On 12/06/2019 09:33 PM, Lukas Haase via USRP-users wrote:
Hi,
I am using the USRP X310+UBX160 with gnuradio to perform very precicse phase
measurements: The TX transmits a CW which is reflected by an object and
received by the RX.
The received phase provides an accurate estimate of the distance to the
reflected object, once the fixed phase relation (between TX/RX-LO, filters,
cables etc.) has been subtracted out.
This works nicely so far.
However, I need my system to work across power cycles, and more importantly,
across different frequencies: The goal is to perform fast frequency hopping and
obtain the phase for each frequency.
Unfortunately it seems that the phase relationship between TX/RX is lost when I
tune the USRP to a different center frequency and back. For example, I have the
center frequency set to 900 MHz and the phase I measure (by computing the angle
of the I/Q samples) stays constant. But when I set the center frequency to 950
MHz and then back to 900 MHz, the phase has a random value again.
Is there any way to avoid this?
Or is there any way to lock the LO phase to a particular phase when tuning back
to the original frequency?
Thanks,
Luke
It *might* be possible to phase-synchroniez the RX and TX LOs using
timed commands combined, possibly with INTEGER_N tuning.
There's an APP Note on phase-synchronization here:
https://kb.ettus.com/Synchronization_and_MIMO_Capability_with_USRP_Devices
My gut tells me this is going to be hard, though, since the requirement
is to bring a synthesizer back to the same relative phase it had when it
was previously tuned to the same frequency.
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