In article <[EMAIL PROTECTED]>, [EMAIL PROTECTED] (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 _______________________________________________ questions mailing list [email protected] https://lists.ntp.org/mailman/listinfo/questions
