Hi,
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 out the reason for this and I suspect it is because the samples from the USRP1 are shifted so that what the computer thinks is the I1 sample is actually the Q0 sample. (When the samples are interleaved like this: I0 Q0 I1 Q1 I2 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 this 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?