Re: [Discuss-gnuradio] FM demodulation block performance ?

[mleech]

On 07 Mar 2013 11:19, jmfriedt wrote:

> I am still trying to receive NOAA APT signals using the RTL-SDR DVB dongle. What I did
> so far:
> 1/ using a wideband discone antenna, I receive the NOAA APT characteristic signal during passes
> on a AOR3000 scanner. I know the NFM mode of the AOR3000 is too narrow for full decoding of the
> image (saturated whites), but at least I know my antenna is good enough to recover an audible and
> usable signal,
> 2/ on the antenna I put a splitter (Minicircuits ZFDC-10-5) with 10 dB loss on the coupled output
> (to the AOR3000) and 1.5 dB loss on the direct path (to the DVB receiver).
> 3/ I use http://picasaweb.google.com/lh/photo/2QDS5vwPra5Q7yQbSdUOng as an example of NBFM receiver
> usage (extending the demodulation bandwidth to 30 kHz instead of the 5 kHz in the example), or I 
> try a WBFM receiver module with a 30 to 40 kHz bandwidth, both of which were preceded by a 100-kHz
> low-pass filter (either as in the screenshot using the FIR coefficients in a Xlating block,
> or using the LPF block)
>
> In all cases I get a clear signal on the AOR scanner (even after the 10 dB loss on the coupler) while
> I cannot recover any usable signal on the DVB receiver, with RF gains in the 33 to 39 dB range and
> IF gains in the 25 to 40 dB range. A friend of mine got usable signals using the NFM demodulator from
> SDRSharp (which I have not yet managed to get running) with the DVB dongle.
>
> Any idea whether the FM demodulation blocks require some minimum signal to noise ratio to lock ?
> more generally is there any other description/documentation on the blocks other than the doxygen
> generated information and source codes ?
>
> Thank you, Jean-Michel

Your FM demod blocks are configured for quadrature-rate == audio_rate     This doesn't work very well.  I generally have the quadrature rate be some small multiple (2 or 4) of the audio rate.

Look at the way multimode, for example, handles NBFM demodulation for various "standard" NBFM bandwidths.