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

Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Jan 9 20:52:32 EST 2009


Warren

Another limitation of such phase detectors is that the 2 frequencies
being compared have to be within a small fraction of 1Hz of one another.
This rules out using a low noise reference that happens to have
drifted/aged out of the adjustment range but which is otherwise OK.

Bruce

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
>
> *************:
>   
Bruce
>>>> What would it take as a minimum for ordinary time-nuts  to be able  
>>>> to perform an ADEV test on their ocxo's and gpsdo's for phase stability at "home", 
>>>>         
>
>   
>> Warren wrote:
>>     
>>> I have noticed that Given enough expertise, anything can be made more complicated than need be.
>>>
>>> For doing noise testing, there is an option to an expensive osc reference, 
>>> that has been pointed out many times before. Its advantages is, that unlike other reference 
>>> standards this one does not have a limit in how low it can measure, and most time-nuts seem 
>>> to already one or more laying around. 
>>> The alternative is to just use another one of the same things you are testing (or ANY thing better).
>>> When comparing two independent noise sources, you get an answer that is the RMS sum of the two. 
>>> That is the answer will be up to 1.414 times the noise of the worse one. It's not too hard to find 
>>> which is the worse one if you need to with a few more test.
>>>
>>>   
>>>       
>> 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.
>>     
>
>   
>>> There are also some simple analog alternatives for measuring Phase noise that do not need high 
>>> resolution Digital TIC, time stamp etc. and can give higher resolution results. 
>>> I use a XOR phase detector, an analog filter and a radio shack multimeter with PC interface capability.
>>>       
>
>   
>> Like all digital phase detectors its best to avoid, if possible, the nonlinearity inherent at the ends of the range.
>>
>>     
>
>   
>>> The ADEV, ODEV and MDEV can then be calculated from the text file data using any of the 
>>> many great downloadable programs that are available .
>>>
>>> The 2G test with a strip chart record of the EFC can be used as a simple way to measure 
>>> the control loop Time constant and see how the control loop responses to an Osc step function error.
>>>
>>> Another interesting and useful effect that can be used if one is careful interrupting the results is the 
>>> fact that common errors will tend to cancel.
>>> If you compare the noise of two different PLL controlled Osc driven by the SAME 1PPS signal, 
>>> you will see Just the effect of the control loops and Osc and NOT the effect of the 1pps GPS noise 
>>> itself. Not what you really want to know when matching an OSC's noise to a GPS signal, but it can 
>>> provide some interesting insights and results about the control loop and Osc.
>>>
>>> I do acknowledge that there are limitations in any of the above and many ways that it can 
>>> be done wrong, But it can provide a Simple usable test, and in some cases near state of the 
>>> art testing, for the beginning time-nut that has not yet collected all the great test equipment 
>>> that is so often referred to.
>>>
>>> WarrenS
>>>       
>
> *****************
>   
>> ----- Original Message ----- 
>> From: "Steve Rooke" <sar10538 at gmail.com>
>> To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
>> Sent: Friday, January 09, 2009 3:18 AM
>> Subject: Re: [time-nuts] ADEV test setup [was GPSDO TC & Damping]
>>
>>
>>   
>>     
>>> Bruce,
>>>
>>> Thanks for the detailed rundown. Looking at the picket-fence method,
>>> this looks possible for me but I will have to get hold of the
>>> reference standard. I have a Racal-Dana 1992 with IEEE488 but need to
>>> get an interface card for the PC end. These are fairly cheap to buy.
>>>
>>> You spoke about some types of rubidium standards being suitable, would
>>> you care to elaborate on that please? Would something like an Efratom
>>> FRS be suitable?  Generating the picket-fence itself should not be
>>> hard as long as care is taken not to introduce noise. Do you have any
>>> links to articles on the design for the
>>> mixer/zero-crossing/square-wave beat circuit? One question, assuming
>>> that I have a 10MHz reference standard and I'm measuring a 10MHz dut,
>>> how do I arrange for them to be about 1Hz apart, given that we are
>>> measuring for accuracy here? 1HZ different would make the accuracy
>>> 1E-7 out anyway, or am I missing something here?
>>>
>>> So the real thing for the budget-conscious time-nut seems to be the
>>> reference standard.  The ocxos you spoke about do seem to be on the
>>> rare/expensive side and are an order of magnitude or two better than
>>> the Option 4E I have in the 1992.
>>>
>>> 73, Steve
>>>
>>> 2009/1/9 Bruce Griffiths <bruce.griffiths at xtra.co.nz>:
>>>     
>>>       
>>>> Addendum:
>>>>
>>>> Timestamping using a conventioanl gated counter is easily accomplished
>>>> using Greenhall's picket fence technique:
>>>> http://horology.jpl.nasa.gov/papers/picket_uffc.pdf
>>>>
>>>> The Acam TDC ICs  (http://www.acam.de) have a resolution of a few tens
>>>> of ps and a range of up to 200ms or so depending on the chip.
>>>> These can easily be interfaced to most micros.
>>>>
>>>>
>>>> Bruce Griffiths wrote:
>>>>       
>>>>         
>>>>> Steve
>>>>>
>>>>> If we take TvB's measurements on a Thunderbolt as some guide as to what
>>>>> to expect:
>>>>> http://www.leapsecond.com/pages/tbolt-tc/
>>>>>
>>>>> Then to make meaningful measurements on a Thunderbolt for example one needs:
>>>>>
>>>>> 1) An independent frequency standard with an MDEV better than 1E-12 or
>>>>> so for  1 s <Tau<1000 s
>>>>>
>>>>> 2) A means of measuring MDEV with a resolution and internal noise <<
>>>>> 1E-12 1s < Tau < 1000 s
>>>>>
>>>>> If one relaxes the Tau range to say 100s < tau < 1000s, then a wider
>>>>> range of techniques that have adequate resolution are available.
>>>>> For most GPSDOs the relevant loop time constant will be somewhere within
>>>>> the (100 - 1000) s range.
>>>>>
>>>>> One point often missed when quoting/plotting MDEV, ADEV measures is the
>>>>> measurement system noise bandwidth.
>>>>> The ADEV and MDEV measures are, in general, dependent on the measurement
>>>>> system noise bandwidth.
>>>>> Different systems with different noise bandwidths measuring the relative
>>>>> ADEV or MDEV of the same pair of OCXOs will produce different results
>>>>> for ADEV, MDEV.
>>>>>
>>>>> Possible measurement systems:
>>>>>
>>>>> 1) Phase comparator directly comparing phases of the 2 (10MHz?) sources.
>>>>> The system can have a well defined noise bandwidth together with
>>>>> adequate resolution if the phase comparator output drives an ADC with a
>>>>> resolution of 12 bits or more ( a sigma delta ADC is perhaps the most
>>>>> suitable). However the frequencies of the 2 sources must match closely
>>>>> and in the case of digital phase detectors the non linearity at the ends
>>>>> of the range should be avoided.
>>>>>
>>>>> 2) Heterodyne system where a low noise offset oscillator is used to mix
>>>>> down to a beat frequency in the audio range.
>>>>> The beat frequency output is low pass filtered and amplified before
>>>>> driving either:
>>>>>
>>>>> A) a sound card  the samples from which are processed to  derive  the
>>>>> phase  of the beat frequency.
>>>>>
>>>>> B) A well designed cascaded amplifier limiter low pass filter system
>>>>> that progressively amplifies the beat frequency signal. The output stage
>>>>> is a linear comparator and line driver which drives a conventional time
>>>>> interval counter with a resolution of 100ns or better. Using the beat
>>>>> frequency output to drive the counter directly results in excessive noise.
>>>>>
>>>>> 3) Dual mixer system with an offset oscillator the performance
>>>>> requirements of which are relaxed somewhat because only the differential
>>>>> phase shift between the 2 beat frequency outputs is of interest.
>>>>>
>>>>> Whilst in principle a high resolution (100ps or better) counter with
>>>>> interpolator could be employed to measure the phase of the divided down
>>>>> output of the UUT with respect to the standard, the system noise
>>>>> bandwidth is large and ill defined unless one resorts to crystal and/or
>>>>> passive RC or LC filters etc with their attendant phase stability problems.
>>>>>
>>>>> Lacking a suitable frequency standard the best you can do is log the
>>>>> phase and frequency errors of the thunderbolt when the OCXO is free
>>>>> running and plot the resultant MDEV.
>>>>> The best value for the loop time constant should be somewhere in the
>>>>> close to the value of Tau corresponding to the location of the minimum
>>>>> value of MDEV.
>>>>> Perhaps TvB can help by making measurements of the free running MDEV of
>>>>> a Thunderbolt as measured by the Thunderbolt itself to check the
>>>>> viability of this method of setting the loop TC.
>>>>>
>>>>> NOTES:
>>>>>
>>>>> 1) Assembling a high resolution timestamping counter with 100ps or so
>>>>> resolution should be reasonably practical.
>>>>>
>>>>> 2) Designing a optimised bandpass slope amplifier limiter cascade is
>>>>> relatively straightforward.
>>>>>
>>>>> 3) Optical or equivalent isolation is critical. Where mixers are used
>>>>> selecting one which allows the IF ports to be isolated at low
>>>>> frequencies is best - Minicircuits have several through-hole models that
>>>>> allow this.
>>>>>
>>>>> 4) The real stumbling block is obtaining a suitable reference.
>>>>> An FTS1200 or an OSA8607 may be suitable, however these are either rare
>>>>> or expensive.
>>>>> Some rubidium standards are also suitable.
>>>>> TvB only appears to have ADEV plots for the LPRO, however since MDEV is
>>>>> somewhat lower than ADEV an LPRO may well be suitable.
>>>>>
>>>>> 5) Using a sound card to timestamp beat frequency zero crossings or an
>>>>> equivalent technique is the most flexible and reliable provided that a
>>>>> high resolution sound card is used.
>>>>> Such a sound card can also be used for phase noise measurements for
>>>>> offset frequencies in the 20Hz to 20kHz range.
>>>>> Some care is required to keep mains related spurs sufficiently low. I
>>>>> have obtained mains related spur levels below 1uV rms by suitably
>>>>> arranging the 6m input cables for a balanced input PCI sound card. Since
>>>>> this sound card has a full scale input of 4Vrms the effect of 1uV spurs
>>>>> is negligible (< 5 fs with 10MHz mixer inputs) for these purposes.
>>>>>
>>>>> 6) A relatively low noise offset source can be assembled from a DDS
>>>>> based system provided that a truncation spur free output frequency is
>>>>> chosen.
>>>>>
>>>>> Bruce
>>>>>
>>>>> Steve Rooke wrote:
>>>>>
>>>>>         
>>>>>           
>> This would enable us (the
>>   
>>     
>>>>>> other half) to see the results of our experiments and tuning of the
>>>>>> gear we have otherwise it is a lot like working blind. I appreciate
>>>>>> that what is normally used is a counter which can continually
>>>>>> timestamp a dut as opposed to a gated counter but what would be the
>>>>>> cheapest way we could achieve this sort of setup?
>>>>>>
>>>>>> Thanks and 73, Steve
>>>>>>
>>>>>>
>>>>>>           
>>>>>>             
>>>>> _______________________________________________
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>>>>> and follow the instructions there.
>>>>>
>>>>>
>>>>>         
>>>>>           
>>>> _______________________________________________
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>>>>       
>>>>         
>>> -- 
>>> Steve Rooke - ZL3TUV & G8KVD & JAKDTTNW
>>> Omnium finis imminet
>>>
>>>
>>>
>>>     
>>>       
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>>
>>   
>>     
>
> Bruce
>
>
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