[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.
--
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