[time-nuts] Measuring gpsdo vs itself

[Bob Camp]

Hi

As a practical example - a SR620 will look much better reading it's own reference than it will looking at almost anything else. That said, it's still a good idea to make sure the counter looks good reading it's own reference. If it doesn't look good, then you need to fix something. 

Bob

On Nov 5, 2012, at 5:42 AM, Bill Dailey <docdailey at gmail.com> wrote:

> 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
>> 
>> 
>> ______________________________**_________________
>> time-nuts mailing list -- time-nuts at febo.com
>> To unsubscribe, go to https://www.febo.com/cgi-bin/**
>> mailman/listinfo/time-nuts<https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts>
>> and follow the instructions there.
>> 
> 
> 
> 
> -- 
> Doc
> 
> Bill Dailey
> KXØO
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.