Getting a higher resolution bode plot

 Hi,

 I am drawing a bode diagram and trying to find the frequency with the peak magnitude. My low resolution bode plot gives me an inaccurate answer, how can I plot more points (smaller step size) so I can see the correct answer?

 My code is 
  [num400, den400]=feedback([0 0 10], [1 11 10], [0 400],[0 1]);
  bode(num400, den400)

 and I am trying to prove the peak is not at 62.8rads.

 Thanks

 Will

Answer 1

[Fangjun Jiang]

You can specify the frequency vector for the bode plot. But more seriously, you are not using feedback() and bode() correctly. You probably mean:

Sys=feedback(tf([0 0 10], [1 11 10]), tf([0 400],[0 1]));
bode(Sys);

If you want to specify the frequency vector, use

W=logspace(f1,f2,N_Of_Points);
bode(Sys,W);

See document for the correct usage of feedback and bode

help feedback
help bode

    BODE(SYS,W) uses the user-supplied vector W of frequencies, in
    radian/second, at which the Bode response is to be evaluated.  
    See LOGSPACE to generate logarithmically spaced frequency vectors.

Update:

Surprisingly, the OP's syntax works too! It recognizes the numerator and denominator and construct them as a LTI system using tf().

[num400, den400]=feedback([0 0 10], [1 11 10], [0 400],[0 1]);
bode(num400, den400)

To be honest, I don't like this sneak feature at all, especially when it is not documented. Neither can be found in doc feedback or doc bode. I can see that in bode.m which deal with the different case for input arguments. There is a warning in the obsoleted comment text.

Any Mathworker has any comment?