I am using the FMCOMMS2 card on the ZC702 EK. My application consists of evaluating the received signal phase after reflection on a target. I am working at 2.4GHz and I transmit an unmodulated CW signal using DMA. The Tx and Rx LOs are set to the same frequency at successively 2.4GHz, 2.404 GHz, and 2.434GHz.
I use one Tx/Rx pair to evaluate the phase difference between the Tx LO and Rx LO by looping the Tx and Rx ports with a cable. I use the other Tx/Rx pair to evaluate the phase difference between the received signal and the Rx LO. I disable all the tracking features (RF Quad, RF DC, and BB DC) as the phase information I seek is found in the Rx signal DC level. I also make sure to set the Tx attenuation properly not to saturate the receiver.
My questions are the following:
1) For a target at the same distance, sometimes I obtain a phase difference (phase Rx1 - phase Rx2) around -1rad, sometimes around +1 rad. Is there a way to explain this difference? Can you tell me how the PLL locks and how its lock time and previous state affect its current phase?
2) When I move the target at a constant speed, the phase doesn't follow. It is somehow calibrated to stay the closest to -1 rad (or in certain cases +1rad). Do you know if a calibration paramater might cause this?