[time-nuts] DMTD phase shifter

[Corby Dawson]

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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