The use of reciprocal interpolating counting in this new counter results in excellent relative resolution: 12 digits/s for all frequencies.

The measurement is synchronized with the in­put cycles instead of the timebase. Simulta­neously with the normal "digital" counting, the counter makes analog measurements of the time between the start/stop trigger events and the next following clock pulse. This is done in four identical circuits by charging an integrating capacitor with a constant current, starting at the trigger event. Charging is stopped at the leading edge of the first follow­ing clock pulse. The stored charge in the inte­grating capacitor represents the time differ­ence between the start trigger event and the leading edge of the first following clock pulse. A similar charge integration is made for the stop trigger event.

When the "digital" part of the measurement is ready, the stored charges in the capacitors are measured by means of Analog/Digital Converters.

The counter's microprocessor calculates the result after completing all measurements, i.e. the digital time measurement and the analog interpolation measurements.

The result is that the basic "digital resolution" of + 1 clock pulse (10 ns) is reduced to 100 ps for the CNT-90 and 50 ps for the CNT-91(R).

Since the measurement is synchronized with the input signal, the resolution for frequency measurements is very high and independent of frequency.

CNT-91/91R features gap-free back-to-back frequency measurements, ensuring no missing periods The counters have 14 display digits to ensure that the display itself does not restrict the res­olution.