[time-nuts] DMTD phase shifter

Stephan Sandenbergh stephan at rrsg.ee.uct.ac.za
Fri Jul 24 18:20:15 UTC 2009


Corby,
Thanks - it makes sense. I was suspecting that such a low-frequency phase
shifter would be done using cable lengths. However, I can't say that I've
got much experience with this kind of thing.

I have another question:

The rule of thumb would be that the reference oscillator must be 3x more
stable than the DUT. I haven't got much to measure my standards against, but
I have got 3 identical units. According to literature the separation of
variances method provide a reasonable estimate.

How well does a measurement, based on separation of variances, compare to
one that was done using a sufficiently stable reference?

Regards,

Stephan.



2009/7/24 Corby Dawson <cdelect at juno.com>

> Stephan,
>
> I have a DMTD system that was built by NBS (now NIST) in the early 80s.
>
> The coarse phase shifter is made up of ten lengths of miniature coax
> cable bundled up in a shielded box with toggle switches on the front
> panel to select the delay used.
>
> The LSB is about 2us.
>
> It also has a fine adjust that consists of a couple varactors in a phase
> delay circuit.
>
> I have found that with a good quality offset L.O. I can get the precision
> I need (<1X10-13th at a 1 second interval) without using the fine adjust
> circuit.
>
> As far as the small time interval value I usually start my measurements
> at around 2 to 4 us delay with the DUT set for a slowly increasing phase
> shift,
>
> The accuracy required depends on what your measuring but with a 10Mhz
> input and a 1hz beat note +-1us is equal to +-1X10-13th so most counters
> will work just fine. Just be advised that the us and below digits will be
> jumping around quite a bit!
>
> Also make sure that the delay is adjusted so the counter updates every
> second, you can be updating every two seconds which nullifies the
> cancellation of the noise in the L.O.
>
> Corby Dawson
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