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

[Bruce Griffiths]

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
>