2010-02-07
G3XBM's XBM80-2 Trivial 80 Metre CW Transceiver
As a keen subscriber to Roger's blog, I've been following his and his audience's work on the XBM80-2. It is such a neat little transceiver, I just had to build one myself eventually.
Of course it is a little ironic that I built a CW transceiver, when I can't RX CW! More incentive to learn.
I initially bread boarded the vanilla circuit which works fine. Investigating the performance of the circuit as specified by Roger gave me something to compare any customisations against.
Naturally though, I ended up making my own changes in the more permanent version after fiddling with the bread board version:
My Changes
- Raised the lower feedback capacitor which improved the power output for my unit quite a bit. Unfortunately this also decreased the RX sensitivity, so I moved the extra capacitance into the keyline to prevent it affecting RX. You may need to RF bypass the keyline as a result. I added a microswitch on the board to act as a rudimentary Morse key, but also provide a keyline jack.
- Added a low pass filter to the antenna line. Michael AA1TJ used a band-pass filter in his version which is probably an ever better idea, but I was being lazy and used commercial RF chokes in a simple Q = 1 Pi configuration. Without the filter the output waveform was positively square! The harmonic energy is vastly reduced with this modification. The losses of the tiny chokes and vanilla capacitors are minor, there is little reason not to add these 6 parts, no toroid winding required, no tuning. (Although you can use 22 turns on T50-2's if you don't have the chokes in stock).
- Used a 10 uH commercial choke in the oscillator collector instead of a tuned value. The value is not critical, it should just be large with respect to the 50 Ohm load seen. 10 uH is about 200 Ohms, larger values are OK, but self resonance can be a problem with commercial chokes - I tried a 500 uH homebrew inductor at one point; worked fine. I did try tuned circuits in the collector but my oscillator was seriously upset as resonance was approached, even with de-Q-ing resistors.
- Added a LPF in the audio path to keep the RF out of the audio amplifier. Without this mod the audio amplifier was misbehaving on TX and worsening the harmonic radiation of the circuit. I toyed with the idea of making the filter series-tuned at 700 Hz to give some AF selectivity, but the required values I did not have and the large inductors required (88 - 100 mH) would likely be largely capacitive at 3.5 MHz. Still this might be worth a try, maybe fronted with the smaller RFC and cap to remove the RF first, or perhaps Michaels method of transformer matching into sensitive headphones and omitting the AF stage altogether is better?
- Added an automatic TX/RX frequency shift circuit. It adds complexity and a tiny bit of chirp to the circuit, but avoids the hardware T/R switch. The off-keying is a bit sharp IMO, adding a large capacitor across the keyline helps but worsens the chirp associated with my auto T/R change, so I omitted it. The on-keying is naturally soft because of the circuit time constants. The T/R transients in the AF output are annoying, but I couldn't do much to suppress them without making the circuit much more complex.
- I used 470 nF coupling caps because I have a lot of them. The value is valid where it is used, but frequently larger than the minimum required.
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Attachments
| title | type | size |
|---|---|---|
| Circuit Diagram Source | application/postscript | 18.068 kbytes |