Fluke 87V-MAX Product Manual - Page 34

Table 6. Frequency Counter Operation With Current Inputs Input Range Approximate Sensitivity (0.5 Hz - 20 kHz) Approximate Trigger Level AC Current DC Current µA 300 µA mA 30 mA A 3A 0 µA 0 mA 0A 400 µA 40 mA 4A Measuring Duty Cycle Duty Cycle (or duty factor) is an alternate Frequency Counter mode that displays, in percent, the time the input signal is above the trigger level (or below the trigger level if the negative trigger slope is selected). The Duty Cycle mode is optimized for measuring the on or off time of logic or switching controls. Many industrial control systems (electronic fuel injection in automobiles, for example) are pulse-width modulated, and duty cycle measurements provide a quick check on their performance. For logic level signals, use the 4V dc range. For 12V switching signals in automobiles, use the 40V dc range. Applications For sine waves, use the most sensitive range you can without getting double triggering. (Normally, a clean signal can be up to ten times the amplitude of the range you are on.) Duty cycle measurements can also be used as an indication of potential triggering problems on sine wave or near sine wave signals. If you do not measure approximately 50% duty cycle, you may have a distorted waveform. In Duty Cycle (and Frequency Counter) mode, the slope (or edge) on which the counter triggers is selected by pressing T. The slope selected is indicated by a + or - annunciator in the lower-left corner of the LCD. The waveform shown in Figure 3 represents the duty cycle measurement of a typical logic signal. The manner in which your meter takes duty cycle measurements allows it to be very tolerant of aperiodic (repetitive but not periodic) signals. Duty cycle measurements on low frequency (