[time-nuts] Measuring gpsdo vs itself

[Bill Dailey]

Thanks guys,

Like usual more complicated than I thought.   I was hoping that this would
cancel any stability issues common to both the reference and the signal
thus giving me best case ability.  I seem to be getting numbers too good to
be true so there must be a hitch.  I get an ADEV 5x10-13 at 1 s mostly
linear to 7x10-16 at 10,000 s with a small hump at 20s-80s.  Figured there
was some kind of gotcha.

Doc


On Mon, Nov 5, 2012 at 2:26 AM, Magnus Danielson <magnus at rubidium.dyndns.org
> wrote:

> On 11/05/2012 06:30 AM, Tom Van Baak wrote:
>
>> Ah, I see what you mean now. Yes, that setup can give you a rough
>> estimate of the counter's noise floor.
>>
>> I can't give you specific numbers but one danger with this sort of test
>> is that the input and the timebase are artificially locked together (i.e.
>> fixed phase relationship) through the common reference. Your measurements
>> may thus show artificially less noise than a real-life case of independent
>> input(s) and reference.
>>
>> This can happen if your sub-ns counter is based on interpolators. Because
>> the input and the timebase are locked in phase, the counter lands near the
>> same point of the interpolator scale on every single measurement, rather
>> than experiencing the noise (and non-linearity) of the entire scale.
>>
>
> It's a little more complex than interpolator non-linearities alone. You
> also need to include cross-talk between the signals. This cross-talk is
> usually higher between A and B inputs than from reference, but never the
> less.
>
> You would need to sweep the trigger input delays to illustrate these
> non-linearities. From a single measurement you can get both a better or
> worse number compared to the average which is what you would expect to see
> for free-running signals.
>
> So, you can get a rough idea about the baseline, but it is not a
> sufficient method.
>
> See the SR620 manual for a plot of non-linearities.
>
> Cheers,
> Magnus
>
>
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-- 
Doc

Bill Dailey
KXØO