[time-nuts] GPSDO "accuracy"

[Bob kb8tq]

Hi

Depending on your definition of “accuracy”, and the actual device, the answer is likely
in the 1 ppb to 10 ppt range based on a 10 second measurement. 

What’s going on?

First up you have the basic accuracy of GPS. Over many days of averaging (with a fairly
fancy averaging device) that can be quite good ( << 1 ppt). Most GPSDO’s run averaging 
in the (maybe) 100 second range. That gets you into a whole different arena. ( = long term
accuracy is not the issue).

Next, you have noise. GPS at 1 second likely has several ppb of noise (as measured by ADEV). 
At 10 seconds you should be below one ppb, but maybe not by a lot. 

Your local reference also has noise as does the loop. Is the loop + reference 1 ppb or 1 ppt at 
one second? (again measuring with ADEV). It depends a *lot* on just what you have. A good guess
is 10 ppt. That may also apply at 10 or even at 30 seconds. 

Now comes the question of your “accuracy” definition. ADEV is a “one sigma, delta between readings”
measure. If you are after absolute frequency, this is not what ADEV is measuring. If you are after a
six sigma “frequency accuracy” number, you might well need to multiply the ADEV by 10 or 100X. 

Often GPSDO’s that are intended as frequency references are spec’d out as “better than X 99% of
the time ( typical ).” You have a sort of three sigma, with the qualifier that it is typical. ( So not all the 
time … hmmm …..). 

So what are you trying to do? What sort of “accuracy” do you need? What are you looking for?
Is ADEV an adequate measure? 

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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