Measuring Analog Frequency

To measure the frequency of an AC signal, you need circuitry that produces triggers of the same frequency as the measured signal. The parameters that determine when to trigger are Threshold Level and Hysteresis. Every time the signal passes from (Threshold Level – Hysteresis) to Threshold Level, a trigger occurs.

Note  (Threshold Level – Hysteresis) must be between –0.5 and 4.48 for the SCXI-1126 module.

A pulse generator uses these triggers and produces a pulse once every frequency cycle. The width of this pulse is set by the input frequency range. As the input frequency range increases, the pulse width grows smaller.

This pulse train is then converted to a DC signal that has a level proportional to the duty cycle of the pulse train. The duty cycle is the fraction of a period of the pulse train when the pulse is occurring. The DC signal has a voltage that is proportional to the input frequency and can therefore be scaled to that frequency value.