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 _______________________________________________ questions mailing list [email protected] https://lists.ntp.org/mailman/listinfo/questions
