[ntp:questions] Dual Mixer Time Difference (DMTD) instruments sought

jlevine jlevine at boulder.nist.gov
Thu May 15 13:52:56 UTC 2008


Hello,

> 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?

   We (the time and frequency division of NBS/NIST) designed and built
a dual-mixer systerm in 1980 (more or less). This same system is the
one
that still runs the atomic clock ensemble in Boulder. You can get the
publications
that describe this instrument from the publications database on our
web site.
Go to tf.nist.gov and click on the publications menu. When the menu
appears,
look for author Glaze. The stuff was published in about 1983 or so.
There were
several papers as I recall with various combinations of the folks who
built the
system and the software drivers for it.
   The system we built was totally analog, but a modern system would
probably
be fully digital. Our system had a resolution of about 0.2 ps and a
stability of
about 3-4 ps. A digital system could do better, mostly because the
temperature
sensitive stuff could be confined to the analog front end whereas we
had to
worry about temperature pretty much everywhere in the system.
However,
the job is not trivial, since even tiny impedance mismatches can
cause
problems at this sub-picosecond resolution. You should watch
especially
for the connectors and the cables. We typically use SMA connectors and
rigid coax. The inputs are buffered with distribution amplifiers with
a reverse
isolation that is as good as we can make it. About -165 db, I think,
although I
have not looked at that recently. (Note that the problems are not
adequate
digital computing power but plain old analog electronics.)
   Even so, we have a detectable sensitivity to temperature at the
level of ps. This noise level tends to be too small to affect the
data  from
cesium standards, but it could be a problem if you were trying to
calibrate
the long-period performance of a device or a transmission system that
had a
small delay, since the residual diurnal temperature sensitivity could
come to
get you. If you are in this business then you need professional help.

Judah Levine
Time and Frequency Division
NIST Boulder




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