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

[Magnus Danielson]

Bruce Griffiths skrev:
> 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.

I think it is of interest to measure the 10s - 10 ks 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.

There is another mistake being done regularly is not to compensate for 
frequency drift. ADEV, MDEV and TDEV measures is insensitive to phase 
offset and frequency offset, where as drift goes straigh thru, which is 
even very easy to show by manually insertion of a linear model into the 
respective formulas. On several occasions I have shown that the linear 
drift apparent in peoples measurements is where their naive ADEV 
calculations "floors out" where as a compensated dataset keeps going 
further down. When using a Rubidium as reference for longer taus, such 
considerations needs to be done, but it remains purely a post-processing 
aspect on the TIE data.

> 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.

What I have been pondering over is what story does the time deviations 
of the ThunderBolt itself says. Just recording that over time and do the 
ADEV/MDEV dance would be kind of interesting.

> 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.

I migth add that it may not be apparent from their datasheets. I have 
Minicircuits mixers which is not documented to be isolated, but when 
looking at them closely you discover that they are isolated and that 
their foot-pattern is also very nice. Some of them is open on the PCB 
side so you can actually see exactly how they are wired.

> 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.

Another thing to recall is that there exist sound cards with more than 
two channels, allowing for 3 or 4 channels to be sampled simultaneously, 
allowing for a fairly cheap three-corner hat solution among other 
things. FFT based cross-correlation techniques can be swooped up with a 
few ten lines of wrapping code around FFT libraries such as FFTW for 
arbitrary FFT lengths of choice.

> 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.

It could be added that soundcards having balanced signal input is used 
and should be an assumed level.

> 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.

Another choice for smaller offsets is a standard OCXO. Mixing the output 
signal with an external frequency reference (if not being one of the 
measured signals) allow for separate locking. Actually, any input could 
be used as frequency reference, but if a separate is wanted, it is just 
"one more of the same". The locking loop would be the only actual 
processing that needs to be done in real time, and it is a fairly 
low-intensive processing.

Cheers,
Magnus