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" <[email protected]> >> To: "Discussion of precise time and frequency measurement" >> <[email protected]> >> 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 <[email protected]>: >>> >>> >>>> 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 >>>>>> >>>>>> >>>>>> >>>>>> >>>>> _______________________________________________ >>>>> 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. >>>> >>>> >>>> >>> -- >>> Steve Rooke - ZL3TUV & G8KVD & JAKDTTNW >>> Omnium finis imminet >>> >>> >>> >>> >>> >> _______________________________________________ >> 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. >> >> >> > > Bruce > > > _______________________________________________ > 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.
