log mag versus phase plots_7
TRANSCRIPT
-
8/17/2019 Log Mag Versus Phase Plots_7
1/1
log‐magnitude‐
versus‐
phase plot
• Another approach to graphically portraying the
frequency‐response characteristics
• is a plot of the logarithmic magnitude in decibels
versus the phase angle or phase margin for a
frequency range of interest
• phase margin is the difference between the
actual phase angle and ‐180◦; i.e. ‐(‐ 180◦)
=180◦+
• The curve is graduated in terms of the frequency
• Bode plot – Bode plot, uses semi‐log paper with two separate curves;
log‐magnitude curve and the phase‐angle curve
• log‐magnitude‐versus‐phase plot: – combines the two curves in Bode diagram into one.
– can be constructed by reading values of the log magnitude and phase angle from the Bode diagram or evaluating them.
– a change in the gain constant of G(jω) merely shifts the curve up (for increasing gain) or down (for decreasing gain).
– relative stability of closed loop system can be determined quickly and compensation easily worked out
• The log‐magnitude‐versus‐phase plot for the
sinusoidal transfer function G(jω) and that for
1/G(jω) are skew symmetrical about the
origin, since
CL Bode
Plot
Nyquist
PlotMagnitude‐
Phase Plot
Plots for 2nd Order system:
; 0 1
-90º -0º-180º-270º-360º
ω→∞
0 dB
|G(jω)|in dB
Phase
margin
Gain
margin
ω1
ω2
ω4
ω5
ω1
Typical response of a type 1,
third order system
Solid curve
system stable;
dotted curve
system unstable
Changing gain raises or
lowers curve without
changing angle
characteristics. K↑