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 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 measuring an 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, a 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 harmonic measurement.
The WT500 uses the filer used as a line filter in normal measurement as an anti-aliasing
filter in harmonic measurement.
Selecting an Anti-Aliasing Filter (Line Filter)
• You can select from one of the following cutoff frequencies.
OFF, 500 Hz, and 5.5 kHz
• If any of the measured elements is set to a setting other than OFF, the LineFilt
indicator at the top of the screen lights.
•
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 14.6.
Note
• The anti-aliasing filter (line filter) setting is common to all measurement functions.
The filter setting does not change for different measurement functions.
• Accuracy may not be defined depending on the measurement function. 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 would thus not be
appropriate. Therefore, accuracy is not defined for the 500-Hz line filter.
• If you select 500 Hz as the cutoff frequency, the anti-aliasing filter (line filter) is turned on
automatically, and the FreqFilt indicator lights.
6.6 Setting the Anti-Aliasing Filter
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