Re: QCX-SSB: SSB with your QCX transceiver

Guido PE1NNZ

For those who are interested; there is now an improved version of the QCX-SSB experiment, it can be found here:  In the meantime, I got quite a number of experimenters who reported a successful modification of their QCX and could make QRP SSB contacts.
Note that if you plan to upgrade, it is important to remove C31 and add a 10uF capacitor at C32. This will resolve an issue with Q6 (which was not digitally switching resulting in degraded IMD and thermal instability). This fix together with the more accurate signal processing of the new firmware, improves the IMD performance, carrier+side-band rejection and spectral purity considerably. Please let me know how this works out for you. Note there is still room for further optimizations like mitigating the PA/key-shaping introduced phase-errors (that occur especially at smaller amplitudes), and the effect that reducing C32  improves the IMD.
Another interesting thing to try out is the use of constant-amplitude SSB feature (set drive parameter to 8); this modulation scheme will solely transmit speech-information via the phase of the carrier (PM). This of course degrades the IMD performance, but quite unexpectedly this seems to be quite beneficial in weak-signal conditions when your signal just needs to be popping out above the noise-floor to make yourself heard: it occurs to me that using a constant-amplitude with worse IMD will actually improve the intelligibility in these cases.
73, Guido

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