[amsat-bb] SDR transmitter asa Doppler dig-gen

Robert Bruninga bruninga at usna.edu
Mon Apr 17 14:37:51 UTC 2017

Some students built an HF inverting SSB transponder simulator.  (4 diodes,
and a siggen at 58 MHz) so they can then use two FT-817’s on the 21.4 MHz
uplink and 29.4 MHz downlink) for our next satellite:
http://aprs.org/hfsat.html )

Now we want to use an USRP 2922 SDR as the programmabel oscillator around
58 MHz so we can simulate Doppler.

Can anyone whip out some code to make the oscillator follow a Doppler Ramp
in freuqncy from say 58.100 to 58.900 MHz over a ten minute pass.  That
should be easy enough for one-observer.  Can just be linear for now
(simulating a low pass).

But then once that is done, we need the driver to then FLIP the Doppler
shift instantly in the opposite direction to simulate the shift seen by
another observer at the other end of the pass.  This is another layer of
complexity, but we will simply have a push button input to shift between
the two states when one station or the other is transmitting.

But for now, the students just need some handholding to get the basic SDR
acting as a signal gen at 58 MHz and how to input the freqncy ramp to it.



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