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 meas-
urement 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 meas-
urement 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.
Keysight DAQ970A User's Guide 209
4Measurement Tutorials
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