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. 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. Bob -----Original Message----- From: Ben Gamari [mailto:bgamari@gmail.com] Sent: Wednesday, March 21, 2012 3:30 PM To: Bob Camp; 'Discussion of precise time and frequency measurement' Subject: Re: [time-nuts] Experience with THS788 from TI? Bob Camp <lists@rtty.us> writes: > Hi > > Are you after 12 ns or 12 ps? > Bah, yes, my bad: picoseconds is the relevant timescale here. Cheers, - Ben

Bob Camp <lists@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

Hi, The NIM Negative pulse is indeed problematic when trying to interface to FPGA logic! The solution I have used in the past is a board based around a Maxim MAX9601[1] acting as a comparator on the incoming NIIM pulse, and configuring the output for connection to the FPGA. Judging by the results that I have seen from this chip, the timing uncertainty is certainly not single figure picoseconds, but in the range of 30ps - not great, but still better then nanoseconds. If the input to this signal is a NIM timing pulse, there are minimal issues with timing walk as the pulse height is always the same. It's certainly a lot easier then building a CFD! Tristan [1]http://www.maxim-ic.com/datasheet/index.mvp/id/3400 On 22 March 2012 08:58, Ben Gamari <bgamari@physics.umass.edu> wrote: > Bob Camp <lists@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 > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. >