WARP Project Forums - Wireless Open-Access Research Platform

Hello,
I would like to ask you some problems about some measurements of the board “WARP v3”.
We have added a “FMC-RF-2X245” module in order to build a MIMO OFDM system. We call the RF ports (RF A & RF B) on the original “WARP v3 board” as Port_1 & Port_2 and  we call the RF ports (RF A & RF B) on the additional “FMC-RF-2X245” module as Port_3 & Port_4.
In our system, we want the exact same phase of the clock for all the DAC/ADC at each port. However, when we measure the signal delay between each port, we found that the delay is not same for each port. The process of the delay measurement is shown as follows:
We transmit the OFDM signal with pilot at all subcarriers. Then we use an attenuator to attenuate the signal. Then the signal is received by another port. We calculate the phase change of each subcarrier and estimate the delay of the transmission between each port (Measured results are averaged by 500 OFDM symbols).
The measured delays (samples) are shown as follows:
(1 ) Port_1 -> Port_2 = 38.454 samples;        (2 ) Port_2 -> Port_1 = 38.454 samples
(3 ) Port_1 -> Port_3 = 38.303 samples;        (4 ) Port_3 -> Port_1 = 37.512 samples
(5 ) Port_1 -> Port_4 = 38.307 samples;        (6 ) Port_4 -> Port_1 = 38.531 samples
(7) Port_3 -> Port_4 = 36.718 samples;        (8 ) Port_4 -> Port_3 = 37.777 samples
In the results, we can see that we can only get exact same delay for Port_1 and Port_2(For example results (1) and results (2)). It can be explained that Port_1 and Port_2 are on the same board (WARP v3). For other ports, they cannot keep the same delay for each measurement (Such as (3) and (4)). In addition, we found that for Port_3, it has some problems about the DAC. The measured results show that the DAC of the Port_3 always has shorter delay than other ports (For example comparing the results in (4) & (3) and the results in (7) & (8)).
We would like to ask following questions:
Why the DAC of the Port_3 has a significantly different delay than other ports? Can we solve this problem by hardware configurations?
Besides the Port_1 and Port_2, why other ports cannot keep the same delay? Can we solve this problem by hardware configurations?
Is it normal for this problem? Do we need something to take care when “FMC-RF-2X245” is connected with the “WARP v3”?
Thank you very much for your response!
Linning