[USRP-users] Phase and frequency locking to an external source

Cochenour, Brandon M CIV NAWCAD, 4.5.6 brandon.cochenour at navy.mil
Wed Sep 18 16:40:32 EDT 2013

In a recent experiment I tried to phase lock an external source to a USRP210 with a WBX using the NI LabView driver.  With both the external source and the USRP referenced to a 10 MHz source...I set both to the same frequency, and then observed the I and Q outputs.  I and Q exhibit a slow drift vs time.  My impression is that even though the USRP and external input are phase locked to the same 10 MHz reference...there's no way to get them exactly frequency locked.  Each VCO/PLL gets to the desired frequency in different ways with different tolerances and resolutions.  Is this a reasonable statement?

For example...if I had two DDS sources...I could play with their synthesis equations (sample rates, resolution, multipliers, etc) and likely find exactly common frequencies between them.  Given the analog front end of the USRP...this doesn't seem possible.  

If I could provide an LO directly...then I could be assured that I was both phase and frequency locked.  Is this possible?  


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