National Instruments 370753C-01 Manual De Usuario

Referring to Figure 5-4, notice the additional lines drawn on the plots at 
the frequencies where the gain crosses the 0 dB line and where the phase 
crosses the 180

° line. When the gain crosses the 0 dB line, the phase is 

about –168

°, 12° away from –180°. So the phase margin is approximately 

12

°. Similarly, when the phase crosses the –180° line, the gain is about 

–44 dB (44 dB from the 0 dB line), and thus the gain margin is 44 dB.

[H,dB,Phase] = bode(Sys,{F,keywords})

The 

bode( )

 function uses 

freq( )

 to compute the frequency response 

of a system. By default, the 

freq( )

 keyword 

track

 is on, but it can be 

overridden. Refer to the 

 section for more details. When the 

frequency response 

H

 is found the decibel magnitude and the phase angle 

in degrees are computed as follows:

dB=20*log10(abs(H); phase=(180/pi)*atan(H)

bode( )

 then produces the standard Bode format plots showing response 

magnitude and phase as functions of frequency. Unlike 

freq( )

, 

bode( )

does not require a frequency range or a pair of maximum and minimum 
frequencies; if no range is specified, it uses 

deffreqrange( )

 to 

calculate a default frequency range.

bode( )

 often generates more than one set of plots. For MIMO systems, a 

plot is made for each output with multiple curves, one per input. If there are 
multiple outputs, a menu will appear which allows you to select an input to 
view. 

If you want to see the response of the system from Example 5-2 to input 
frequencies ranging from 0.01 Hz to 10 Hz, you can analyze a frequency 
response using 

bode( )

, as shown in Example 5-3.

sys = polynomial(-0.5)/polynomial([0,0,-2,-10]);

[H,dB,phase]=bode(sys,

{Fmin = 0.01,Fmax=10,npts = 300,!wrap})

You obtain the gain and phase plots as shown in Figure 5-4.