Noise on the output of AD9361 ADC (might appear in one of I or Q components, only)

Dear AD9361 folks,

I'm working on a TDD system with the pin-control ENSM. My problem is a signifficant noise affecting samples received from ADC. What you can see in the figure below are the samples received from ADC through util_ad9361_adc_fifo on both Rx channels. The last line is a valid in line, so you can see that the head of my Rx frame reaches my IP core near 26,080 offset. As the tranmitter does transmit anything, only the noise should be visible in all I,Q components. However one of them is contaminated by some extra noise.

The noise would also appear if the receiver gets some useful data from the transmitter. Below you can see the moment when the head of the signal preamble starts, but the I-component of Rx1 (the first line) is degraded by the noise, again.  

My question is what can cause this kind of noise? It can appear in I, Q, or both signal components, and can affect either Rx1 or Rx2 channel.

It is visible if the signal is transmitted over the air or by a cable (in the second case one Tx channel is splitted to both Rx channels)

Some details about my system:

  • Both results were generated with the gain control set to MGC, but it can also appear in Fast Attack mode.
  • I'm using PicoZed AES-PZSDRCC-FMC-G with BD-Z7PZ-SDR2-G doughterboard and AD-PZSDR2400TDD-EB-Z board (to realize 2x2 space-frequency diversity)
  • The axi_ad9361_l_clk period is 6.125 ns.
  • The system carrier frequency is 2,5 GHz and the signal bandwidth is 28 MHz (I'm using the built-in FIR filter).

Best regards,

described Tx-Rx system configuration
[edited by: Matt139 at 6:14 PM (GMT 0) on 21 Aug 2019]
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