[time-nuts] GPSDO "accuracy"

[Bob kb8tq]

Hi

Well …. 

A reasonably good DOCXO will come in around 2x10^-10 over 0 to 70C. Over a 
typical 4 C “lab wander” you should see < 1x10^-11. After being on power for a month
or three, aging should be below 1x10^-11 per day. 

ADEV wise, it should do better than the GPSDO …..parts in 10^-13 are not unusual at
10 seconds on a typical design. 

Bob

> On Sep 11, 2020, at 3:35 AM, donald collie <donaldbcollie at gmail.com> wrote:
> 
> 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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