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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: Tue, 1 Dec 2015 10:12:38 -0700
User-agent: Mutt/1.5.23 (2014-03-12)

On Tue, Dec 01, 2015 at 02:14:59PM +0100, Michael Kramer wrote:
> 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.

I've never heard of FFSK, but it looks to me like you have a secondary
frequency modulation.  In other words, you have a frequency modulated
AFSK signal; the data bits are frequency modulated (FSK) onto a carrier
in the audio range, and then that signal is frequency modulated onto an
RF carrier.  It doesn't look like MSK.

Try using a second frequency demodulation in your flowgraph.  One way
would be to insert Hilbert->Quadrature Demod after your second LPF.
Scope the output of the second Quadrature Demod, and you should see a
squarish signal where data zeros are high and data ones are low (or vice
versa).  You should be able to verify the symbol rate by analyzing this
output and then pass it to a clock recovery block.  You'll probably need
an additional step to center the signal around zero before clock


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