[time-nuts] Multiple Time Interval Counters to measure Transients?

Marek Peca marek at duch.cz
Sat Sep 8 19:56:45 UTC 2012


Hello,

> Well, for the CMOS stuff 100ps should do just fine. Of course, less
> is better, but there's only so much one can reasonably do for so many
> channels... Even a PICTIC should be able to do better than 500 ps for
> a single channel. From what I've read, a few hundred units of HP5370
> should solve the problem as well, but then there's only so much space...

Indeed, with such a precision, several dozens of TDCs could easily fit 
into one FPGA chip. The task is to design clever data processing to handle 
all the events.

We may be able to deliver such a custom product, but you have mentioned 
your need to save as much money as possible -- it should have be made more 
precise by means of technical specification to say more about time/price.

> I'd be very much interested. 7ps to me seems to be even more than I'd
> need for the CMOS stuff. Just out of curiosity: Does this work already
> from 0ps on or is the minimum count higher than that? And what's the
> limiting factor to accuracy you experience with your current design? Is
> it actually the FPGA and its properties or something else?

Of course, the 7ps _RMS_ is the jitter of the TDC. The measurement is 
continuous w.r.t. reference clock, i.e. "0ps" is of course possible.

There is (almost) no other component than the FPGA. So, the limit is 
indeed the FPGA itself, you may include its power-supplies and 
comparators, as well.

However, you have been probably asking, how far from the limits imposed 
bhy single FPGA cell's inherent jitter are we -- this is what I exactly 
can not answer with certainty, but according to our indirect measurements, 
best cells of the given FPGA exhibited estimated jitter about 1..3ps, 
whereas our last complete design has overall 7ps RMS, under the assumption 
of asynchronous measured signal. Under deterministic worst case, the 
precision will drop to 21ps abs max. (Note: this last number may be a too 
pesimistic value; stay tuned for newer revision after soldering recent 
pcbs).

Feel free to ask more, if you were interested in testing our device.

Best regards,
Marek



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