Fluke 87 TRUE RMS MULTIMETER User Manual page 36

87
Users Manual
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Trigger Point
The precision and resolution of the duty cycle
measurements are achieved by averaging many
repetitions of the input signal. In rare cases, this
averaging technique (which is similar to pulse-width
averaging in a conventional counter) may cause a
measurement problem called "aliasing." Aliasing results
when the frequency of the input signal happens to be
exactly synchronized with the reference crystal oscillator
of the meter. This occurs when the frequency of the input
signal can be exactly divided into the frequency of the
oscillator (131,072 Hz) or one of the oscillator's
harmonics. When they are nearly synchronized, the
meter is "blind" to the correct duty cycle, and the display
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Trigger Point
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100%
Figure 3. Duty Cycle Measurement of Typical Logic Signal
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will alternate between incorrect readings. If this occurs,
and the frequency reading was stable, press Mto
select the MIN MAX Record mode and scroll to the
average display. The average display will stabilize on the
correct duty cycle.
A common duty cycle measurement is the "dwell" angle
in an automobile. Dwell is the number of degrees of
distributor rotation that the points remain closed (or
current is flowing in the coil). Use the following to convert
a dwell angle to duty cycle (in percent):
ep3f.eps