[time-nuts] Question about frequency counter testing

[Bob kb8tq]

Hi

The degree to which your samples converge to a specific value while being averaged
is dependent on a bunch of things. The noise processes on the clock and the measured
signal are pretty hard to avoid. It is *very* easy to over estimate how fast things converge.

Bob

> On Apr 26, 2018, at 5:28 PM, Oleg Skydan <olegskydan at gmail.com> wrote:
> 
> 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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