[USRP-users] B200-mini does not respond to fast frequency changes

David Rose David.Rose at govsco.com
Tue Nov 14 18:35:35 EST 2017


I'm trying to implement a spectrum analyzer that spans the 2.4 GHz ISM band.  The band I want is about 90 MHz wide, which is too wide to handle at one frequency setting.  So my design is to step the receive frequency across the band, pausing at each step to collect enough samples for an FFT.  I'm sampling at 10 Msps, and only need 1024 complex samples at each frequency step.

My flowgraph has the following components:

  *   Source block: (Controls the B200-mini)
  *   Stream-to-vector block: (Groups the samples in to 1024-sample vectors)
  *   Signal processing block: Custom block, written in Python, that does the FFT, computes the magnitude, and writes the results to file.  Also tells the source when to change frequency.

The problem is, if the dwell time at each frequency is less than about 3 mS or so, the B200mini doesn't respond to the "set frequency" command.  It adds a tag to the stream that says it changed to the new frequency.  And if I interrogate it using the command "new_f = self.uhd_usrp_source_0.get_center_freq()", it returns the new frequency.  But based on looking at known emitters in the resulting spectrum, the frequency does not in fact change-it just continues generating samples at the original center freq.  If the dwell at each frequency step is more than 3 mS or so, it all works as it should.

The 3 mS limit doesn't seem to be related to the settling time of the PLL in the receiver.  After a successful frequency change, I see valid samples (and valid spectrum) within about 0.1 mS.

Bottom line is, I only need one vector of 1024 samples at each frequency, but this issue is forcing me to wait for around 30 vectors at each freq.  That's too slow for the intended app.  Has anyone run into this before (I assume they must have), and is there a solution?

Misc info:

  *   GNURadio version 3.7.9
  *   GNU Radio Companion 3.7.12
  *   UHD version: 3.11.0.git-191-g1cd96dde
  *   Python 2.7.12
  *   Ubuntu 16.04.3 LTS - Desktop version
  *   Running on Oracle VM VirtualBox Manager (version 5.1.28) under Windows


Dave Rose
Valkyrie Systems Corp.

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