I am using an fmcomms3 + zedboard. I am attempting to run the ad9361 at a lower sample rate e.g. < 10 Mhz. Using the sd card shipped with the fmcomms3 (using AD provided fpga + linux drivers etc), setting sample rates down to 9 Mhz on the iioscope demo spectrum analyzer yields what i would expect for a spectrum. Setting it to 8 Mhz or lower doesn't appear to work correctly, the gain of the receive path is reduced by >40 dB and unexpected high frequency noise comes up much higher than the desired passband (centered around 0).
For what its worth - Using our own design which includes the ad9361 hdl component and the noos driver running under userspace linux i see the same result.
I did an experiment to determine what the minimum sample rate that 'works' was - i found that a sample rate of 8215001 Hz gives a 'good' spectrum. Going down by 1 Hz to 8215000 results in the 'bad' spectrum - gain is reduced by > 40 dB and odd high frequency noise components become apparent.
I there some other setting i need to change along with the rx sample rate to support sample rates of less than this magic 8215001 Hz?
I wonder if there is some crossover of clock configuration / divisors / etc that is occurring here?