[time-nuts] ADEV test setup [was GPSDO TC & Damping]

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Jan 10 16:35:34 EST 2009

Bill Janssen wrote:
> WarrenS wrote:
>>> Bruce said:
>>> The critical requirement is that the 2 standards being compared are statistically independent.
>>> Comparing a pair of Thunderbolts GPSDOs with similar time constants and
>>> damping will give optimistic results for Tau comparable with or greater than the loop time constant.
>>> Its is even better is to use 3 or more similar standards simultaneously
>>> logging phase differences between the various pairs (0.5*N(N-1) pairs for N standards).
>>> It is then possible to obtain estimates for ADEV, MDEV etc for each standard.
>> The optimistic results at and above the loop time constant, that results even when 3 or more units are used, 
>> is because the noise is then mostly due to the GPS signal itself and NOT the local oscillators in the GPSDO.
>> In effect you are then using the same 1PPS signal into each unit, and any common noise on the 
>> GPS 1PPS signal will cancel and not be seen.
>> So I think what Bruce is saying is that you can not (or is it should not?) use the GPS signal to 
>> measure the GPS's noise.
>> But the point is, if you want to compare your GPSDO with different settings, or compare it to 
>> another OCXO, It can be done this way, if you do not have a better ref to use.
>> You could then add the noise of the GPS nose above the control loop time to your 
>> optimistic results if you want true results at high Tau values.
>> Also note that having the GPS noise cancle is not necessary a bad thing,  It can be a good thing 
>> especially if the GPS noise is not what it is that you want to measure. 
>>> Like all digital phase detectors its best to avoid, if possible, the nonlinearity inherent at the ends of the range.
>> Using a phase detector near its end point (or at its crossover point if there is any deadband) 
>> is something that needs to be avoided. 
>> The two basic standard ways to insure that just the center of the phase detector's range is use:
>> 1) Divide the signals down just enough before sending them to the phase detector so that 
>> the end points is not an issue.  This works when both signals are from devices that are 
>> locked to a common signal such as the GPS.
>> 2) When one of signals is from a non locked source such as a OCXO whose phase can drift 
>> any amount overtime, One of ways to limit phase detector issues, and use just the very accurate zero phase point, is to use the Phase detector's output to lock the OCXO in a fast control loop and then by knowing the gain of the EFC input, the filtered EFC voltage can be use as freq drift information to find the ADEV's.
>> WarrenS
>> *************:
> What I am doing to ovoid the "end of range" problem is;
> First I divide the signal by two to get a 50% duty cycle. Then when the 
> phase difference gets to
> 10% or 90% of the full scale value I switch the phase detector (or 
> counter) to respond to
> to the opposite zero crossing. I keep track of those switches in 
> software. I use a computer to control things and to keep a log of the 
> phase difference.
> Bill K7NOM

At this level of precision the waveform duty cycles are never precisely
50% so some correction for this also needs to be made.


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