[amsat-bb] Re: Automatic doppler tracking of DSB

[John Magliacane]

> It started when I read that AO-16 had been placed in a test mode
> with an FM uplink and DSB downlink.  I remembered a diagram of a
> double sideband suppressed carrier ( DSB ) demodulation technique
> that used a variation of a phased locked loop to recover the
> frequency and phase of the original carrier.  I realized that it
> should be able to track the "original carrier" through the doppler
> shift caused by the relative motion between the satellite and the
> receiving station.

I think this can be made to work, although maybe not as well as you'd
like.

Given a receiver passband wide enough for the full DSB bandwidth,
plus some additional "wiggle room" to chase the signal if the
demodulator should temporary fall out of lock, it probably can be
done PROVIDED the modulation of the DSB signal never goes to zero for
any extended period of time (especially around TCA).  Since people
will be taking turns using the transponder, that last requirement may
be difficult to achieve.

The reason Doppler tracking of AO-16's digital transponder worked so
well is   because the downlink was ALWAYS modulated with BPSK, even
if it was AX.25 NULLS.

Such is not the case with analog audio.  :-(


73, de John, KD2BD


Visit John on the Web at:

	http://kd2bd.ham.org/
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