11-9
IM 253710-01E
Waveform Analysis
11
Explanation
The power spectrum of the voltage, current, and active power can be displayed by taking
the FFT (Fast Fourier Transform). You can check the frequency distribution of the
voltage, current, and active power from the result. For example, if the equation is set to
FFT(C1) when setting the equation as described in section 11.2, the FFT of CH1 is
performed. For a detailed explanation of the FFT, see section 1.8, “Waveform Analysis.”
Operand
C1 to C8, which correspond to CH1 to CH8, can be used as operands when
constructing the equation as described in section 11.2. For FFT functions, only one
operand can be used from the following: C1 to C8, C1
∗
C2, C3
∗
C4, C5
∗
C6, or C7
∗
C8.
Selecting the number of computed points
Select the number of points from the following choices. The FFT is taken over the
specified number of points from the start point of the waveform computation set in
section 11.1.
1000, 2000, 10000
Selecting the time window
Select the time window from the following choices.
• Rect
The time window is set to a rectangular window. This window is best suited to
transient signals such as impulse waves in which the signal attenuates completely
inside the window.
• Hanning
The time window is set to a hanning window. This window is best suited to
continuous signals.
Note
•
The FFT cannot be performed when the display record length is less than the number of
computed points.
•
If a channel of an element that has no module installed is used as an operand in the equation,
the computation result displays [-------] (no data).
•
The maximum frequency displayed on the horizontal axis of the FFT display is the Nyquist
frequency, which is half the frequency of the module’s sampling rate. When the
measurement is made at rates of 5 MS/s and 100 kS/s, the displayed frequencies are 2.5
MHz and 50 kHz, respectively. If the input signal contains a frequency component that is
higher than half the sampling rate, aliasing occurs. Note that a display containing an aliased
signal does not represent data that are actually measured.
11.3 Performing the FFT