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

[Bruce Griffiths]

Magnus Danielson wrote:
> Bruce,
>
>   
>> 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.
>>     
>
> Indeed.
>
>   
>> 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.
>
> Cheers,
> Magnus
>   
Hej Magnus

Of course such enhanced resolution techniques arent restricted purely to 
frequency measurement.
An analogous technique can be used in an integrating DVM where the 
integrator output is sampled periodically during the signal integration.
Even when a "multislope" technique is used during integrator runup, such 
a technique can be used.
In this case a form of sigma delta modulation is used to keep the 
integrator output within bounds and minimise the effect of dielectric 
absorption.
The accumulated quantised feedback is combined with the sampled 
integrator residue to form the equivalent of samples taken from an ideal 
integrator with a very large output range.
One can then use regression techniques to fit a straight line (or other 
curve) to the integrator output.
This will be most effective when the noise of the ADC used to sample the 
integrator residue is the dominant noise source and the input signal is 
relatively quiet with no significant spurious frequency components.

Bruce