Measuring Frequency and Periodic Signal Frequency

The DMM can measure periodic signal frequencies, up to 500 kHz, from a variety of signal sources, ranging from millivolts to hundreds of volts.

The DMM measures frequency by counting the number of zero-crossing rising edges of the input signal, using an onboard 108 MHz timebase.

Frequency (Hz) = timebase (Hz) x Ns / Nt

Period = 1 /Frequency (Hz)

where

timebase = 108 x 10⁶ Hz

Ns = number of rising edges detected in the measurement window

Nt = number of timebase clock periods between the first and the last rising edge

To measure a signal of frequency f in Hz, the DMM needs a minimum aperture of (2/f) s. In other words, the DMM requires an aperture that is greater than or equal to at least two periods of the input signal. Therefore, as a general rule, the aperture should be set to a value greater than or equal to 110% of (2/f) s. The resolution of the frequency/period measurement can be improved by increasing the aperture time enough to allow for an integer increase in the number of periods.

Another way to measure frequency is to perform waveform acquisitions and then to use the signal processing functions in the ADE to extract the frequency.