[ntp:questions] Dual Mixer Time Difference (DMTD) instruments sought
John Ackermann N8UR
jra at febo.com
Tue May 13 12:32:13 UTC 2008
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.
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).
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).
John
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