UHD GRC loopback glitches

I have a simple GRC flowgraph that transmits a complex sine wave on a
UHD USRP2 with a WBX, receives on a USRP2 with a DBSRX, and displays the
output on a scope. The two boxes are connected via a 30dB Mini-Circuits
attenuator. Mostly the output looks as I expected, but every 350
microseconds or so there’s a glitch that looks like the receiver stops
and starts. You can see the effect in the screen capture below (or in
the PNG at https://public.me.com/mepard in the gnuradio folder, where
you’ll also find the flowgraph).

I’m using current UHD and gnuradio code via git on a very capable
machine running Ubuntu 10.04. System Monitor shows one CPU at 100%, one
at 25 - 40% and the others blow 25%. Wireshark shows a lot of activity
on the control port for the receiver, but I haven’t deciphered the
packets (anyone have a UHD Dissector?).

Any idea what is causing this or how I should troubleshoot it?

Thanks.

-Marc

This could very well be underflow on the tx or overflow on the rx. Are
you seeing Us and Os printed to stderr while running? I suggest:

  1. reduce the sample rate to see of that fixes or reduces the problem
  2. isolate the problem as either an rx or tx problem
  3. if this in linux, make sure you have enabled a large rx buffer

-Josh

On Sep 17, 2010, at 3:44 PM, Josh B. wrote:

This could very well be underflow on the tx or overflow on the rx. Are you seeing Us and Os printed to stderr while running?

Ah, I forgot to mention that nothing is printed to stderr, nor to the
UART.

I suggest:

  1. reduce the sample rate to see of that fixes or reduces the problem

It reduces the frequency of the glitches, but they’re still there even
at 250,000 sps. Suppose it’s related to the amount of data?

  1. isolate the problem as either an rx or tx problem

Do you think a tx problem could cause these scope results? I’m a
software guy, but my analog colleagues suspect the rx. The png is at
https://public.me.com/mepard.

  1. if this in linux, make sure you have enabled a large rx buffer

500000000 bytes send and recv.

-Marc

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