About synchronize two USRP2s using external reference (SMA)

I tried to synchronize two USRP2s using external reference (SMA).
I input 10-MHz signal to REFCLOCK pin and a trigger in PPS IN.

I modified rx_streaming_samples.cc and wrote the following code
u2->config_mimo(usrp2::MC_WE_LOCK_TO_SMA);
I use txrx_raw_eth_20100608.bin and u2_rev3-20100603.bin.
but I failed to synchronize two USRP2s.

I synchronized start timing using PPS but two USRP2’s signal
have phase difference. This phase difference change every observation.
Some figures are attached.
In this case, I input RF signals of 130 MHz + 0.05 MHz to RF1 and set as
follows:
Center frequency: 130 MHz
Decimation: 4

When I don’t set center frequency, two USRP2’s signal don’t have phase
difference.
So I suppose this is due to the phase difference between NCOs of two
USRP2.

What should I do?

Many Thanks.

Youhei Wakisaka.

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