Time Windows
You can use a rectangular, Hanning, flattop, Hamming, or exponential time window (you can use an exponential
time window on models only with the user-defined computation option).
The rectangular window is suited to transient signals, such as impulse waves, which attenuate completely
within the time window. The Hanning, flattop, and Hamming 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, they
are suited to continuous signals. The Hanning window provides a higher frequency resolution compared to the
flattop window. However, the flattop window has a higher level of accuracy. The Hamming window is a corrected
Hanning window. The frequency resolution of its main beam is greater than that of the Hanning window. The
Hamming window is best suited for dividing close signals. When the waveform being analyzed is a continuous
signal, consider the above characteristics in selecting the proper window to be applied. 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
Time windows Integral Power spectrum
Rectangular
Hanning
T
T
Hamming
Rectangular
Hanning
Flattop
Hamming
: W(t) = u(t) – u(t – T) u(t): Step function
: W(t) = 0.5 – 0.5 cos( 2π )
: W(t) = {0.54 – 0.46 cos( 2π )}
: W(t) = 0.54 – 0.46 cos( 2π )
T
T
Flattop
t
T
t
T
sin{2π(1 – 2t/T)}
2π(1 – 2t/T)
t
T
The exponential window is used to eliminate noise components from the signal. It is suited for the signals of
impulse-excitation frequency-response tests and other similar signals. On the DL850E/DL850EV, 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, with the weight of the first data point in the
specified number of FFT points taken to be 100% (= 1). You can set the damping rate 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 fully within the record length. When the damping rate is set to 100%, the window
functions like a rectangular window.
Appendix
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