Measuring Frequency - Fluke 87 TRUE RMS MULTIMETER User Manual

87
Users Manual

Measuring Frequency

In the Frequency Counter mode, the frequency display
autoranges to one of five ranges: 199.99 Hz, 1999.9 Hz,
19.999 kHz, 199.99 kHz, and greater that 200 kHz. For
frequencies below 10 Hz, the update rate slows and
follows the input signal. For frequencies between 0.5 Hz
and 0.3 Hz, the display may not be stable. For frequencies
below 0.3 Hz, the display shows 00.00 Hz.
For most frequency measurements, turn the rotary switch
to , connect the meter to the signal being measured, and
then press FConnecting the meter to the signal
before pressing Hz will normally allow the meter to
autorange to an appropriate range, but the minimum input
signal required to trigger the frequency counter varies,
depending on the range and frequency (see
Specifications). If the input signal is below the trigger
level, frequency measurements will not be taken. If your
readings are unstable, the input signal may be near the
trigger level for that range. You can usually correct this by
selecting a lower range. In the Frequency Counter mode,
the range (displayed in the lower-right corner of the LCD)
will only change when you press the Kbutton.
If your readings seem to be a multiple of what you
expected, your input signal may have distortion or ringing.
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(For example, electronic motor controls distort both
voltage and current waveforms.) Select a higher input
range if you suspect multiple triggering. An alternative is
to turn the rotary switch to F or l, which will shift the
trigger level from 0V to a positive voltage that changes
with each range. In general, the lowest frequency
displayed is the correct one.
In the Frequency Counter mode, the input range acts like
an attenuator, the G function ac-couples the input signal,
and the F and l functions dc-couple the input signal.
The F function is optimized for triggering on logic and
switching signals. The 4V dc range is optimized to trigger
on all common 5V logic families (triggers at 1.7V
High frequency logic signals may require the use of the
400 mV ac range. The 40V dc range is optimized to
trigger on automotive switching signals (triggers at 4V
1V). All ranges in the F function trigger at approximately
10% of range, except for the 4V range.
Frequency measurements can be made on current inputs.
The inputs are always dc-coupled. The triggering
characteristics are shown in Table 6.
0.1V).