[time-nuts] ADEV test setup [was GPSDO TC & Damping]
bruce.griffiths at xtra.co.nz
Fri Jan 9 10:24:06 UTC 2009
Timestamping using a conventioanl gated counter is easily accomplished
using Greenhall's picket fence technique:
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:
> If we take TvB's measurements on a Thunderbolt as some guide as to what
> to expect:
> 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.
> 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
> Steve Rooke wrote:
>> Maybe this should be the subject of a separate thread but to enable
>> ordinary time-nuts to be able to test their ocxo's and gpsdo's for
>> phase stability at "home", what would it take as a minimum to be able
>> to perform something like an ADEV test? 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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