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

From: Matteo Campanella
Subject: Re: [Discuss-gnuradio] FM demodulation of a real signal
Date: Wed, 5 Apr 2006 17:49:13 +0200 (CEST)
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I have tried building the analytic by hilbert, but I think that, because
of the many near DC components of the signal, I do not get a nice
quadrature demod output...I will try with a pll, thanks for the


> 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 data.)
> Greetings,
> Martin
>> Any suggestions?
>> Matteo
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