On 29/06/2023 09:36, zhou via USRP-users wrote:
Hello All,
I am using X310 USRPs. I did a loopback test with this USRP, that is,
Tx was connected to Rx (with a 20dB attenuator between them). This is
for checking the channel status.
In my test, Tx is free running without stop, and Rx is occasional.
Both transmission and capture are time-based, that is, signals are
transmitted at specified time, and capture starts at specified time.
Master clock rate: 184.32MHz
Sampling rate: 184.32MHz
Using the captured signals, I can calculate the channel coefficient. A
few captures were made.
I expected constant channel because it was cable connection and it was
in the same USRP, but I found that the constellations of the pilot
signals rotated when comparing different captures. I think this can be
caused by the phase difference between Tx LO and Rx LO, but not sure.
Questions:
1. are there independent LOs for Tx and Rx in a USRP?
Yes. In fact, in any realistic on-the-air scenario, your RX and TX
will never be phase aligned, or even phase-coherent.
2. Is the Rx LO on and off in test, that is, it starts when capture
starts and stops when capture is completed? So, the LO phase offset
between Tx and Rx is random?
I would expect the RX LO phase to random with respect to the TX every
time you start/stop the RX.
Any feedback will be highly appreciated.
Kind regards,
Hongwei
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