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: > >> 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 >> >> > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
