[time-nuts] Sub 20 ps time detection

Attila Kinali attila at kinali.ch
Sat Mar 25 14:49:10 EDT 2017

On Sat, 25 Mar 2017 09:32:02 -0700
Mohammad-Hadi Sohrabi <mhsohrabi at ucdavis.edu> wrote:

> I have been following the posts here for a while and learned alot. I
> thought I could get some help as well.
> I need to detect less than 20 ps between two incoming pulses from a
> detector. pulses are 5ns wide and the rise time and fall time is around 2ns
> each.

Do you mean the pulses are less than 20ps apart or that you need
a measurement precision of better than 20ps?

The latter can be achieved using TDC chips of various kinds.
The former not so much. The only TDC chip I am aware of, is the
new 130nm HEP TDC that CERN is currently verifying (ie they are
in the last stages of the design, before they can go into mass

What is your measurement range and what precision do you need?
What is your requirement for absolute accuracy and do you have
any provisions to calibrate for that?

You write from an .edu address, so I assume you are a researcher.
Do you need measurement equipment or are you building a demonstrator?
If it's the latter, do you need something that can be produced in
quantity or is one-of-a-kind good enough?

> I am planning to use a constant fraction discriminator, simply a fast
> comparator with less than 500 MHz bandwidth for zero crossing detection and
> providing a very sharp edge. Then this can be fed into a TDC such as GPX2
> or anything of this sort. I think these can be fairly easily implemented on
> FR4.

The PCB base material is usually not the limiting factor, unless you
have long traces. Anything below 10cm should be stable enough to
measure down to 1ps. But beware that your electronics might not be
and that the circuit itself can show a high temperature coefficient.

> Recently I stumbled upon LTC6957-4 logic converter from another discussion.
> Can anyone guide me if this can be used for producing a sharp edge from a
> pulse that I mentioned? Datasheet suggests 0.5 ns rise time is possible but
> I think this is for single tone sine wave and not a short pulse.

To precisely masure time, you don't need sharp edges. The edges only have
to be stable. Yes, the LTC6957 can be used as a comparator, but I am
not sure whether that would be the right choice.

			Attila Kinali

You know, the very powerful and the very stupid have one thing in common.
They don't alters their views to fit the facts, they alter the facts to
fit the views, which can be uncomfortable if you happen to be one of the
facts that needs altering.  -- The Doctor

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