Fluke 189 - Manual (Page 59)

Making Measurements
Measuring Duty Cycle
3
3-23
tc026f.eps
Figure 3-14. Hz Display
The meter beeps to indicate when a particular pushbutton
is not allowed when measuring frequency. The following
are some general rules.
•
Relative ( D ), Hold ( Q ), and MIN MAX
( M )can be used.
•
FAST MN MX ( O M ) cannot be used.
The following are some tips for measuring frequency:
•
If a reading shows as 0 Hz or is unstable, the input
signal may be below or near the trigger level. You can
usually correct these problems by selecting a lower
range, which increases the sensitivity of the meter.
•
If a reading seems to be a multiple of what you
expect, the input signal may be distorted. Distortion
can cause multiple triggerings of the frequency
counter. Selecting a higher voltage range might solve
this problem by decreasing the sensitivity of the
meter. In general, the lowest frequency displayed is
the correct one.
Measuring Duty Cycle
Duty cycle (or duty factor) is the percentage of time a
signal is above or below a trigger level during one cycle
(Figure 3-15).
The duty cycle mode is optimized for measuring the on or
off time of logic and switching signals. Systems such as
electronic fuel injection systems and switching power
supplies are controlled by pulses of varying width, which
can be checked by measuring duty cycle.