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

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Jan 10 05:32:30 UTC 2009


Steve

Steve Rooke wrote:
> 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?
>
>   
The best article I've come across on zero-crossing detector design is:

The Design of Low Jitter Hard Limiters" Oliver Collins, IEEE
transactions on Communications, Vol 44 No 5, May 1996 pp 601-608

Unfortunately its not free, however you may be able to access it via a
Library.

However if you only want to use the technique described in the paper, I
have a couple of spreadsheets that calculate the stage gains and low
pass filter time constants both for the simplified analysis in the paper
and the more general case where the input noise spectral density differs
for each stage.
Some pointers on what to include in the noise calculations for each
stage can be found at:

http://www.ko4bb.com/~bruce/ZeroCrossingDetectors.html
<http://www.ko4bb.com/%7Ebruce/ZeroCrossingDetectors.html>

Some care is required, in that if the spreadsheet predicts a gain of
less than unity for the input stage, it is in fact better to use a
passive RC low pass filter in front of the first amplifier limiter stage
(without a clamp as typically the IF signal amplitude at the mixer
output is insufficient to cause the clamp diodes to conduct - more
complex clamps are too noisy).
The amplifier limiter chain is then redesigned to accommodate this change.

Don't be taken in by those who would insist that everything should be
linear as long as possible, the resultant deign is suboptimal.
Such comments sprang from the fact that no one at that time had worked
out how to include the effect of the clamps on the performance.
Oliver Collins solved that problem, so there is no longer a valid excuse
for such misguided recommendations.

> 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
>   
Bruce



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