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ad9361 SLOWATTACK AGC noos/linux problem at < 8.215 Msps

Question asked by sbjaver on Jul 11, 2014
Latest reply on Jul 15, 2014 by sbjaver

Hello,

 

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?

 

Regards,

Joe

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