Shifted samples from USRP1


I’m using the USRP1 and the UHD driver together with gnuradio. Often,
especially at high sample rates (8MHz), the spectrum observed in the
fftsink becomes distorted after a while. It becomes almost mirrored.
Typically it happens after a couple of “overruns”. I have tried finding
the reason for this and I suspect it is because the samples from the
are shifted so that what the computer thinks is the I1 sample is
the Q0 sample. (When the samples are interleaved like this: I0 Q0 I1 Q1
Q2 I3 Q3 I4 Q4 I5 Q5 …)

I have tried modifying the FPGA image so that the FPGA sends a constant
positive sine wave to the computer and the same things happens also in
case. This narrows down the problem quite a lot… I have also tried
modifying “rx_buffer.v” so that the I-channel is delayed one clock cycle
more than the Q-channel. This causes the same distortion to be apparent
immediately when starting the USRP.

I wonder if you guys have any ideas about what may cause this and how it
can be fixed?


(I’m using the git version of uhd and gnuradio as of today)