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Re: [Discuss-gnuradio] FFSK/MSK Demodulation in Gnuradio

From: Michael Ossmann
Subject: Re: [Discuss-gnuradio] FFSK/MSK Demodulation in Gnuradio
Date: Wed, 2 Dec 2015 15:53:13 -0700
User-agent: Mutt/1.5.23 (2014-03-12)

On Wed, Dec 02, 2015 at 02:41:29PM +0100, Michael Kramer wrote:
> I've tried your suggestion but again I'm a little bit confused by the
> results. The attached Wave2 is produced if I decimate by 100 in my Low
> Pass Filter right before the Hilbert -> Quadrature Demod. If I don't
> before, but after the second Quadrature Demod I get Wave3. So I guess
> I'm doing right by decimating within the Low Pass Filter.

Agreed.  Wave2 is very close to the result I was hoping for.  Try
increasing the number of taps in the Hilbert block by 2x to 4x.

> I can even more or less see my data inside Wave2 (even though it seems
> to be inverted. it looks like its 00000111111111001110110 instead of
> 111111000000000110001001), but I'm not really able to center it since
> the upper spikes are crossing zero sometimes.

You might try using DC Blocker for this.  You'll probably have to
increase the default length by about 3x.

> Also I wanted to ask if my calculation is correct: I have sampled with
> 2MHz. The bitrate (should be) 2400. So if I want 4 or 5 samples per
> symbol for the Clock recovery I need to resample to 4*2400=9600 or
> 5*2400=12000.

Yes.  For best clock recovery results, I usually aim for 2 to 4 samples
per symbol.  If you decimate by 400 instead of 100, that should give you
5 ksps or 2.0833 samples per symbol.  (And, actually, if you do that,
you may not need to set the parameters for Hilbert and DC Blocker so


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