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

Steve Rooke sar10538 at gmail.com
Sat Jan 10 04:46:38 EST 2009


Bruce, thanks, I'm soaking it all up.

73, Steve

2009/1/10 Bruce Griffiths <bruce.griffiths at xtra.co.nz>:
> 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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-- 
Steve Rooke - ZL3TUV & G8KVD & JAKDTTNW
Omnium finis imminet




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