[Elecraft] An idea for the K2

[Geoffrey Mackenzie-Kennedy]

This is a horse of a very different colour!!  We are now talking 'RFCompression/Expansion' rather than 'RFClipping' after the modulator, a little bit like AGC in the receiver or ALC in the Tx, but with low level signals boosted. One approach is to generate a good SSB RF signal (modulator plus the 7 crystal filter) and then play with Compression / Expansion of the RF signal, which can be difficult. One method is to "fast detect" the level of SSB signal and pass back a control voltage to a variable gain amplifier in the audio circuits. Another and prefered method is to use a "Forward AGC" circuit block all at the SSB IF. The VERY BIG danger with RF Compression /Expansion systems is that all the time constants have to be absolutely right otherwise you can have a mess or oscillation. Audio Companders are easier. For RF processing, RF Clipping is very much simpler. If I had to make a choice and did not have time nor the right test equipment, I would go for RF Clipping with two filters. The BIG filter at the output. Perhaps a properly designed 3 or 4 crystal filter after the modulator so that a SSB signal is clipped might be enough, but the results would have to be checked out with a RF Spectrum Analyser or equivalent along with means to check audio quality.

73,
Geoff     GM4ESD