2-30
IM 701310-01E
FFT Analysis ►For the procedure, see section 10.7
This executes a Fast Fourier Transform (FFT), and displays the power spectrum.
You can select the trace for the real part or the trace for the imaginary part. If the trace
for the imaginary part is not set, the real part only is used for calculation, and negative
frequencies are not displayed.
You can select the time window from Rectangular, Hanning, and Flattop.
The rectangular window is best suited to transient signals, such as impulse waves, which
attenuate completely within the time window. The Hanning and flattop windows allow
continuity of the signal by gradually attenuating the parts of the signal located near the
ends of the time window down to the zero level. Hence, it is best suited to continuous
signals. With the Hanning window, the frequency resolution is higher than that of the
flattop window. However, the flattop window has a higher spectral level accuracy. When
the waveform being analyzed is a continuous signal, select the whichever of the Hanning
window and flattop window is more suitable for the application.
The number of points in the FFT can be selected from 2.5 k, 6.25 k, 12.5 k, 25 k, 62.5 k,
125 k, and 250 k. The FFT range is specified in the waveform area (Main/Zoom 1/Zoom
2). If the waveform area record length is more than the number of FFT points, the data is
downsampled for computation.
Marker measurement or peak value measurement can be used on the FFT waveform.
T
T
T
T
t
Sine wave
Window
Integral
Power spectrum
Rectangular
window
Hanning window
Rectangular window:
Hanning window:
Flattop window:
W(t)=u(t)–u(t–T) U(t) : Step function
W(t)=0.5–0.5cos(2
P
W(t)={0.54–0.46 cos(2
P
)}
T
T
Flattop window
t
T t
T
sin{2
P
(1–2t/T)}
2
P
(1–2t/T)
FFT Function
Given that the complex function resulting after the FFT is G = R + jI, the power spectrum
can be expressed as follows:
( )
10 log R
2
+ I
2
2
R: Real Part, I: Imaginary Part
Reference value (0 dB) of the logarithmic magnitude (Log mag): 1 Vrms
2
AC component
( )
10 log R
2
+ I
2
DC component
2.8 Analyzing and Searching
