[time-nuts] Measure GPSDO stability with minimum resources?

Jim Harman j99harman at gmail.com
Wed Oct 5 22:14:27 EDT 2016

On Wed, Oct 5, 2016 at 6:46 PM, Bob Stewart <bob at evoria.net> wrote:

> What I'm really looking for is a way to do everything in the PIC.  I've
> been experimenting with saving the DAC value in a circular queue every 20
> seconds for 60 minutes, and plotting the difference value between the head
> and tail of the queue every second.  After posting this question, I took
> another look at the overall behavior and decided to cut the queue size down
> from 60 minutes to 5.

Hi Bob,

Unless the oscillator is still warming up, 5 minutes or even 60 is way too
short a time to look at aging. For aging, you will want to look at the
change in DAC values over several days at least.

Looking at the current value and one in the past will give you a feel for
what is going on, but then you are discarding all the intermediate data.
You really want to do a least squares fit using as many data points as you
can handle, Check Wikipedia under Linear Least Squares for an example.

What sort of phase detector are you using? If you want to see whether the
system is locked, you may be better off looking at the phase detector
signal.and declaring a lock if the low pass filtered phase error stays
within a pre-determined range.


--Jim Harman

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