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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