[time-nuts] DMTD Question

[Brian Kirby]

DMTF is used to "see" time interval info in the very "near" term, say 
the 0.01 to 10 second range, with high accuracy and more measurement 
complexity.

Most folks use a time interval counter and compare 1 PPS signals between 
standards and references.  The problem with this method is limited by 
the gating of the counter and the counters time base.  A HP/Agilent 
53131A time interval counter is in the 500 picosecond one shot class, so 
it will resolve to 5x10-10 in one second.  An older HP5370B is rated at 
20 pS one shot or 2x10-11.  This is why a lot of counters will not be 
accurate until they hit 100 second or so averages. 

You can measure your own counters baseline line by feeding the same 1PPS 
signal to the input and record and analyze its data.  I would advise it 
to know the limitations of your measurements, as it sets the noise floor 
for your system.

Brian - KD4FM

John Ackermann N8UR wrote:
> 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
> ----
>
> 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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