Agreed. The DAC resolution only needs to be a little better than the short-term noise of the OCXO. For example, there's no point at all stepping the DAC by 1e-13 if the OCXO's own noise is on the order of 1e-11 or 1e-12. On the other hand, what you want to avoid is having the DAC/EFC increase the short-term noise of the OCXO. Sadly this seems to happen a lot. Just measure the stability of your favorite GPSDO with and without the DAC/EFC operating.
I've not seen graphs but I assume there's also a cross-over where DAC resolution/noise as a function of update rate meets OCXO stability as a function of tau. /tvb ----- Original Message ----- From: "Bob Camp" <li...@rtty.us> To: "Discussion of precise time and frequency measurement" <time-nuts@febo.com> Sent: Saturday, September 15, 2012 1:41 PM Subject: Re: [time-nuts] GPSDO control loops and correcting quantizationerror > Hi > > The real answer is that you don't need anything near 32 bits. Anything with a > <1.0x10^-13 AVAR isn't going to have much of a tuning range on the EFC. 22 > bits will do just fine for a 1 ppm EFC range part with 1.0 x10^-12 at 1 > second. With that sort of sensitivity you will have a *very* hard time > getting the voltage into the OCXO without a gotcha right at the terminals, > unless the unit has a fully isolated EFC input. That rules out roughly 99.99% > of all OCXO's. > > Bob _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
