In article <[EMAIL PROTECTED]>, jlevine <[EMAIL PROTECTED]> wrote:
> > I may need a Dual Mixer Time Difference (DMTD) instrument, to measure > > picosecond changes in electrical length in a coax plus amplifier time > > reference signal distribution system with total delays in the hundreds > > of nanoseconds, currently operating at 10 MHz (sinewave), but with 100 > > MHz likely at some future date. > > > > 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. > > We use dual-mixer systems in our primary time scale and also to > calibrate and evaluate oscillators and timing hardware. So far as I > know, the only units that are commercially available are made by Timing > Solutions, which was recently acquired by Symmetricom. There > are a number of different configurations, depending how how many > devices you want to measure, whether they all run at the same > frequency, etc. That's been what I'm finding, and now this is being confirmed. I don't know why Symmetricom keeps the 5120 under their hat. It's really a strange story - the only way to find out that the 5120 is a DMTD instrument (done up in all-digital DSP form) was by knowing that TSC used to make an analog DMTD instrument, and following TSC's (and specifically Dr Stein's) trail in the literature. > It is possible to build these devices on your own, but it is not > trivial to get pico-second resolution and stability. Almost everything > is temperature sensitive at this level of resolution. I think such instruments are also sensitive to user mood. While it's unlikely that I will soon get to build such an instrument, I am quite interested in how they are built, if only to understand what can happen and why. Can you suggest some articles and/or books and/or patents delving into both the theory and the practicalities of building DMTD instruments? Thanks, Joe Gwinn
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