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Re: Demodulating slow phase-modulated data

From: Patrick Sathyanathan
Subject: Re: Demodulating slow phase-modulated data
Date: Wed, 3 Nov 2021 22:18:09 +0000

Hi Philip,

Converting real valued signal to I-Q is a simple transformation:

  1. Pick an LO frequency, F_lo,  greater than 200Hz, the signal bandwidth
  2. Pick a sample interval, deltaT, smaller than 1/(2*F_lo)
  3. Generate I-Q digital local oscillator signal: [cos(2*pi*F_lo*i*deltaT), j*sin(2*pi*F_lo*i*deltaT] for i depending of the length of your sampling interval, i*deltaT
  4. Multiply each real valued signal value, s, from your WAV file, by each of the above LO values to generate the I-O signal
Then use whatever I-Q signal processing you to to process the samples.

Note that there is nothing to be gained by performing the above transformation. Unless you have some existing I-Q processing code, you might be better off analyzing the real valued signal.

Good luck,


From: Discuss-gnuradio <discuss-gnuradio-bounces+wpats=hotmail.com@gnu.org> on behalf of Philip Pemberton <philpem@gmail.com>
Sent: Wednesday, November 3, 2021 9:28 AM
To: discuss-gnuradio@gnu.org <discuss-gnuradio@gnu.org>
Subject: Demodulating slow phase-modulated data
Hi all,

I'm working on a project to reverse-engineer the data format used by an old navigation system called Datatrak. I've got a project page on my website about it: https://www.philpem.me.uk/datatrak/start

The data is sent by phase-modulating a ~133kHz or ~146kHz (longwave) carrier at a very slow rate -- the modulating wave is sinusoidal, either 37.5Hz or 50Hz depending on the part of the signal being sent. The signal bandwidth is about 200Hz.

I've been given some single-channel WAV format recordings from LW SDR receivers, 44100Hz sample rate, with a LO frequency of 128kHz. Sadly they're not I-Q recordings. I'd like to try and demodulate the signals in these recordings to recover the 64 bits of "Goldcode" synchronisation code (from the Trigger slot) and the 128 bits of Clock signal (from the Clock slot). I already know what the Goldcode value should be.

Can anyone suggest a way I could do this with Gnuradio/GRC or some other tool?

I figure I need to convert the WAV file into a complex (I-Q) signal, mix the signal of interest down to baseband, then filter it with a 200Hz bandpass filter. I'm not sure what I need to do after that to recover the modulating phase signal.


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