[time-nuts] Sub 20 ps time detection

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Mar 25 16:43:45 EDT 2017


The LTC6957-4 is inputs are intended to be AC coupled.

With  AC coupling a variable rate pulse will experience varying switching thresholds.

A constant fraction discriminator minimises timing walk when the pulse amplitude varies whilst the ristetime remains constant. 

If the pulse risetime also varies a more complex amplitude and risetime compensated discriminator is necessary to minimise timing walk.

For minimum timing jitter there will be an optimum zero crossing detector bandwidth.


>     On 26 March 2017 at 05:32 Mohammad-Hadi Sohrabi <mhsohrabi at ucdavis.edu> wrote:
>     Hello,
>     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.
>     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.
>     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.
>     Bests,
>     Mohammad
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