1-21
1
IM 701830-01E
Functions
Power Spectrum Display
≡
Page 9-17.
≡
FFT (Fast Fourier Transform) computation can be performed on the input signal to display its
power spectrum. This is useful when you want to check the frequency distribution of the input
signal.
Power spectrum
waveform
The following windows are available: rectangular/Hanning/flattop windows.
The rectangular window is effective for transient signals, such as an impulse wave, which
attenuate completely within the time window. The Hanning window allows continuity of the
signal by gradually attenuating the parts of the signal located near the ends of the time window
down to the “0” level. Hence, it is effective for continuous signals. With the Hanning window,
the accuracy is relatively low, but the frequency resolution is relatively high. The frequency
resolution of the flattop window is less than that of the Hanning window. However, the flattop
window has the advantage of high level accuracy.
FFT computation generates 1000, 2000 or 10000 measurement data points, but only half points
are displayed on the screen.
T
T
T
T
Rectangular window
Hanning window
Hanning window
: W(t) = u(t)–u(t–T) U(t) :
Step function
: W(t) = 0.5–0.5cos(2
π
)
: W(t) = {0.54–0.46 cos(2
π
)}
T
T
t
T
t
T
sin{2
π
(1
–
2t/T)}
2
π
(1
–
2t/T)
Sine wave
t
Window
Integral
Power spectrum
Rectangular
window
Hanning window
Flattop window
[FFT function]
When the complex result of FFT computation is G = R + jI, the power spectrum can be
expressed as follows.
Power spectrum = 10 log
R : Real Part I : Imaginaly Part
Reference (0dB) for Log magnitude: : 1Vrms
2
( )
R
2
+ I
2
2
1.5 Analyzing the Waveform
