[USRP-users] LO frequency offset

Francois Quitin fquitin at ulb.ac.be
Fri Jun 8 17:41:32 EDT 2012

Dear all, 


I’m designing a frequency synchronization algorithm on our USRP-N200 with a
WBX daughterboard, and I’ve been observing some strange behavior with it. So
I’ve run the following test: 


I use one USRP-N200 to transmit a constant pilot tone, and I use another
USRP-N200 as receiver. I store the received pilot tone to a file, and
post-process this data to extract the LO frequency offset  (Center frequency
of 964 MHz, sample rate of 200kHz). In some cases there is no problem (first
picture), the LO frequency offset changes over time, but nothing surprising.
However, sometimes (second picture), the LO frequency offset “jumps” several
dozen of Hz. 


I asked a colleague from another university to run the same test with their
USRPs-N200, and they have observed the same behavior: the LO frequency
offset sometimes “jumps”. Even stranger, when I use our older USRP-2, I
never observe this behavior. 


Has this happened to someone before? Have I broken something on our boards?
Or is there some flaw in the clock distribution on the USRP-N200? 


Thanks a lot, 


François Quitin, Ph.D.


BAEF Post-doctoral research fellow

Electrical & Computer Engineering Dept. 

University of California

Santa Barbara, CA 93106-9560

Phone: +1-805-883-8599

Email: fquitin at ece.ucsb.edu

Home: www.ece.ucsb.edu/~fquitin 


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