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

Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Jan 9 22:47:50 EST 2009


Warren

Yes its a real problem several of us have such OCXOs.
They are typically a few Hz off.
If they can be shown to have low noise and low drift they are worth keeping.
Its a pity to discard such OCXOs just because ones measurement system
cant verify their performance.

The only way to know if such an OCXO is still good enough is to compare
it with a better standard or with a pair of low noise OCXOs using a 3
cornered hat technique.
Unfortunately one can't easily use 3 digital phase detectors plus ADCs
for this unless the associated low pass filter cutoff frequency is
increased.
One would then also need to use something like a sigma delta ADC rather
than a cheap DVM to ensure that the low pass filter output sampling rate
is high enough.
An ADC with an input multiplexer could also be used (several
microprocessor have such ADCs built in).

Such a setup is not that much simpler than using an array of timestamp
counters based on a suitable microprocessor.
A dual mixer scheme would require 6 timestamp counters or 6 simultaneous
sound card inputs.

In principle one could use 3 dividers setup so that the output
transitions of each divider are well separated from the output
transitions of the other dividers.
A standard time interval counter with an external input multiplexer
could then be used in a variation of the picket fence technique to
sequentially time stamp the divider output transitions.


Bruce

WarrenS wrote:
> Bruce
>
> True, but not a problem for me, because I don't have any like that.
> I know you have brought this up many times. Is it a real problem that 
> exist enough to even think about? What kind of Osc are you referring to
> that is worth keeping even though it can not be adjusted to be on freq. 
> How far off is it? 
> How do you know it is good enough?
> Trick question, why not just continue to use what it is that you tested it with? 
> Maybe could use the dual 90 deg phase detector system, 
> so there is at least one phase signal near the center of the range at all times.
> There is probable some simpler solution also.
>
>
> WarrenS
>
> *****************
>
> 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
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