There are many variables that could affect this - what version of the programs are you running?

The most common possible causes are

 1) the USRP RF/IF gain setting(s) must be tuned for optimum reception.  Later versions of scope.py have a --gains [-N] parameter - you can use the osmocom_fft application to adjust the gains to find the best values.

 2) there is a frequency offset in all USRP's unless you have a GPSDO - the offset must be accounted for when tuning

 3) there is a DC offset (at zero IF) which can be "cured" (circumvented) using the -o option (I usually use -o 50e3)

 4) Trying to receive LSM/CQPSK in disctap mode

For more help we'll need to know the versions of op25 and gr, the command line typed, and ideally post screen captures of your spectrum, eye, and constellation plots...

Max

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