[time-nuts] GPSDO "accuracy"

[donald collie]

Thankyou to all who responded. It looks as if 2 parts in 10^12 is about
what can be expected - certainly much better than the 1 part in 10^9
available from  a typical double ovened quartz crystal oscillator barefoot.
Cheers!...............................................................................................................................Donald
Brett Collie

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On Fri, Sep 11, 2020 at 1:15 AM Bob kb8tq <kb8tq at n1k.org> wrote:

> 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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