[time-nuts] What is the best counter for a Time Nuts?

Magnus Danielson magnus at rubidium.dyndns.org
Thu Oct 9 13:52:34 UTC 2008


> When measuring the frequency of a signal using a conventional counter,
> increasing the gate time decreases the noise contribution of trigger
> jitter to frequency measurement noise although jitter is invariant with
> gate time. The jitter simply becomes a smaller fraction of the total
> gate time. Similarly the frequency measurement resolution increases with
> increasing gate time although the resolution (measured in picoseconds)
> is invariant with gate time it simply becomes a smaller fraction of the
> total gate time.

True, I was being a bit sloppy there...

> However if one timestamps a few more signal zero crossings within the
> gate time interval than just those and the start and end of the gate
> time lower noise estimators of the frequency are available.


> The HP53132 averages the frequency estimates for a series of identical
> duration overlapped gate times.
> Enrico's paper analyses this case.
> The Australian paper corrects some of the errors in Enrico's analysis
> and hints how the analysis may be extended to cover the case of counters
> that use other resolution enhancement techniques.

Recommended reading!

> Counters like the Pendulum CNT91 in effect time stamp every Nth zero
> crossing of the signal and fit a linear regression line to the sequence
> of time stamps. The frequency is then estimated from the slope of the
> regression line. However the CNT91 has limited memory and processing
> power so that full advantage isnt taken of the resolution offered by
> this technique.
> AFAIK no one has yet done a similar analysis to that done for the
> "triangular" averaging counter (by Enrico and the Australians) for a
> counter that does a regression line fit.

Actually, as I recall it, there was a note in one of those articles
relating to it, but your could figure out that it was rendering about the
same conclusion. It also makes sense.


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