11-2
IM WT3001E-51EN
FFT Measurement Examples (Conceptual Diagrams)
• Input signal:
52.2-Hz sine wave
• Number of FFT points: 200 k (FFT measurement time: 1 s)
• Left end of the graph: dc, right end: 100 Hz
Rectangular
Hanning
Flattop
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)
W(t)
=
0.5–0.5cos(2π
)
W(t)
=
{0.54–0.46 cos(2π
)}
T
T
Flattop window
t
T t
T
sin{2
π
(1
–
2t/T)}
2
π
(1
–
2t/T)
U(t): Step function
• Number of FFT Points and Frequency Resolution
The frequency range of the FFT is DC to 100 kHz. FFT is performed on 20000 points
or 200000 points of measured data, and the result is displayed using 10001 data
points or 100001 data points, respectively.
11.1 FFT Function
