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Re: [Discuss-gnuradio] FM demodulation of a real signal

From: Martin Dvh
Subject: Re: [Discuss-gnuradio] FM demodulation of a real signal
Date: Wed, 05 Apr 2006 17:32:12 +0200
User-agent: Debian Thunderbird 1.0.2 (X11/20051002)

Matteo Campanella wrote:
> Hello, I am quite stuck at a problem, that is, how to demodulate a FM
> signal that is NOT in complex form using existing gnuradio blocks. The
> quadrature demodulator works only with complex input; I have tried to
> build an analytic signal out of my real one, but the result of the
> demodulation is then quite poor.
How did you try to "build an analytic signal out of my real one".
The easiest way of doing that is to use gr.hilbert_fc (numberoftaps)
The numberoftaps in combination with the samplerate determines the lowest 
frequency for which the hilbert transform is accurate.
(The higher the number, the more accurate the transform)
Try and experiment with the numberoftaps value.
Most of the time a value like 256 will do.

You can also try to use the usrp_wfm_pll.py example.
This one uses a pll to recover the frequency modulation which should perform 
much better with a real source.

The pll it uses is gr.pll_freqdet_cf
You could build a gr.gr_pll_freqdet_ff block to go straight from real data to 
frequency detection.

(You can look in the old code atsc/GrAtscFPLL.cc
This is the PLL which was used for atsc demodulation. It works with real input 
> Any suggestions?
> Matteo
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