YOKOGAWA DL7440 User Manual

2-22 IM 701450-01E
Power Spectrum Computation <For the setup procedure, see section 9.6>
The power spectrum of the input signal can be computed and displayed by taking the
FFT (Fast Fourier Transform). This is useful when you wish to check the frequency
distribution of the input signal.
Power spectrum waveform
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 0 level. Hence, it is best suited to continuous
signals. With the Hanning window, the frequency resolution is high compared to the
flattop window. However, the flattop window has a higher level of accuracy. When the
waveform being analyzed is a continuous signal, select the proper window for the
application. FFT is performed on 1000, or 10000 points of measured data. The data is
converted to half the specified number of points and displayed.
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(2π )
W(t)={0.54–0.46 cos(2π )}
T
T
Flattop window
t
T
t
T
sin{2π(1–2t/T)}
2π(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
2.5 Waveform Computation

YOKOGAWA DL7440 Specifications

General

Brand	YOKOGAWA
Model	DL7440
Category	Test Equipment
Language	English