National Instruments 370753C-01 Manual De Usuario

© National Instruments Corporation

Figure 5-4.  Bode Plot Showing System Gain and Phase Margins

These plots illustrate how the location of the system poles and zeros shapes 
the gain and phase curves. Each pole contributes a factor of –20 dB per 
decade (frequency interval from 

ω to 2ω). The two poles at zero cause the 

magnitude response of the system to start with a slope of –40 dB/decade. 
The zero at 0.5 radians/sec (about 0.08 Hz) contributes a factor of 
approximately 20 dB. These gain magnitude factors add, so the slope of the 
gain plot changes from –40 dB/decade to about –20 dB/decade until you 
begin to see the influence of the poles at 2 radians/sec. (0.318 Hz) and 
10 radians/sec (1.59 Hz), each of which contribute another –20 dB/decade 
to the slope of the magnitude plot.

The phase is a function only of the pole and zero locations. Notice that in 
creating the phase plot with 

bode( )

, you specified the 

!wrap

 keyword. 

This created a phase plot where range goes down to the full angle value of 
the phase, rather than wrapping the phase between +180

°. Each pole at zero 

contributes –90

° of phase. 

The remaining poles are called first-order poles because they are of the 
following form:

+