I'm seeing an odd problem with the data received from the ADC (AD9361 Rx data interface). I'm examining the ADC data with ChipScope within a modified version of the FPGA reference design and I see good data (expected waveform and magnitudes) for a while then it suddenly drops to near 0 values for both I and Q. I'm using a two-board setup with the ZedBoard and FMCOMMS2, where one board is transmitting to the other. Here are the details:
- The data dropout only occurs when transmitting over the air, a cabled connection works flawlessly
- The AD9361 ENSM is set to FDD mode
- When our custom packet is not being transmitted, null (0) data is being sent to the DAC
- The Rx dropout always occurs at the same sample point in the packet
- The dropout is temporary and good data returns for the next frame
- AGC is disabled, the MGC setting is 0x14 (Register 0x109). I've tried various MGC settings and enabling AGC but this doesn't change the behavior
- No software is running on either system
- The ADC data captured with ChipScope emanates from an untouched portion of the FMCOMMS2 FPGA reference design (adc_data_i1, adc_data_q1 output from axi_ad9361_rx.v)
- ADC sample magnitudes are in the expected range when the data is good
Since we are at both ends of the link, I realize the problem could be on the Tx or Rx side. I'm not suspecting the modifications made to the FPGA since everything works as expected when using a cabled connection.
Any suggestions on what to look at next would be appreciated.