[time-nuts] Time interval counters

Hal Murray 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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