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. Any suggestions? Matteo

on 2006-04-05 15:48

on 2006-04-05 17:35

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

on 2006-04-05 17:51

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 suggestion! MC

on 2006-04-05 19:35

On Wed, Apr 05, 2006 at 03:44:59PM +0200, 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. > > Any suggestions? > Matteo Is your real signal at a non-zero IF? If so, just multiply by a complex sinusoid to translate to baseband, then use existing code path. We come from the complex is simple school of signal processing...

on 2006-04-05 22:57

It will not be possible to recover the signal if the real signal is at zero IF (and predetection) so we will assume that it is not at zero. The simplest possible FM detector is a zero crossing counter. That is, you output a signal that is proportional to the distance between zero crossings. After filtration, this will provide a usable FM detection. It works much better if you do a bandpass filter first, count distance between zero crossings and then lpf the "distance" signals. The zero crossing should not be the number of samples. Given a sample N and N+1 where we have a zero crossing between, one can do (S(N+1)-S(N)) --------------- = slope 1 is the slope and the zero crossing then is the solution to: slope*Xnew + S(n) = 0 or Xnew = - S(n)/slope. Where we have decided to find the place where the connecting line crosses. Xnew - Xold is the zero crossing distance and is output as the detected FM. now set Xold = Xnew and go do it again. Bob Eric Blossom wrote: >> > > Is your real signal at a non-zero IF? > > If so, just multiply by a complex sinusoid to translate to baseband, > then use existing code path. We come from the complex is simple > school of signal processing... > > > -- AMSAT VP Engineering. Member: ARRL, AMSAT-DL, TAPR, Packrats, NJQRP/AMQRP, QRP ARCI, QCWA, FRC. ARRL SDR Wrk Grp Chairman Laziness is the number one inspiration for ingenuity. Guilty as charged!