[time-nuts] Experience with THS788 from TI?

Ben Gamari bgamari at physics.umass.edu
Wed Mar 21 21:58:29 UTC 2012


Bob Camp <lists at rtty.us> writes:

> Hi
>
> Ok, for a legit 12 ps with 0.1 ps drift and 200 mega samples per second -
> not to many alternatives. The FPGA stuff will get you to 50 to 100 ps on the
> same basis this gets you to 12 ps. They will get you to 20 to 40 ps on a
> good day - sort of the way this chip gets 8 ps. The FPGA will do it at a
> much lower data rate. 
>
In our experiments, we are typically observing very low count rates
(100kHz at absolute most). I've occassionally stumbled upon a paper
which claims to get 10ps on a standard FPGA, but naturally they never
show the code. Given that I'm a relative novice at high-speed
electronics and FPGAing in general, I'll consider myself lucky if I get
the 50ps advertised by the CERN core.

In particular, one issue I've been struggling with is the
discriminator. Our fast detectors produce a NIM negative-current pulse
which will ultimately need to become suitable input for the FPGA. Of
course, the most precise time measurement in the world is useless if the
discriminator front-end has a nanosecond jitter. Unfortunately, I have
yet to find any open, high precision discriminator designs. In principle
a constant fraction discriminator doesn't seem to difficult to
implement, but when it comes to preserving the high-speed signal
integrity, it seems like it could get pretty hairy. Comments?

> If you average over many samples, all of these will get you a better
> estimate. How much better depends on a bunch of things. The TI part *could*
> do very well if you have a 200 MHz signal to look at.
>
For time-correlated single photon counting (our primary use for
precision timing), having high temporal resolution is quite
important. That being said, all of those arrival times all get combined
into a correlation function so shot-per-shot jitter will be in large
part averaged out.

Cheers,

- Ben





More information about the Time-nuts_lists.febo.com mailing list