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

Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Jan 9 17:34:16 EST 2009


The Efratom FRS may be OK, its hard to say without some MDEV measurements.
The specifications only give ADEV for 1s, 10s and 100s.

One way to find out is to compare 3 of them in a 3 cornered hat arrangement.


Steve Rooke wrote:
> 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

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