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

James Whitfield n5gui at cox.net
Wed Feb 6 08:19:04 PST 2008

---------- From: "Jim Towler"
>In general terms, this is largely true, and for a 2m uplink, little
>or no tuning is required on AO-51/Echo etc. However, a
>number of earlier emails spoke of the more critical need for
>AO-16 as it appears to have tight filters compared to a typical
>FM uplink. Careful control of Doppler for the uplink would
>allow a more full use of the entire filter bandwidth, rather than
>using FM-Narrow, and having weak audio.

Hmmmmm. I would like to claim that I am a victim of the news article about
the test mode for AO-16 on the ARRL website, but that would be admitting
that I am gullable enough to believe the FM uplink did not need Doppler
shift compensation.  That it works at all on a system that was intended to
be digital is a blessing.

>I still like the idea of attempting CTCSS....

I think it is a great idea.  I can imagine a comparison of three modes using
it.  The first is to use CTCSS in the uplink to model what was previously
tried with SSB and pilot carrier.  That is of course on the assumption that
the pilot carrier did not need to be zero beat with the SSB signal.  If you
can find out about their experiements you might find suggested frequency and
power levels to use.

The second mode would be to use the Costas Loop to compensate for Doppler
shift and synchronously detect the DSB signal.  That would give you
comparison of the effectiveness of the Doppler tracking and maybe compare
the noise benefit of the synchronous detector.

The third mode would be Costas Loop without CTCSS to find out if it really
helps, and how much.  As I indicated previously, I am guessing that the
system will acquire lock on voice power peaks, so it may not require much
CTCSS power to maintain the system lock.  It may well be that the power
required to maintain lock depends on the rate of change in the Doppler
shift.  Said another way, it may require more power to hold the signal at
the time of closest approach than when the Bird is near the horizon, comming
or going.

>Not related to AO-16, but something I intended to test on FO-29
>at some point, was "two-station Doppler correction"....

It is an interesting set of control systems, one for transmit and an
independent one for receive.  The transmit system would seem to be open loop
and based on the known position of the other station and path of the Bird.
The receive system could be either open or closed loop, perhaps both with
the ability for the operator to select the "better" channel.   It might also
be possible to feed the receive side closed loop parameters into the
transmit system.  I will have to think a little more, but there does not
seem to be any way to close the transmit loop.

Best Wishes


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