Demodulating a MPT1327 signal

GNU Radio
1

MPT1327 uses MSK on its control channel. I am attempting to use NBFM
Receive to demodulate the signal into a WAV file that I can play back
and use sdrtrunk to decode it. So far my best attempt is set up as
follows:

USRP
Sample Rate: 250k
Center Freq: 652.625M
Gain: 100

Frequency Xlating FIR Filter
Decimation: 1
Taps: firdes.low_pass(100,samp_rate,10e3,1e3)
Center Frequency: 0
Sample Rate: 250k

Rational Resampler
Decimation: 250k
Interpolation: 192k

NBFM Receive
Audio Rate: 192k
Quadrature Rate 192k

Wav File Sink
Sample Rate: 192k
Bits per Sample: 16

The output of this seems to be just noise and is not working.

Thank you.

–tyler

2

Post your GRC file. I tried it just as you implemented and it is
working
here. I would have choose different sample rates and topology, but
nonetheless it works. Gain of 100 (dB) should not be a valid UHD value
as
it tops out at 81 for the B200, and 31 for the WBX. It still works
though.

Can you trace the signal down the flow with the GUI instrumentation?

Weaver, Tyler [GCS] wrote

Decimation: 1
Quadrature Rate 192k

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3

OK got your file, and it is working fine here on a N210+WBX, and a B200,
when
fed with a signal generator.

  1. You must be using GR 3.6 or earlier as some blocks don’t import into
    3.7.5. I replaced the rational resampler and nbfm_rx with your same
    settings.

  2. I assume you are using an USRP2 or N2x0 since you are using an IP
    address. The gain should be 31 maximum for the WBX. I measure the
    minimum
    detectable signal at this gain is about -90 dBm, for 1 kHz tone on NBFM
    with
    5 KHz deviation.

  3. I also added a resampler to 48k, 10x gain, and an audio sink. I did
    verify the wav file is being written.

Are you sure you are on frequency and signal is strong enough? Any tune
warning from UHD? Can you tune a strong signal like 162.55 MHz NOAA, or
equivalent in your area? At least you are getting noise.

http://gnuradio.4.n7.nabble.com/file/n49889/uhd_rx_sample.png

Thanks,
Lou
KD4HSO

madengr wrote

use sdrtrunk to decode it. So far my best attempt is set up as follows:
Sample Rate: 250k
Sample Rate: 192k
Bits per Sample: 16

The output of this seems to be just noise and is not working.

Thank you.

–tyler

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