Difference between bode and my calculation of amplitude of the frequency response

Hi every one, I want to calculate the frequency response of a system, for example :

I dont know why my answer is different from tf and bode function of MATLAB: I substitute s with jw and use abs function as below: fre=linspace(1e-1,100,100); omg=2*1i*pi.*fre; H=tf([1,0],[1,2,3]); bode(H) figure Hm=(omg)./((omg).^2+2.*omg+3); semilogx(fre,20.*log10(abs(Hm))) (I mean the amplitude part). It seems my calculation is wrong but I need to use my method instead of tf function. How can I fix it? Sorry. I am not an electrical engeering student and perhaps my question is very simple!! many thanks in advance

Answer 1

[Star Strider]

The ‘fre’ vector needs to be created in logspace and it then needs to be divided by because the frequency axis is in rad/sec. Then, it matches reasonably closely to the bode result.

fre=logspace(-1,2,100)/(2*pi);
omg=2*1i*pi.*fre;
H=tf([1,0],[1,2,3]);
bode(H)

figure
Hm=omg./((omg).^2 + 2.*omg + 3);
semilogx(fre*2*pi,20.*log10(abs(Hm)))

.

[raha ahmadi]

OMG!!

Dear Star Strider, you ve helped me alot. I don't know how to thank you. I didnt recognise the problem and it took me in the trouble.

I really appreciate and hope you the best

[Star Strider]

As always, my pleasure!

The solution was not immediately obvious to me, and it took a bit of experimenting to figure it out.

[raha ahmadi]

I spent 3 days!!!

as always very thanks

[Star Strider]

As always, my pleasure!