NI USRP-2920 Clock stability: Baseband frequency jumps

[xarteaga]

Hello,

I am trying to do a labview program which is able to send a message from one USRP (2920 model) to other one. I programmed it using labview 2012.

I programmed a quadricorrelator for measure the frequency offset.

I programmed another VI to test it. This VI measures the frequency using the quadricorrelator and also a NI Phase-Lock Loop (PLL). After a while senging the symbol '1+1i' I get the following image:

The measured frequency sometimes jumps from one frequency to another one. I have no explanation. The USRPs are connected each other using the included cable and the attenuator. The ports used are the TX1 and the RX1.

Why is occurring that? Have I configured properly my pair of USRP?

I attached the VI I used.

Thanks in advance,

Xavier