Keysight DAQ970A Guia Del Usuario página 248

4 Measurement Tutorials
Frequency and period measurements
The internal DMM uses a reciprocal counting technique to measure frequency and period. This method generates
constant measurement resolution for any input frequency. The internal DMM's AC voltage measurement section
performs input signal conditioning for frequency and period measurements.
The timebase is divided to provide a gate signal. The gate signal and input signal are combined to enable the
counter. During the "on" time, the counter counts the 6 MHz timebase signal. At the end of each gate period, the
total count is latched and the result is divided by the known timebase frequency to determine the input frequency.
The counter is then reset before the next gate period. The resolution of the measurement is tied to the timebase, and
not the input frequency. This increases measurement speed especially at low frequencies.
The reciprocal counter has the advantage of a constant number of digits of display, no matter what the input
frequency is. With a reciprocal counter, the number of digits of resolution scales with the gate time. If a 1-second
gate gives six digits of resolution, a 0.1-second gate gives five digits, and so forth.
248 Keysight DAQ970A/DAQ973A User's Guide