[amsat-bb] Re: Automatic doppler tracking of DSB
n5gui at cox.net
Thu Feb 7 13:03:00 PST 2008
Well at least sort of. I received an informative message from PE4WJ, Wouter
Jan Ubbels, who is part of the team working on Delfi-C3, which is scheduled
to launch in April. As part of that project he has made available a sound
card based program, called Warbler, to demodulate the 1200 baud telemetry
downlink. It uses a Costas Loop to demodulate and for Doppler correction.
He suggests that the program could be expanded, or at least adapted for
voice DSB signals. While he is busy with Delfi-C3, he seems interested in
our discussions. Perhaps we can come up with some ideas that are worth
trying to add to future versions of Warbler.
I had originally thought that the idea should have been considered before,
and I am pleased to know that it is actually in use. The fact that it is in
use by the digital "community" but I could not seem to find any reference to
it, is a yet another example of a communications problem that has spanned my
three decade engineering career: different technical fields have such
specialized languages that the members of each cannot recognize when they
are talking about similar things. I should have recognized that I needed
better understanding of just why AO-16 has a DSB transmitter. It makes a
lot more sense after I read an article by G3RUH called "the shape of bits to
come" which Wouter suggested. I still have a lot to learn, but I am
starting to comprehend the following quote from the article:
Binary data DSB modulating an RF carrier" and "binary phase shift
keying (BPSK)" are exactly the same thing.
What I might add to try to clarify the statement for those, like me, for
whom it was not immediately obvious is that "Binary Data" in this context
comes in the form of +1 and -1 instead of 1 and 0 . For some of us it makes
the difference between BPSK and OOK.
If I am understanding Warbler, it brings in the 1200 baud BPSK, which
"looks" like DSB with 2400 Hz bandwidth, into a sound card. The signal is
tuned so that the "carrier" would be between 1400 and 1800 Hz. Once the
Costas Loop is locked, it will then compensate for 400 Hz of Doppler shift.
If the "carrier" drops below 1400 or above 1800, the extremes of DSB signal
would get attenuated by the passband of an SSB receiver, which is assumed to
be used to pick up the signals from satellite.
Modifications for voice DSB, would start by the use of a wider receiver,
perhaps an AM mode instead of SSB. Assume for the moment that we have a
receiver audio bandwidth of 10 KHz. If the audio used to modulate the DSB
transmitter is limited to 3 KHz, then the DSB signal will be 6 KHz. Set the
Costas Loop to track "carrier" from 3.3 KHz to 7.0 KHz then you get a 3700
Hz Doppler correction range. The sound card would then process the upper
and lower sidebands into the original speech which would then be produced by
the sound card. You could whittle that down some by communication quality
audio trimmed to 2.7 or even 2.5 KHz. The Doppler range increases to 4300
and 4700 Hz. A wider receiver band would also be a benefit, limited by the
high frequency limit of the sound card. And of course a wider receiver lets
in more noise. The additional noise suggests digital processing.
Wouter also suggested that software defined radio could be used instead of
the scheme outlined above. SDR is the concept that I had in mind. Perhaps
a simple system that has the I and Q channels fed into the L and R sound
card inputs, Costas Loop and additonal processing in the computer, sound
card or serial output to control the VCO. The bandwidth would be limited by
the sound card audio inputs, but the Doppler tracking correction would then
be limited only by the range of the VCO and its update rate.
I think this is worth the time to experiment. The situation with AO-16
certainly makes it easy to get voice DSB signals on the downlink. One only
needs FM transmissions. It could be used on a translating repeater, but I
would suggest that it would be courtesy to not try it on a crowded area. If
DSB gets a bad reputation for "hogging" limited bandwidth, it will never get
fair consideration for current or future satellites. DSB does not use as
much spectrum as FM, and I hope that it can demonstrate a benefit for
Doppler compensation. It has yet to prove its value the amateur satellite
community. I think it will be fun to try.
I certainly hope that I have presented some ideas that encourage thinking,
learning, and experimentation.
More information about the AMSAT-BB