[time-nuts] Experience with THS788 from TI?
SAIDJACK at aol.com
SAIDJACK at aol.com
Wed Mar 21 22:52:33 UTC 2012
Two suggestions:
How about using an off-the-shelf Wavecrest counter?
These have a 0.8ps resolution, and typically have a noise floor of around
3ps when averaging. They certainly can do 10ps single shot. There is a
DTS-2050 on Ebay now for $700: item number 120647180882
You can't get much lower than that for the resolution and accuracy that
system provides in a working off-the-shelf solution.
Alternatively, if you want to design your own, you could use a
time-expander. It only requires a small micro with counter/timer, and a little bit of
external circuitry for charging/discharging a precision cap. You charge
fast gated by the signal to measure, then you discharge slowly (expanded time)
and measure the amount of charge deposited on the cap. The Linear Appnote
I mentioned earlier already has most of the capacitor charge pump circuitry
in it that you would need for this.
The idea is to design the cap discharge and charge cycles at different
time scales, say 1000x to 1, so that the capture time get's expanded out to
intervals that the micro can measure. If the micro has say 60MHz counter
resolution (16.66ns) then a 1000x to 1 expansion would allow a 0.016ns (16ps)
resolution. Using say 2000x expansion and a 100MHz counter in the micro
would get you to 5ps resolution.
This setup works very well without having to buy a $170 TI chip and
designing with an FPGA, and only requires a little bit of software in the micro,
and a small number of analog components. I think the operating principle
and circuitry is explained in great detail in the service manuals for the HP
5334A counters, and the PRS-10 rubidium service manual as well.
bye,
Said
In a message dated 3/21/2012 14:58:57 Pacific Daylight Time,
bgamari at physics.umass.edu writes:
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
mailing list