Phase difference of GPSDO 10 MHz signal

[Annam]

Hi everyone,

I am comparing the pahse difference of two GPSDO 10 MHz signals on an oscilloscope.

I think that if the GPSDOs are locked onto GPS signal, the pahse difference is very small and almostly stationary.

But, when I measured, the phase difference was chaning from -107 to 260 constantly.

I am sure that they are locked onto GPS, because I have seen the message "GPS is locked" by "query_gpsdo_sensors" command.

What is wrong?

[ErikL]

Hello!

Can you describe what you are attaching to the oscilloscope?  The output of the GPSDO is a 10 MHz waveform.  This should be aligned between the GPSDO's.  The derrived local oscillators from the two devices will not be phase algined.

Phase Alignment vs. Phase Coherence

Sharing the common 10-MHz reference among USRPs allows a phase-coherent Local Oscillator (LO) to be synthesized using a fractional-N frequency approach. During synthesis, as the reference is divided, the phase may lock on either rising or falling edges producing a constant but arbitrary phase offset on each channel. In MIMO communications, these fixed yet arbitrary phase offsets are corrected by the same algorithms that perform multipath estimation and correction.

Beam forming applications however require phase alignment of the LO, so that phase can be controlled or captured through baseband I and Q. Clever methods have been used to detect the phase offset in the USRP using a know reference signal and then apply the correction to the received signals for receive direction finding and beam forming research.

[sczhili]

Hi,

have you solved this problem untill now? could you share the causes with us? thanks!