[USRP-users] LO frequency offset

Nick Foster nick at ettus.com
Fri Jun 8 17:46:29 EDT 2012


The behavior is due to the TCXO used in the N200 series. Small TCXOs
typically use digital correction steps; the step size in this particular
TCXO is relatively large. You can avoid the problem by using an external
reference. You can also configure the N210 to select the "external"
reference, but don't connect a reference. In this case, your absolute
frequency error will go up several orders of magnitude (comparable to the
USRP2), but you will not see the "jumps".

--n

On Fri, Jun 8, 2012 at 2:41 PM, Francois Quitin <fquitin at ulb.ac.be> wrote:

>  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 ****
>
> ** **
>
> _______________________________________________
> USRP-users mailing list
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> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
>
>
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