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[Discuss-gnuradio] FFSK/MSK Demodulation in Gnuradio

From: Michael Kramer
Subject: [Discuss-gnuradio] FFSK/MSK Demodulation in Gnuradio
Date: Tue, 01 Dec 2015 14:14:59 +0100
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my name is Michael Krame and I'm studying Information Engineering right
now. For a studyproject I'm working with Gnuradio-Companion right now.

I have the following problem. I've got a signal and I know it's FFSK
modulated with a bitrate of 2400. I've tried to read some tutorials or
find other information about FFSK (which is the same as MSK apparently),
but I found very little. Most said that the GMSK Demod Block should work
for MSK (and therefore my signal) as well. But since this block didn't
worked I've tried to use Quadrature Demod, Low Pass Filter and Clock
Recovery as can be seen in the attached flowgraph.png. But I still can't
get the bits through Clock Recovery.

I've attached a Wave.png which shows the signal after the second Low
Pass Filter. I can see the data in there (Preamble of 1111110000001,
followed by 10001001). which is the beginning of the data I'm trying to
get. So a long, high pulse is a 1 and two fast pulses is a zero. With
this scheme I'm able to read my data from the signal. But Clock Recovery
still fails to get my data. I've already tried with many values for
Omega (started at 833 without decimation, then went down to 8,33 through
decimation and also tried values of 4 or 5 with the right decimation,
cause I've read that these values proved to be the most useful).

So my question is: Is there any other way to demodulate FFSK signals? Or
am I doing something wrong?

Btw I'm using GNU Radio Companion 3.7.9git-99-gaae64454.

Michael Kramer

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Attachment: Wave.PNG
Description: PNG image

Attachment: flowhraph.PNG
Description: PNG image

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