[time-nuts] Phase shifter circuit for DTMD

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Jul 25 00:32:51 UTC 2009


Magnus Danielson wrote:
> Stephan Sandenbergh wrote:
>> Hi,
>> Thanks for the replies. I agree. One is interested in the timing of beat
>> notes.
>>
>> But, I'm slightly confused now. It might be a language problem on my
>> side.
>>
>> Quoting Howe, Allan and Barnes, 1981, "...adjust the phase so that
>> the two
>> beat frequencies are nominally in phase; this adjustment sets up a nice
>> condition that the noise of the common oscillator tends to cancel..."
>>
>> I can see a few issues here:
>>
>> 1) if the beat frequencies are in phase, there will be a very small time
>> interval between their zero crossings. This might be difficult to
>> measure
>> with accuracy.
>
> No, it is not a particular issue, unless you have problem with
> isolation. The method to workaround isolation problems that I
> described is just one possible approach out of many, including
> designing it out of being an issue.
>
Most time interval counters have problems with crosstalk if the START
and STOP edges are too close together.
However one can always insert a suitable length of coax (> 10ns or so
delay between the START and STOP inputs should work well with the
5370A/B time interval counters) to delay the STOP signal sufficiently to
avoid this problem whilst maintaining near coincidence at the DMTD
system outputs.
> What they are after is that as coincidence in time makes signals from
> a DTMD perspective correlate better in the regard as supressing the
> transfer oscillator phase stability. If you keep the coincidence time
> to say within 100 us much of the low-frequency noise of the transfer
> osicllator beyond 100 us correlate quite well, so cross-correlation
> will perform well to supress it where as instabilities with shorter
> times will not correlate and thus polute the measurement. As
> time-separation increases, more integrate time-noise of the transfer
> oscillator will be exposed and induced into the measurement results,
> for which only time-averaging will help.
>
JPL papers on their latest DMTD mixer system use a beat frequency of
around 100Hz or so to allow some compensation for the phase noise of the
offset oscillator to be made when the 2 beat frequency zero crossings
aren't coincident.
>> 2) I agree that reference oscillator noise will cancel to some extend
>> because the measurements are done closer to the same time, which
>> makes the
>> reference oscillator noise better correlated between the start and stop
>> edges.
>
> Correct.
>
>> 3) Also, one would like to compare both clkA and clkB at the same
>> time. Not
>> one at t=0 and the other at t=1day to exaggerate a little. I'm not
>> planning
>> to measure atomic standards, or the best OCXO available, so I doubt this
>> will bother me.
>
> The closer together they are, the better cross-correlation gain can be
> achieved. On the other hand, tbe closer the edges is and thus
> cross-correlation losses may also be found.
>
>> Other than the above, I agree: it is better to have a greater phase
>> offset
>> between the beat notes.
>>
>> Does my thinking my sense at all?
>
> Sure thing.
>
> Cheers,
> M'agnus
>
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Bruce




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