[time-nuts] TPLL secret reveled

[Ulrich Bangert]

Jim,

pardon if I correct you: The reciprocal counters were an intermediate thing
between the counting only and the subclock interpolating ones. A reciprocal
counter would notice when a frequency measurement would be too imprecise due
to an arkward relationship between that gate time and the frequency to be
measured.

With only a few zero crossings within the gate time a reciprocal counter
would use the frequency to be measured as the source for its gate time and
measure the frequency of its reference with that. The result is then back
computed to the frequency to be measured.

This principle has nothing to do with sub clock interpolation. Nevertheless
it is true that once that the reciprocal principle has been introduced it
has been used in all following technologies.

Best regards
Ulrich Bangert

> -----Ursprungliche Nachricht-----
> Von: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von jimlux
> Gesendet: Donnerstag, 10. Juni 2010 15:55
> An: Discussion of precise time and frequency measurement
> Betreff: Re: [time-nuts] TPLL secret reveled
> 
> 
> Ulrich Bangert wrote:
> > 
> > 
> 
> > 
> > The next improvement to the old fashioned pure counter was the 
> > invention of subclock interpolation schemes. A counter using this 
> > works so: After the beginning of the gate time it waits of the next 
> > zero crossing and then measures the time up to the last 
> zero crossing 
> > within the gate time with a fixed resolution of say 1 ns (like the 
> > well known Racal Dana 1992/1996/1998). The frequency value 
> is then the 
> > result of a computation. If you consider this working principle you 
> > notice that this is even more a phase meter like thing than the 
> > original counter only thing. For that reason frequency measurements 
> > with a counter like that are suited as well for ADEV calculation.
> > 
> 
> 
> I've always referred to these style counters as "reciprocal" 
> counters.. 
> (because the frequency is calculated as the reciprocal of the 
> length of 
> N periods of the input signal).  They've been around at least 
> since the 
> 80s, especially for applications where you need short gate time, but 
> measurement precision greater than 1/gate time. It was very 
> popular for 
> applications like intercept receivers in the signals 
> intelligence area 
> before straight digital processing (ADC and FFT) was practical.
> 
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