[ntp:questions] Dual Mixer Time Difference (DMTD) instruments sought

Joseph Gwinn joegwinn at comcast.net
Wed May 14 03:33:50 UTC 2008


In article <48298A4D.7090401 at febo.com>,
 jra at febo.com (John Ackermann N8UR) wrote:

> Joseph Gwinn said the following on 05/12/2008 10:38 PM:
> 
> >> What DMTD instruments are commercially available?  A google search was 
> >> not successful - all noise no detectable signal, probably because DMTD 
> >> instruments are not that common, and many people build their own.
> > 
> > The silence, the silence.  I have not found too many commercial DMTF 
> > units, but I have found one, although the maker does not market it a 
> > such:
> > 
> > The Symmetricom 5120 
> > <http://www.symmttm.com/products_pn_adev_test_sets_5120A.asp> is at 
> > heart a digital DMTD instrument, and will make all the usual DMTD 
> > measurements, although it is marketed primarily as a phase noise test 
> > set.
> > 
> > What else is available?  
> 
> The 5120A is truly a wonderful box, but it's also not cheap (about
> $30K).  It's fully DSP based so all the interesting stuff is done in
> software.  One huge advantage is that the reference and
> device-under-test do not have to be at the same frequency.  There's an
> older version, the 5110A, that has been discontinued but should sell
> used for less than $10K if you can find one.  It's more of a pure DMTD
> box and doesn't do phase noise in a useful way.

The 5110A is analog, I think, although I never did get a users guide.


> I don't know of other commercially marketed products that provide a DMTD
> function.  However, there's been quite a bit of discussion about this
> over on the time-nuts list, and that's probably a better place for your
> question (https://www.febo.com/mailman/listinfo/time-nuts).

I joined, but will lurk for now.

 
> The single most critical piece of a DMTD system is the zero crossing
> detector.  Unless you have a way to increase the slew rate of the low
> frequency beat note by a million or so, trigger jitter in the counter
> will eat up almost all the advantages of the down-mix.  Again, there's
> been some discussion about this on time-nuts, and there are some folks
> there working on designing and building bits of the hardware (at least,
> a couple of months ago there was a fair bit of discussion on the point).

Yes.  And don't forget ground loops.  Noise at 1 Hz is very difficult to 
shield.

I bet one big advantage of the DSP approach is that math is cleaner than 
practical analog hardware.

Joe Gwinn




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