AD9361 - I and Q channels

Hello.

I have tested the AD9361 with 2 FMCOMMS3, both configured with 50 MSPS and 12,5 Mhz RF, setting in the device tree 1 Channel mode. The emitting AD9361 has a Tx LO Freq = 2,4 GHz and Rx LO Freq = 2,45 GHz, while the receiving signal has Tx LO Freq = 2,45 GHz and Rx LO Freq = 2,4 GHz. I set different frequencies in the Tx and Rx channels of each AD9361 in order to avoid the VCO pulling effect.

Tx:


Rx:


I send the sequence [011111111111, 100000000000], each value repeated 4 DAC_Valid pulses, through the DAC_Data_I0 input of the AXI_Ad9361 IP core and set the rest of the DAC_Data channels to 0.

I receive the emitted signal through both I0 and Q0 and I encounter several phase shifts (180º) between them. Sometimes they occur in I0, sometimes in Q0, sometimes one phase shift is followed by the next one only ~50 samples later, sometimes it occurs hundreds or thousands of samples away.



Next, I tried feedbacking the Tx and Rx channels of only one FMCOMMS3 and I found that those phase shifts do not occur within the same transmission as before, but each time I initialise the board I find a different phase between I0 and Q0. Note that I still find the emitted signal at both I0 and Q0 channels of Rx.


Why do I receive DAC I0 through both ADC I0 and Q0? Why those phase shifts? Do I need to synchronise both AD9361 in order to solve these problems? If so, why do I still find some of those problems when feedbacking one AD9361? In the case I needed to synchronise both AD9361, how could I perform a wireless transition when they are far away from each other?