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

Jim Towler jtowler at xtra.co.nz
Fri Feb 1 17:24:48 PST 2008

Hi John,

What if someone turns on a CTCSS subtone for their FM uplink?

If set high enough, it might make it thru the audio filters, so could be
tracked with a PLL at the receive end.
(But not so high it's a problem for human ears :-)

1) Would this work?

2) Will it cause any problems to AO-16, by raising the duty-cycle of the DSB
(Clearly this would be a bad idea if it might cause harm or unacceptable
power-budget issues. Mind you, it can't be too much worse than the original
BPSK can it?)

3) If this is a good idea, does we need a standard tone?

4) How would it cope with multiple overlapping uplinks?

If someone was able to automate the downlink Doppler correction, that can
then feed right back into the uplink correction too.

It would be interesting to them compare the corrections against that
calculated from the Keps.

I'd love to hear ideas re above, but most critical is to be sure it's not
harmful to the sat in any way.

I think everyone should hold off testing this until some feedback has gone a
round or two here first.

Jim, ZL1TYF.

-----Original Message-----
From: amsat-bb-bounces at amsat.org [mailto:amsat-bb-bounces at amsat.org] On
Behalf Of John Magliacane
Sent: Saturday, 2 February 2008 11:31 a.m.
To: amsat-bb at amsat.org
Subject: [amsat-bb] Re: Automatic doppler tracking of DSB

> 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

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

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