[time-nuts] DMTD question

Bruce Griffiths bruce.griffiths at xtra.co.nz
Wed Jun 17 18:45:37 EDT 2009


Corby Dawson wrote:
> Hi!
>
> I have been running a bunch of oscillator plots using a DMTD unit built
> for the NBS in the early 80s that I picked up last year and repaired.
>
> I am using an offset oscillator with a 1Hz offset (an FTS1200) for the
> L.O. and select the 1Hz BW on the unit.
>
> I have fed both inputs from the following oscillators thru a power
> splitter to evaluate the baseline.
>
> The Allen Deviation at 1 second for three different oscillators driving
> both inputs at the same levels and having the delay set to the same small
> value are.
> Delay  100us
> Osc 1 - 1.05E-13th
> Osc 2 - 9.50E-14th
> Osc 3 - 2.00E-13th
>
> At larger delay values the Allen Deviation at 1 second for Osc 3 rises
> much faster than the other two.
>
> Delay 100ms
> Osc 1 - 3.10E-13th
> Osc 2 - 3.50E-13th
> Osc 3 - 3.00E-12th
>
> Can I deduce any useful information  from these numbers? 
>
> Obvoisly Osc 3 is not as good as the other two at the 1 second tau!
>
> Plots run against other oscillators also show that Osc 3 causes a rise in
> the value at 1 second.
>
> Thanks,
>
> Corby Dawson
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>   
Corby

The noise contribution of the offset oscillator is incompletely canceled
when the delay between the zero crossings for both channels isn't zero.
The effect increases with the delay between zero crossings as your tests
demonstrate.
You can deduce little from the test apart from the fact that Osc 3 has
worse performance than that of the other 2 oscillators.

Bruce





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