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From: "Talbot Andrew" <ACTALBOT@dstl.gov.uk>
To: rsgb_lf_group@blacksheep.org
Subject: LF: RE: RE: GPS Locked PSK
Date: Thu, 14 Feb 2002 13:23:56 -0000
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A difficult one, if you are after software that automatically decodes the
message.
 
But the beauty of GPS locked signalling is that the need for carrier and
clock recovery has been removed.  Provided the transmitted frequency is
known exactly and the timing intervals are also known - with GPS locking
both of course are very precisely defined - then a straightforward off-air
phase plot, after filtering to a narrow bandwidth is all that is needed to
'see' phase modulated transmissions.

Wolf DL4YHF is adding a vectorscope and time / phase plot to Spectrum lab
and I believe the first versions may be available for testers, so for a
simple fully coherent session you could just set that to its narrowest
setting (after calibrating your soundcard oscillator and real time clock !)
and plot out the received phase of a transmission over a period of an hour
or so and look at the pattern you get.   One of the options I have built
into my GPS locked DDS driver software is CW coding so that a message can be
read visually from a time / phase plot.  This technique does not have the
advantage of a proper coherent integration over a symbol period, but will
still give many of the S/N advantages of coherent reception in the decimated
bandwidth.

Alteratively, you could try this :  Build a divider with 2 quadrature
outputs to get a 90 degree shifted pair of very accurate audio frequency
tones, say 1kHz, from your locked source;  a PIC could probably do this very
well.  Apply this to the LO port of a pair of mixers that will work at
audio, such as the ubiquitous MC1496.  Connect your  receiver producing a
tone at exactly the same frequency to both mixer signal inputs.  Low pass
filter the outputs to some low value - well below 1Hz.  Initially just
connect the two outputs which are now an analogue complex representation of
the received signal in a narrow bandwidth to a scope in X/Y mode.  Voila  -
a vectorscope for narrow bandwidth reception with no software in sight.  It
is difficult to get the actual phase without calculating it in real time
using teh arctan function from the I/Q voltages, but plotting either I or Q
as appropriate on some sort of chart recorder arrangement will show 180
degree changes.  

I find it very sad that there is so much reliance these days on Soundcards
with their inherent inaccuracies and difficulty in producing driving
software.
Any other interface, from the simple external A/D on the serial port, right
up to a separate DSP card, allows the entire system to be locked to one
reference and removes a substantial amount of the processing overhead.  The
serial port A/D is so straightforward to build and programme for, that I'm
truly amazed a lot more use is not made of it !  The VE2IQ PSK software will
still run satisfactorily on a 10MHz 286 machine, whereas most of the latest
Soundcard software needs a Pentium machine.  Is there a reluctance to build
extra hardware these days ?   

But I supppose that anyone who doesn't use the Windoze operting system now
is behind the times and "Not with it!".  An attitude seen more and more
these days until the advantage of simple software performing real time
decoding is demonstrated to them.

Andy  G4JNT


-----Original Message-----
From: Paul A. Cianciolo [mailto:paulc@snet.net]
Sent: 14 February 2002 11:37
To: rsgb_lf_group@blacksheep.org
Subject: LF: RE: GPS Locked PSK

>There a few of us here in the states with GPS locked systems.
>The references have been becoming more and more available.

>My system is locked to GPS.  The question is however is there any
>software avaiable to take advantage of the system you are using.

>I would really like to try and hear you or others transmitting this mode
and
>I have all the hardware to do it,
>but lack the software.

>Any idea's


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