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Re: [Discuss-gnuradio] Demodulating a MPT1327 signal

From: madengr
Subject: Re: [Discuss-gnuradio] Demodulating a MPT1327 signal
Date: Sat, 9 Aug 2014 14:47:15 -0700 (PDT)

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

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.



madengr wrote
> 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
>> 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:
>> 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
>> _______________________________________________
>> Discuss-gnuradio mailing list

>> Discuss-gnuradio@

>> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio

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