[USRP-users] N200 Factory Clock

Nick Foster nick at ettus.com
Wed Jun 27 13:30:49 EDT 2012


On Wed, Jun 27, 2012 at 9:17 AM, Nowlan, Sean
<Sean.Nowlan at gtri.gatech.edu>wrote:

>  >For compatibility with other TCXOs that may be installed and require a
> pullup, a pulldown, or a divider for enable and frequency control. On the
> FOX924B installed on the N210 those components are not required.****
>
> >** **
>
> >--n****
>
> ** **
>
> Can I assume the bias network is not needed on the N200 either, and that
> it has no effect on its operation? Also, is the latest revision of N200
> still shipping with this same F924B part?****
>
> ** **
>
> The reason I’m asking is because we observed quite a bit of drift using a
> freq counter driven by a Rubidium reference. I used N200 with RFX900 and a
> simple GNU Radio app (Const source --> USRP) to transmit a 900 MHz CW. We
> downmixed the 900 MHz CW to 100 MHz by using an 800 MHz CW from a signal
> generator running off the same Rubidium reference. The freq counter only
> goes up to 225 MHz. So we observed a range of values within 350 Hz on the
> 100 MHz CW. This is within spec for the F924B part, and I know there’s a
> lot of freq synthesis, D/A conversion, and other stuff going on in the
> transmit chain, but what concerned us most was that the drift had high
> rates of change and near-discontinuities, rather than steady drift over a
> long period of time. Hence we were wondering whether the bias network was
> affecting the factory clock at all. A plot is attached. Thanks!
>

The FOX924B uses a digital loop with high and low digital step sizes. We've
noticed the same behavior, and it's definitely not optimal. The accuracy of
the part is well within specifications, but the steps produce the
occasional phase discontinuity. Future revisions of N200/N210 will use an
analog TCXO from another manufacturer that doesn't exhibit this phenomenon.

HTH,
Nick


> ****
>
> ** **
>
> Sean****
>
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