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IM WT1801-01EN
Anti-Aliasing Filter
When an FFT is taken through the performance of 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 is called aliasing.
Aliased signal
Input signal Sampled points
Aliasing causes problems such as increased errors in measured values and incorrect measurements of the
phase angles on each harmonic. An anti-aliasing filter is used to prevent aliasing and eliminate high frequency
components that are irrelevant to the harmonic measurement.
For example, when an input signal with a fundamental frequency of 50 Hz is measured up to the 50th
order, the frequency of the 50th order is 2.5 kHz. Thus, a 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 harmonic
measurement.
The WT1800 uses the line filter as an anti-aliasing filter for harmonic measurements. For information about how
to configure the filter, see “Line 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 appendix 6.6 in the getting started guide, IM WT1801-03EN.
3 Harmonic Measurement Conditions (Option)
