[time-nuts] Time interval counters
hmurray at suespammers.org
Tue Sep 26 02:57:53 EDT 2006
> Suitable TDC chips are available from http://www.acam.de/ <http://
> www.acam.de/> In particular the TDC-GP1 has a resolution of better
> than 250picosec with a range of 200 millisec, which should be more
> than adequate for measuring the time delay between GPS and local
> standard derived PPS pulses.
> Although these chips are a little pricey at around the 100 euro mark
> they are a lot cheaper than an Agilent/HP 53131 or its equivalent.
Has anybody tried kludging something together without using expensive parts?
I'm guessing that the general idea is something like an R/C feeding an ADC.
Hold the R/C reset until time1, then let it start charging up. At time2,
shut the door on a sample/hold that's probably built into the front end of an
ADC. Then wait until things calm down and read the ADC.
If I start with a 10 MHz clock, that's 100 ns. An 8 bit ADC gives a span of
2.5 clocks for a resolution of 1 ns. Lots of handwaving here.
A 1 ns resolution needs an ADC or S/H with an input bandwidth in the ballpark
of a GHz. My quick scan for a cheap/fast ADC didn't find anything
interesting. So this is probably a wild goose chase.
I wonder if it is reasonable to calibrate a slower ADC. The input signal
will be stable.
Digikey has the TI TLC5540 at under $6. It says 75 MHz input bandwidth but
30 ps aperature jitter. (That's just the first one I looked at. There are
probably better ones.)
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