[time-nuts] GPSDO "accuracy"

[Bob kb8tq]

Hi

Maybe a good idea to “back up” a bit here:

The most commonly plotted data for the performance of a GPSDO is ADEV. 
Very simply put, to do ADEV you take a series of readings at a specific time 
spacing ( called tau ). The delta frequency from one reading to the next is then
computed. You take the standard deviation of that “delta” information. 

What you are looking at is the “good guess” at what the frequency will be in the
next time slot, based on what it is in this time slot. This may or may not be what
your system / measurement instruments are looking as a. spec. 

The big reason ADEV exists is that it is convergent. As you take more data, the 
results don’t move all over the place, they converge to a single value. Measure today, 
then measure tomorrow, you get pretty much the same number. 

You might want to know what the maximum frequency error compared to “absolutely 
correct” is in your 10 second time period. This is a measure that is non-convergent. The 
longer you collect the data, the bigger the number gets. Measure for an hour and you 
get a different number compared to measuring for a day. Measure today / measure 
tomorrow and you may get dramatically different results. 

This is not to say that nobody ever can know what the frequency is. Only that “max” is
not a good limit for this sort of random fluctuation. Again, this is what drove the guys
at NIST to come up with ADEV back in the 1960’s. It’s what keeps us using it as a
means of comparison today.

Bob

> On Sep 9, 2020, at 7:04 PM, donald collie <donaldbcollie at gmail.com> wrote:
> 
> Can any list member please tell me the "accuracy" that can be expected from
> a typical GPSDO
> over, say, a 10 second interval? I have several measuring instruments
> connected to my Trimbal GPSDO, and would like to know what to expect. At
> the moment I am guessing about 1 to 2 parts in 10^12.
> Thankyou,................................................................................Donald
> Brett Collie
> 
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