Dear Stewart, LF group,
Nice to see everyone again at Wimbourne yesterday. Hope
everyone had a good trip home.
In response to Stewart's points, As far as I know, BPSK is good
because you can use, indeed have to use, a coherent detection
scheme. If you were using an FSK modulation scheme, presumably
you would have to find a way of making the two tones have a fixed
phase relationship to some reference to achieve similar advantage.
Generating BPSK at 13.6MHz and dividing it down to 136k would
also divide down the phase modulation from 180degrees to
1.8degrees, which I don't think would do much for the demodulated
SNR.
Using the normal CW keying shaping to amplitude modulate the
BPSK transitions would result in a BPSK signal with roughly the
same bandwidth as the CW signal. However, the amplitude keying
transitions for CW are normally much faster (5 -10ms) than those I
am using for BPSK (=1 bit period = 100ms), otherwise the CW
keying sounds excessively "soft" for aural reception. Looking at
typical CW spectra on the air, they usually have a bandwidth of
around 1-200Hz, wheras my current BPSK signal has about 20Hz
BW.
Cheers, Jim Moritz
73 de M0BMU
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