[time-nuts] Question about frequency counter testing

Hal Murray hmurray at megapathdsl.net
Thu Apr 26 15:28:05 EDT 2018


olegskydan at gmail.com said:
> The plots I showed were made with approx. 5*10^6 timestamps  per second, so
> theoretically I should get approx. 4ps equivalent  resolution (or 11+
> significant digits in one second). 

Is there a term for what I think you are doing?

If I understand (big if), you are doing the digital version of magic 
down-conversion with an A/D.  I can't even think of the name for that.

If I have a bunch of digital samples and count the transitions I can conpute 
a frequency.  But I would get the same results if the input frequency was X 
plus the sampling frequency.  Or 2X.  ...  The digital stream is the beat 
between the input and the sampling frequency.

That technique depends on having a low jitter clock.  There should be some 
good math in there, but I don't see it.

A related trick is getting the time from something that ticks slowly, like 
the RTC/CMOS clocks on PCs.   They only tick once per second, but you can get 
the time with (much) higher resolution if you poll until it ticks.

Don't forget about metastability.


-- 
These are my opinions.  I hate spam.






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