the reverse side of the board underneath the SSB filter were still some room has been available:

IF peamplifier in SMD technology
IF peamplifier in SMD technology

This new part of the circuitĀ  is not marked in the block diagram I’ve posted some days ago. As main amplifier of this stage, MC 1350 is used. Due to space saving reasons the tuned circuitĀ to terminate theĀ if amplifier IC has been made of a very smmall pig-nose core:

IF amplifier detail
IF amplifier detail

The parallel capacitor has been experimentally optimized by putting various capacitors into a 2 pin part of a socket stripĀ and keeping the best valued.Ā The MC1350 is gained controlled by an AGC amplifier and DC rectifier section to be described later. Please noticeĀ the correct termination of the SSB filter with 2 resistors 2.4 kOhms each.

Homemade SSB amateur radio transceiver 40 meters (IF amplifier) Homemade SSB amateur radio transceiver 40 meters (IF amplifier with bipolar transistor and MC1350)
Homemade SSB amateur radio transceiver 40 meters (IF amplifier) Homemade SSB amateur radio transceiver 40 meters (IF amplifier with bipolar transistor and MC1350)

Thanks for watching! Vy 73 de Peter (DK7IH)

A very compact SSB transceiver for 40 Meters with 50 watts of output power (Overview and block diagram)

In my last article I talked about my ideas fo a new transceiver project beyond the QRP level. First pictures of cicuitry were also shown. In the meanwhile the transceiver has been finished, some minor changes had to be made and now it’s time to go to the details.

All construction objectives (compact size, sufficient output power to establish even DX contacts on 40 meters, good stability, good receive performance, rigidness for outdoor use) have been met so far as I can say. I had the rig with me, when I was on vacation on the Island of Jersey (GJ/DK7IH/P) from 12th to 19th of August this year. It was big fun operating the rig. Lots of stations were calling during the two days when I was on 40 meters. ODX was HL1AHS, OM Kun from Seoul. So, this was very nice for 50 watts and a vertical antenna made of a fishing rod.

First, to give an overview, let’s have a look on the completed transceiver. Cabinet size is 7.5 x 16 x 6 centimeters.

SSB transceiver for 40 Meters with 50 Watts of output
SSB transceiver for 40 Meters with 50 Watts of output
SSB transceiver for 40 Meters with 50 Watts of output
SSB transceiver for 40 Meters with 50 Watts of output power (by DK7IH)

As you can see, the rig is very compact in size. The block diagram gives you an overview what is inside. Receiver section is on top, DDS can be found in the center and the transmitter is sited at the bottom of the diagram. As you can see, it’s again not rocket science and SDR-virus could not strike as well. šŸ˜‰

SSB transceiver for 40 Meters with 50 Watts of output (Block diagram)
SSB transceiver for 40 Meters with 50 Watts of output (Block diagram)

The next posts will describe the rig in details step by step. Proceed with the receiver’s front end.

73 de Peter

The semi-automatic antenna tuner – Version 1b

This article is outdated! Please read about the new version of the antenna tuner here!

People frequently reading my blog know that I love to improve the things presented here. This is revision one of my semi-automatic antenna tuner for QRP. I found that the reading of the SWR value on the LCD is a little bit uncomfortable. So I decided to use an “old-style” analogue meter. This one has been pulled from a home stereo amplifier.

Semi-automatic antenna tuner V2 (C) DK7IH
Semi-automatic antenna tuner V2 (C) DK7IH

The problem with the installed directional coupler is that the amount of energy and so the induced voltage is a reverse function of coupled frequency. Thus an operational amplifier has been added to amplify the DC voltage from the directional coupler. A 50k stereo potentiometer regulates negative feedback an so reduces the op’s gain.

Semi-automatic antenna tuner V2 (C) DK7IH
Semi-automatic antenna tuner V2 (C) DK7IH

Thanks for watching! 73 de Peter