Sorry---just one more. I thought I had already sent this response to Carter
and the list.
Garry
Carter:
Going through
> the
> audio chain is inherently less clean and more prone to distortion than
> true FSK which uses a varactor to shift the VFO. However, to the casual
> user, the net effect APPEARS to be the same.
>
> 73/Carter/K8VT
>
This ain't necessarily so! The devil is in the details, specifically how the
two carrier frequencies are switched.
* AFSK: if the SSB modulator is modulated by clean sinusoidal tones, and the
switching between them is phase and phase-slope continuous, the AFSK
performance is as good as the SSB performance. Granted, in some rigs, the
SSB performance is not so hot. Granted also, if a soundcard is used, and the
SSB modulator is overdriven by excessive soundcard gain (which I expect is
quite common), the signal will be degraded.
* "Pure" FSK: if the varactor is switched by a logic-level signal whose
edges are asynchronous with the zero-crossings of the carrier, there will be
glitches at the transitions which will generate spurious spectral components
in the output. IOW, there should be integer numbers of carrier periods at
each output frequency. This is achieved by syncing the data edges to the
carrier, e.g. by applying the data to the D input of a fast D-flop that is
clocked by the carrier. I don't know if the Omni does that, but let's hope
it does.
What we are after is "phase-continuous FSK."
Garry
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