[time-nuts] Comparing PPS from 2 GPS units

Gerhard Hoffmann dk4xp at arcor.de
Mon Dec 17 23:27:03 UTC 2012

Am 17.12.2012 19:56, schrieb David:

> My next pretrigger generator is going the differential comparator or
> differential ECL route with a fast ramp and precision reset.  I expect
> jitter to be significantly better than 10s of picoseconds for delays
> up to about 100 nanoseconds.  If I get down to 10 picoseconds of
> jitter, I will be happy since I have no real way to measure much below
> that.

That should be easy to do. I did a trigger pulse & delay for a
54750A scope. You need abt. 25 nsec delay to see the trigger event.
I did it with ooold 10K ECL I still had around and a few meters of
semi rigid cable. The semi rigid is surprisingly compact when wound
on a piece of plastic tube.

There was not much difference when watching the output of an
ADCMP580 comparator, via trigger delay or via internal trigger
and waiting a few clocks with the built-in delay.

The ADCMP580 is a fine chip, btw. You get a lot of speed for
not too much money. (digikey)

I will rebuild the trigger delay over xmas, this time with
CML to remove most of a systematic error in my scope setup.

CML is much more friendly than ECL/PECL from a probing
point of view. No need to carry Vtt to the scope and have
a problematic bias tee there, or shifting the supplies
until all happens to fit...

regards, Gerhard

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