Bode Diagram

When I plot a bode diagram from a FRD sys (obtained by a measurement data), I encounter phases beyond [-180,180]; actually, their order is about 2000 degrees! What should I do in order to have them inside [-180,180] degree?

Answer 1

[Arkadiy Turevskiy]

You need to use bodeoptions command and set PhaseWrapping to on. Here is an example showing how to do it:

>>s=tf('s'); %define Laplace variable

>>freq=logspace(-1,2,1000); %define frequency vector

>>sys=1/(s^3+2*s^2+3*s+1); %some tf with phase <-180

>>bode(sys,freq) %phase is not within -180:180

>>p=bodeoptions

>>p.PhaseWrapping='on'

>>bode(sys,freq,p) %phase is wrapped inside -180:180

HTH, Arkadiy

[Amir]

Thank you very much. It worked for me :)

Answer 2

[Benjamin Hoepfner]

Hi,

i'm using version 2021a and i have a similar problem but the previous answer doesn't work in my case. I need the values of the bode(sys) function for further calculations or external data storage. But the phase wrap obviously doesn't work in this case.

Here is my Code:

%same system from the previous answer
s=tf('s');
freq=logspace(-1,2,1000);
sys=1/(s^3+2*s^2+3*s+1);

[M1,P1,w1] = bode(sys,freq); %saving the magnitude,phase and frequency from the system via bode(sys)
X1=[]; %saving the datas in an extra variable
X1(:,1)=w1;
X1(:,2)=M1;
X1(:,3)=P1;

p=bodeoptions;
p.PhaseWrapping='on';
[M2,P2,w2] = bode(sys,freq,p); %same step as before but with the phase wrap option from bodeoptions
X2=[];
X2(:,1)=w2;
X2(:,2)=M2;
X2(:,3)=P2;

figure(1)
semilogx(X1(:,1),X1(:,3),X2(:,1),X2(:,3)) %plotting phases from X1 and X2 over frequency 
legend %two curves with same datas..

Is there anybody out there how can help me with this or can give me an explanation for this?

Best regards!

Ben