Technician Handbook ! 673 Electronic & Computer Controlled Systems Frequency Measurement If you only need to determine a signal’s frequency, you could use the frequency measuring feature available with an advanced DVOM. It measures frequency by counting how many times per second a varying voltage or current reading crosses a selected threshold. Duty Cycle Similarly, some DVOMs can calculate duty cycle by measuring the percentage of time a signal is above or below a selected threshold during one cycle. The result is displayed as a percentage. The percent duty cycle reading assumes the first part of the cycle is the ON portion.
The value will be different depending on whether the cycle is interpreted as starting with a positive-going (rising) or negative-going (falling) voltage. In a ground-side controlled circuit, the load is ON when the voltage is low. Therefore you want to set the cycle to start when the voltage falls so the percentage of ON time is measured properly. This requires setting the DVOM for a negative trigger. While measuring duty cycle on the Fluke 87, you can toggle between positive trigger and negative trigger by pressing the Peak MIN/MAX button. Technical Training 31