On 29/06/2023 11:38, Rob Kossler wrote:
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.
Something else to note about X310 is that in order for timed tuning to
work with the DDCs, there has to be a stream running
through it at the time, because it gets its notions of time from the
radio block and uses tagged samples running through it to
time re-tunes precise-to-the-sample.
In this case, I think we're just talking about timed *streaming*, which
shouldn't touch the LOs/DDC/DUC at all. However, if
the TX is continuous, and the RX is discontinuous, the RX will be
picking up the TX at some random phase.
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