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Chirpy - an Ultra Simple 2 Transistor 28MHz CW Transceiver



Updated 29.7.12 (Lesser Chirpy added further down page)

Click images for larger, clearer views.


Introduction

(see lower down the page for revisions to reduce/eliminate the chirp)


This is probably the simplest CW transceiver possible for use on 10m. Based on my XBM80-2 design for 80m, this is essentially the same circuit redone for a 28.060MHz fundamental crystal.  I had named it the XBM10-2 but now decided a more apt name was Chirpy.  Power output is around 200mW, which is more than enough to cross the Atlantic on a good day. Unfortunately the keyed oscillator does produce a fair amount of chirp on the CW. I have not managed to overcome this yet. I guess one cannot expect perfection in something this simple.

Circuit Description

The circuit is a crystal controlled Colpitts oscillator used as a TX oscillator with the key down and as a direct conversion receiver with a single stage oscillator-mixer and separate audio gain stage with the key up. 



Very inexpensive, ubiquitous, 2N3904 transistors are used. One can argue whether this receiver is a direct conversion or a regenerative one: in a single stage it is probably not possible to say it is one or the other.  At least the receiver is crystal controlled and does not need a reaction control, so on balance it is more likely best described as a direct conversion receiver.  The receiver audio output into the high impedance crystal earpiece is low, but it can hear down to around 2uV (-100dBm) in a quiet room. Your ears may be better than mine. This sensitivity has been checked on 3 different examples built. There is no real audio selectivity so you will need to use your "ear-brain filter" to select the 800Hz-1kHz received audio signal.

The frequency shift between RX and TX (about 1kHz) is just about right to listen for replies on the TX frequency.  The rig is full break-in going from RX to TX when the key is pressed with around 1kHz offset.  Broadcast breakthrough does not seem to be an issue at all, which is surprising as there is little front end selectivity or rejection of out of band signals. 

The transmitter really needs the simple 3 component low pass filter (shown on the RHS in the circuit above) adding for serious use, but this was omitted in the basic "no frills" version. It is possible that an ATU will provide the additional filtering in some set-ups.

Notice there is no supply decoupling in the basic version This should be added if the supply impedance is not very low but you can get away without (wishing to keep component count absolutely as low as possible) if using a low impedance battery supply and short leads. Don't try using a mains PSU as audio hum will be a problem.




Results so far

Best DX so far is a QSO with IT9QAU/QRP who gave me RST439. Distance was 1414km.  The main issue is the chirp: arguably OK for something this simple, but not quite good enough. I suspect that with less TX power there will be less chirp.  You can hear the level of chirp on the video clip below.

This is a "for fun" rig, so don't expect incredible performance, but it does work. On the first day of use 2 DXCC countries were worked. On receive it has copied plenty of stateside and European signals on the QRP calling frequency.

If the fixed capacitor in series with the crystal was made variable some small movement of the frequency would be possible allowing around 10-20kHz of movement around the QRP frequency. This might help get a few more contacts although one has to watch the RX-TX offset does not change too much as the crystal is pulled down in frequency.

A Video Clip of Chirpy


Using the Chirpy receiver with an audio frequency SDR


By connecting the collector of TR2 to my PC soundcard I can use a number of simple SDR packages to allow 28.040-28.080kHz to be monitored. Using Johan Bodin's (SM6LKM) VLF receiver that tunes 0-22kHz I can hear CW and other signals over a 40kHz section of the band quite well. Of course, with no I and Q inputs and a direct conversion receiver the two sides of the spectrum either side of 28.060kHz are folded back on eachother. Still, this is a simple way of extending the use of this very simple rig, albeit rather negating its ultimate simplicity.

Making it simpler still?

By replacing the collector tuned circuit with a packaged coil with built-in inductor/capacitor and link winding (like a Toko KANK coil) it may be possible to reduce the effective component count by 2. The capacitor in series with the crystal is there to get the crystal onto frequency, so could be omitted saving another part. The 15k and 82 ohm resistors could be replaced by a small preset resistor with the wiper set at the 82 ohm point and the key connected at that point. So 4 more parts could, at a pinch, be removed reducing the parts count to 10 apart from the crystal, earpiece and key.

Eliminating the chirp? Introducing the "Lesser Chirpy" :-)

Added 27.7.12

SPRAT 151 had my article on Chirpy, an ultra-simple, very basic crystal controlled CW transceiver for 28MHz. As described above, this does work and has even managed a few decent QSOs but, boy, does it chirp! So, today I followed up on an idea I had to reduce the chirp by FSKing the oscillator when on TX. The schematic is attached.

The basic idea is to switch to TX by closing S1 (increases the oscillator output to around 150mW) then key a capacitor in series with the crystal. As the oscillator is on at full power with the key off or on (FSK shift) the chirp goes. There are a couple of disadvantages (a) the rig is no longer full break-in, and (b) when the key is up there is the "opposite" morse about 1kHz up the band, which is wasteful of power and may confuse. Still, this is a "for fun" rig and, with virtually no chirp, a much better radio. As before, a small low pass filter on the output is advisable unless your ATU provides this.

I also adjusted the turns on the main collector inductor finding that a tap 3t from the cold end worked better. You will almost certainly have to experiment with the values of capacitance around the crystal and key to get the right amount of TX-RX offset.

Click on schematic for a larger, clearer version



What if it doesn't work?

This was my reply on the GQRP Yahoo Group to someone who had problems with Chirpy:

Pleased to hear you are having a go at replicating a Chirpy (XBM10-2), but sorry
things are not yet going as expected.

Firstly, an explanation of the circuit:

When the key is pressed, the crystal oscillator stage acts as a single stage TX
producing around 100mW (on the two samples I built) into 50 ohms. This may well
vary with different 2N3904 transistors to some extent.

On receive the 15k resistor in the emitter (rather than 82 ohms) allows the
oscillator to run at a much lower level and the stage is acting as a self
oscillating crystal controlled mixer.

Audio is present across the 15k resistor by mixing the incoming RF signal with
that produced by the oscillator producing an audio beat note. In effect this can
be seen as a direct conversion receiver mixer.

The audio present across the 15k resistor is amplified by the second transistor
which is simply an audio gain stage coupling the amplified signal to the high
impedance crystal earpiece.

Now, what can be wrong? Let's hope I can help....

(1) Firstly, are you sure you are using a FUNDAMENTAL 28MHz crystal? If not, the
oscillator may be oscillating at 1/3rd the frequency. These are not that common
and the G-QRP Club ones I got some time ago were 3rd overtone ones, not
fundamental. I got my fundamental ones from Golledge.

(2) Assuming a fundamental crystal, can you detect the oscillator when the unit
is on receive on your main station rig? It should be about 1kHz off from the TX
frequency. If not, try adjusting the bias resistor (10k) in the base of TR1 to
ensure the oscillator works at the reduced receive current, or reduce the 15k in
value slightly.

(3) Remember that, compared with a conventional receiver with a decent noise
floor and sensitivity, this one WILL be deaf. Around 2uV was just detectable (in
a quiet room) on my 2 samples, but the recovered audio is very low as there is
little audio gain. 2uV is at least 20dB less sensitive than a typical receiver,
possibly worse.

(4) Are you using a very high impedance crystal earpiece? These look like a VERY
high impedance. Do not confuse with other lower impedance magnetic earpieces or
those used with MP3 players which are around 30-40 ohms only. The circuit needs
a very high impedance "deaf aid" type crystal earpiece. These are usually a
beige colour with the words "crystal earpiece" on the back.

(5) Check the collector voltage on TR2. It should be about 1-2V. This can be
raised by reducing the resistor value in TR2's collector. Originally this was
around 5-10k but I found slightly better gain with a much higher value, although
be careful TR2 is not approaching saturation.

(6) It is possible that the audio stage is being overloaded with RF from the
oscillator stage. The only "filter" removing the RF is the 100pF across the 15k
resistor. In my examples this was not an issue. Also, AM breakthrough did not
appear to be an issue in mine, although I was expecting this to be an issue.

(7) Experiment with different values to replace the 15K resistor in TR1 emitter.
the higher the value the lower the oscillator level on receive but the lower the
backwave is on TX. A smaller value may help if the oscillator does not want to
start on RX.

I hope these notes help a bit with diagnosis. Please remember this is a very
simple circuit, full of compromises and you may well have to play with values to
get the best from it with your components. As there are few parts you will not
have to play with too many though.

If anyone else has a go, I would value feedback. There will be ways to make this
simple little circuit function better and with more repeatability, although I
hope without adding any further complexity.



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