[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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