[time-nuts] DMTD question

Bruce Griffiths bruce.griffiths at xtra.co.nz
Wed Jun 17 21:19:58 EDT 2009


Magnus Danielson wrote:
> Bruce Griffiths skrev:
>> Magnus Danielson wrote:
>>> Hi Corby,
>>>
>>>> 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.
>>> I agree with Bruce, the delay removes the cross-correlation gain in
>>> the suppression of the transfer oscillator. Compare the oscillators
>>> pair-wise instead.
>>>
>>> Cheers,
>>> Magnus
>>>
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>> Another question that arises is the stability/jitter of the 100ms delay
>> itself.
>> How was this 100ms delay achieved?
>
> Indeed. The NBS DMTD system I know of doesn't have this feature.
>
> Cheers,
> Magnus
>
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Hej Magnus

This could be done by changing the phase difference between the 2 inputs
to the 2 channels by 10% of the mixer input signal period or by about
20ns at 5MHz.
Even so questioning the stability of this delay is reasonable
considering that an instability of 0.3ps or so would be equivalent to
the observed noise at 100ms delay between the zerocrossings of the 2
mixer outputs. A temperature fluctuation of 1C would produce a 2ps
change in phase shift if ordinary coax were used for the delay.

Bruce





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