[time-nuts] Question about frequency counter testing

Oleg Skydan olegskydan at gmail.com
Thu Apr 26 17:28:20 EDT 2018


From: "Hal Murray" <hmurray at megapathdsl.net>
Sent: Thursday, April 26, 2018 10:28 PM

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

I saw different terms like "omega counter" or multiple time-stamp
average counter, probably there are others too.

> 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.

No, it is much simpler. The hardware saves time-stamps to the memory at
each (event) rise of the input signal (let's consider we have digital logic
input signal for simplicity). So after some time we have many pairs of
{event number, time-stamp}. We can plot those pairs with event number on
X-axis and time on Y-axis, now if we fit the line on that dataset the
inverse slope of the line will correspond to the estimated frequency.

The line is fitted using linear regression.

This technique improves frequency uncertainty as

2*sqrt(3)*tresolution/(MeasurementTime * sqrt(NumberOfEvents-2))

So If I have 2.5ns HW time resolution, and collect 5e6 events,
processing should result in 3.9ps resolution.

Of cause this is for the ideal case. The first real life problem is
signal drift for example.

Hope I was able to tell of what I am doing.

BTW, I have fixed a little bug in firmware and now ADEV looks a bit better.
Probably I should look for better OCXOs. Interesting thing - the counter
processed 300GB of time-stamps data during that 8+hour run :).

All the best!
Oleg 
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