On Thu, Jun 29, 2023 at 11:09 AM Marcus D. Leech <patchvonbr...@gmail.com> wrote: > > On 29/06/2023 09:36, zhou via USRP-users wrote: > 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.
Although there are separate LOs, they are both disciplined to the 10 MHz reference. If they are set using timed commands, they can provide repeatable phase (at least for some daughterboards like UBX and SBX). In radar applications, phase coherent Rx/Tx is common. > > 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. In the description above, you mentioned free-running Tx but you also mentioned timed transmission and capture. If you are using timed Tx and Rx along with the timed tuning commands, I would expect a consistent phase offset (depending again on daughterboard). If you have free running Tx, then I would expect it to be random. _______________________________________________ USRP-users mailing list -- usrp-users@lists.ettus.com To unsubscribe send an email to usrp-users-le...@lists.ettus.com
