Interestingly, now that the modem seems to be working I could play with it a bit today, with various sub-carrier center frequencies and no diversity.
I got 8.1021 dB crest factor with 7 carriers at 112 Hz spacing (Newman phases). Not much change there. With 112 Hz the spectrum used was about 900 Hz. 109 Hz gave the best SNR numbers, and I tried 105 which was still good (couple of dB less), but I don't know if the SNR is programmed to handle the variations. I'll have to look. My theory is that you don't want to go below (8000 sample rate/75 baud) 106.66 Hz anyway. Heck, 900 Hz BW SSB should be a killer with a 1 watt on 2 metres...
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