GRC NBFM Receiver

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natedawg1604

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Hi there,

For the last few weeks I've been pulling my hair out trying to build a simple Narrowband FM receiver with my RTL SDR and gnuradio companion on my Mint Linux box. Does anyone have a list of the minimum modules needed to accomplish this (and perhaps a flow graph of recommended settings)?

Everything is definitely installed correctly, I just can't figure out how to build the proper flow graph. I was able to build a flow graph for Wide-band music FM stations with a 48k audio sink to my soundcard. Also I can hear NFM VFH/UHF channels just fine in GQRX, I just can't figure out how to construct a proper Narrowband FM Flow graph in GRC...
 
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DSheirer

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Hi there,

For the last few weeks I've been pulling my hair out trying to build a simple Narrowband FM receiver with my RTL SDR and gnuradio companion on my Mint Linux box. Does anyone have a list of the minimum modules needed to accomplish this (and perhaps a flow graph of recommended settings)?

Everything is definitely installed correctly, I just can't figure out how to build the proper flow graph. I was able to build a flow graph for Wide-band music FM stations with a 48k audio sink to my soundcard. Also I can hear NFM VFH/UHF channels just fine in GQRX, I just can't figure out how to construct a proper Narrowband FM Flow graph in GRC...

Alex Csete has an example NBFM receiver GRC flowgraph here:
GRC Examples

You can eliminate one of the NBFM receive blocks and the subsequent selector block. It doesn't look like he has a sink attached ... you can add a sound card sink block.

Minimum blocks:
1) Quadrature (Complex) Source
2) Complex Decimate sample rate (down to ~48k)
3) Complex Low Pass filter
4) FM Demodulate
5) Float Low Pass filter (the NBFM receiver block might do this along with demod)
6) Sink - output to speakers or wave file.

The frequency translating (and decimating) FIR filter does steps 2 and 3.
 
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natedawg1604

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On the low pass filter, can someone explain how to calculate the "cutoff frequency" and "transition width"? I don't quite understand what these two items represent, and I can't set them correctly to hear a single narrowband frequency with the NBFM receive block; it should be really simple but I'm obviously missing something...
 

DSheirer

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On the low pass filter, can someone explain how to calculate the "cutoff frequency" and "transition width"? I don't quite understand what these two items represent, and I can't set them correctly to hear a single narrowband frequency with the NBFM receive block; it should be really simple but I'm obviously missing something...

The low pass filter removes any high frequency noise produced during the demodulation process. Humans normally can hear sounds in the range 300 - 3600 hertz. So, you can set the cutoff frequency somewhere between 3000 and 3600 hertz.

The transition width is the region starting at the cutoff frequency and extending up to the frequency where the filter has maximum attenuation. The narrower the transition width the more taps that are required to achieve the filter shape. More taps equals more cpu.
 
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