[time-nuts] GPSDO control loops and correcting quantizationerror

[Tom Van Baak]

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" <lists at rtty.us>
To: "Discussion of precise time and frequency measurement" <time-nuts at 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