[time-nuts] TPLL secret reveled

jimlux jimlux at earthlink.net
Thu Jun 10 13:55:15 UTC 2010

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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