DMTD = Dual Mixer Time Difference
It's a system for improving resolution of frequency stability
measurement. You use a common low noise oscillator to separately mix
both the device under test and the reference source down to a low
frequency (usually between 1 and 100 Hz), then use a time interval
counter to measure the phase of the two outputs. You get an improvement
of resolution equal to the scaling factor, so a counter with nanosecond
resolution can deliver femtosecond results using this method.
The main challenge becomes noise in the zero crossing detector system
that is used to trigger the scope off the low frequency signals. And,
as has been pointed out, the tempco issues that become a big factor when
you're talking about resolution at these levels.
John
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Bill Hawkins said the following on 01/25/2010 04:24 PM:
I'm not learning anything from this thread because I don't know the
purpose of DMTD or how it would produce a beat note for a phase error.
Now, DTMF I understand.
Bill Hawkins
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