Measuring Duty Cycle - Fluke 87 TRUE RMS MULTIMETER User Manual

Table 6. Frequency Counter Operation With Current
Approximate
Input Sensitivity
Range (0.5 Hz - 20 kHz)
A 300 A
mA 30 mA
A 3A

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.
Inputs
Approximate Trigger
Level
AC Current DC Current
0 A 400 A
0 mA 40 mA
0A
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
4A
(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 (<400 kHz) aperiodic
logic signals, especially serial communication signals, is a
simple form of signature analysis. A known pattern will
read the same duty cycle every time (if the pattern repeats
in less than 1/3 second).
Applications
27