[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



More information about the questions mailing list