New frequency counting principle improves resolution

Frequency counters have gone through several evolution stages in their design since their first appearance on the market: Stage 1 until 70-ies Conventional counting Stage 2 1980-ies Reciprocal counting (period measurement + inversion) Stage 3 1990-ies Interpolating Reciprocal Counting Stage 4 2000-ies Multiple Time Stamp Average Continuous Counting. This paper describes the theory and design of frequency counters, and analyses the improvements in the latest generation of frequency counters. The newly introduced high-resolution CNT-90 Timer/Counter/Analyzer from Pendulum Instruments AB in Sweden is used as commercially available example of the latest design technology in this presentation. The advantages of continuous time-stamping technique are discussed, for example regression analysis to reduce effects of measurement noise and to improve resolution to 12 digits for 1s of measuring time. Another example is the ability for seamless back-to-back measurements without missing any period, which is essential for theoretically correct calculation of Allan Deviation.