[ntp:questions] Dual Mixer Time Difference (DMTD) instruments sought
Joseph Gwinn
joegwinn at comcast.net
Wed May 14 13:55:47 UTC 2008
In article
<c3f10405-7010-4a25-8db2-8c6696632290 at k1g2000prb.googlegroups.com>,
jlevine <jlevine at boulder.nist.gov> 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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