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RE: AO-40 data mode



This would not be a problem at all for any computer above 1 Ghz.
This I would be willing to guarantee.  Let me give you my reasoning:

Right now with the SDR-1000 ( http://www.flex-radio.org ), I wrote
the SSB detection and the synchronous AM detector.   We are wasting
many ticks by doing SAM, etc. at the high sample rate, even after
we have basebanded and filtered (so we could decimate the signal
a lot).  Even so, I can do the normal SSB style detection and
the phase locked loop for the synchronous AM detector and have
ticks to spare on a 1 Ghz Athlon (not XP, an old one).  The
PSK "demodulator/detector"  would replace the synchronous AM
detector with a simple Costas loop, be one complex multiply more
expensive and the rest would work normally.  I am already doing
a software mixer for spur reduction (we tune the DDS to "good
spur" frequencies and then do the final tuning with software
mixers).  Taken together, this clearly indicates that it is
within a complex multiply of being the identical number of
operations to do what Jim suggests.  This code is running in
bloody Visual Basic 6 (the SAM is completely in VB code) while
the detection/filter is mostly done using Intel library primitives
which can and will soon be replaced for licensing reasons, but
just have not had time to yet.

73's
Bob
N4HY


-----Original Message-----
From: owner-AMSAT-BB@AMSAT.Org [mailto:owner-AMSAT-BB@AMSAT.Org]On
Behalf Of George Kadziolka
Sent: Friday, October 24, 2003 4:23 PM
To: 'Joe Fitzgerald'
Cc: amsat-bb@AMSAT.Org
Subject: RE: [amsat-bb] AO-40 data mode



> One notion I have been thinking about is as follows:
> digitize a wide bandwidth, say 40 kHz with your soundcard.
> Lock onto the beacon to run your AFC loop.  Then you can
> digitize the desired signal without worrying about doppler anymore.
>
> Of course the problem with this scheme is that you can't use
> a standard IF with 3 kHz bandwidth.  You need at least 40 kHz
> with I/Q outputs.

I've been giving this some thought also (ie looking at a software based
solution with an interface at the IF).  You would need enough bandwidth to
cover the entire passband plus whatever Doppler you would anticipate (on the
order of 1 MHz, say).  Tracking the beacon frequency would eliminate manual
Doppler compensation (as you indicated).  This would allow you to select an
appropriate band (up and/or down) within the transponder passband with
reference to the beacon.  I would also consider a DSP solution rather than a
PC.  Although capable of fast operations, this would require a bit more than
what a Pentium can provide (especially if it is running a GUI).

This project would not be for the squeamish, but a working solution would be
much more automated than what is available at present for working the sats,
which in turn would open up the doors to more satellite users (=more
members, =more revenvues, =more satellites, etc).

My 2 cents worth.

Cheers,

George
VE3LQN


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