7-29
IM WT3001E-51EN
Harmonic Measurement in Normal Measurement Mode (Option)
7
Explanation
When taking the FFT by performing A/D conversion on a repetitive waveform, a
phenomenon occurs in which frequency components that exceed half the frequency of
the sampling frequency are detected as low frequency components. This phenomenon is
called aliasing.
Aliasing signal Input signal Sampling point
Aliasing causes problems such as increased errors in measured values and incorrect
measurements of phase angles on the components of each order of the harmonic
measurement. Therefore, an anti-aliasing filter is used to prevent aliasing and eliminate
high frequency components that are irrelevant to the harmonic measurement.
For example, when measuring the input signal with a fundamental frequency of 50 Hz up
to the 50
th
order, the frequency of the 50
th
order is 2.5 kHz. Thus, an 5.5-kHz anti-aliasing
filter is used to eliminate high frequency components that are greater than or equal to
approximately 5 kHz, which are irrelevant to the harmonic measurement.
This instrument uses the line filter in normal measurement mode as an anti-aliasing filter
in harmonic measurement.
Selecting the Anti-Aliasing Filter (Line Filter)
• The cutoff frequency can be selected from the list of choices below.
OFF, 500 Hz, 5.5 kHz, and 50 kHz
• If any of the target elements is set to a setting other than OFF, the LINE FILTER key
illuminates.
• Selecting OFF disables the anti-aliasing filter (line filter).
The accuracy and the upper limit of the measurement bandwidth change when the
anti-aliasing filter (line filter) is turned ON. For details, see section 7.11.
Note
• The anti-aliasing filter (line filter) setting is common to all measurement modes.
The filter setting does not change even if the measurement mode is changed.
• Accuracy may not be defined depending on the measurement mode. For example,
the harmonic measurement in normal measurement mode assumes 50/60 Hz for the
fundamental frequency. In this case, a 500-Hz anti-aliasing filter (line filter) would eliminate
frequency components that are greater than or equal to 10
th
order, and is not appropriate.
Therefore, accuracy is not defined for the 500-Hz line filter.
7.8 Setting the Anti-Aliasing Filter
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