[time-nuts] Some results of PRS10 and Trimble Resolution

Tom Van Baak tvb at leapsecond.com
Thu Jun 29 02:01:42 EDT 2006

> would you have a schematic of an interpolator with say <5ns resolution  to
> share?


On interpolators:

1) Another good example is found in the op/svc manual
for the SR620 -- which implements (in simple, non-ASIC,
1980's TTL/ECL technology) time interpolators that are
good to 25 ps. Maybe overkill for you, but the design is
worth understanding.

2) I recall this recent paper ($) also made good reading:
    Review of methods for time interval measurements with picosecond

3) Google for words like time interval interpolator and you
will find quite a lot of sub-ns TIC ideas and products. Few
seem to be cheap or available, though.

4) The wonderful US patent database also has design
info on time interpolators (which is good for us home
time-nuts but perhaps not so good for you if you plan to
put any of this into your commercial GPSDO product).

Since you're asking about time interval counters in
relation to your GPSDO design, I want to tempt you
to do some math first:

1) You know, or can measure, the ADEV of your GPS
engine (M12M, or Res-T, etc.).

2) Same goes for the OCXO you have chosen.

3) Decide what GPSDO PLL time constant or PID
parameters to use.

4) And then calculate what minimum TIC resolution is
necessary for you to have to meet your product spec.

For any tau, what limits GPSDO performance is a
combination of GPS, OCXO, PLL, and TIC.

You may in fact find your current 3.33 ns guess is fine.
Or maybe you'll find that it's not near good enough and
you are leaving good performance on the table. Let us
know what you come up with. Perhaps it will shed light
on why the classic HP GPSDO used a ~100 ps TIC.


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