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Re: [Discuss-gnuradio] A medley of questions

From: Steven Clark
Subject: Re: [Discuss-gnuradio] A medley of questions
Date: Wed, 14 Mar 2007 18:54:30 -0400

There is always an unknown phase between the transmitter and receiver.
In addition, the clocks between the transmitter and receiver are never
running at exactly the same frequency.  FWIW, the oscillator on the
USRP is spec'd to 50ppm.


GMSK is a constant amplitude modulation. So unless I am misunderstanding something (which is entirely possible), if you plot the complex baseband signal with real for X and imag for Y, the result should be a constant-amplitude circle regardless of where in the symbol you are sampling, and regardless of any carrier freq discrepancy between the tx and rx. A carrier freq offset will make the phasor spin faster or slower, but it should still have constant magnitude.

I'd seen the 50ppm number before, and that's part of the reason I asked the second question about adding improved carrier recovery. I was noticing a freq difference of ~5 kHz @ somewhere in the 200-300MHz range (that's about 20ppm, so in line with our expectations), and it was really making the M&M clock recovery block in gmsk.py struggle. I'm guessing that M&M depends on zero crossings for clock recovery, so when it's input (which is normally in the [-1, 1] range) gets a DC offset such that it's now [-0.5, 1.5] for example, it has problems.

I'm interested in your IIR suggestion. It seems like what we want to do is essentially high-pass filter the output of the FM demod to get rid of the DC offset. Were you recommending low-pass filtering the FM demod output to estimate the DC bias and then subtracting that off from the original signal? Why an IIR filter rather than a FIR? Which IIR block would you choose?

Thanks again for your continued patience.

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