App-21
IM 701210-06E
Appendix
App
Time Windows
You can select rectangular, Hanning, flattop, or exponential for the time window. The rectangular
window is best suited to transient signals, such as an impulse wave, 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
0 level. Hence, it is best suited to continuous signals. With the Hanning window, the frequency
resolution is relatively high as compared with the flattop window. However, the flattop window
has a higher level of accuracy. When the waveform being analyzed is a continuous signal,
consider the above characteristics in selecting the proper window to be applied.
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
π )
: W(t)={0.54–0.46 cos(2
π
)}
T
T
Flattop window
t
T
t
T
sin{2
π
(1–2t/T)}
2
π
(1–2t/T)
The exponential window is used to eliminate noise components from the signal. It is
effective against signals such as the frequency response test signal generated through
impulse excitation. On the DL750/DL750P, the exponential window and force window
are activated simultaneously.
• Exponential Window
The damping rate is set in terms of the weight of the last data point when taking the
weight of the first data point in the specified number of FFT points to be 100% (= 1).
The value is set in the range of 1 to 100% (1% resolution). The exponential window
damps the signal exponentially along the time axis. It is effective when the signal
does not attenuate all the way within the record length. If the damping rate is set to
100%, the window is equivalent to a rectangular window.
• Force Window
Sets the area over which computation performed in terms of a percentage from the
first FFT point when taking the specified number of FFT points to be 100%. The
areas (force 1 and force 2) can be set in the range of 1 to 100% (1% resolution) of the
input/output signal. If the area is set to 100%, the window is equivalent to a
rectangular window. On the DL750/DL750P, the data outside the area is computed
as an average value of the area.
Force1: The setting applies to the input signal (first parameter) of the one-input FFT
or two-input FFT.
Force2: The setting applies to the output (response) signal (second parameter) of
the two-input FFT.
0
1
Time
Time
Setting
(%)
Setting (%)
1 frame = 100%
100%
Exponential window
Force window
Weight
Weight
0
1
Appendix 5 User-Defined Computation
Appendix 5 User-Defined Computation
