[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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