How performing FRF for complex functions

Hello,

I'm tryiing to plot phase and magnitude plot for below complex transfor function by performing FRF for say -600 to 600 Hz

1257 s + 3.736e05

-------------------------------------------

s^2 + (1554 - 2e02i) s + (3.736e05 - 2e02i)

Please help

Answer 1

[Star Strider]

Try this —

s = tf('s');
H = (1257*s + 3.736e05) / (s^2 + (1554 - 2e02i)*s + (3.736e05 - 2e02i))

H =
 
             1257 s + 373600
  -------------------------------------
  s^2 + (1554-200i) s + (3.736e05-200i)
 
Continuous-time transfer function.

opts = bodeoptions;
opts.FreqUnits = 'Hz';
opts.FreqScale = 'linear';

figure
bodeplot(H, {0 600*2*pi}, opts)                         % Plot Transfer  Function
grid

[mag,phase,wout] = bode(H,  {0 600*2*pi});              % Return Numeric Values ('wout' Will Be In rad/sec, So Divide It By (2*pi) To Get Hz)
mag = squeeze(mag);
phase = squeeze(phase);

.

[Bala Sundaram Nauduri]

Thank you so much, it helped me, but in the paper I'm refering the magnitude plot is not in DB, how can I change scale? Once again thank you so much.

[Star Strider]

My pleasure!

Set the MagUnits option to abs in the bodeplot call —

s = tf('s');
H = (1257*s + 3.736e05) / (s^2 + (1554 - 2e02i)*s + (3.736e05 - 2e02i))

H =
 
             1257 s + 373600
  -------------------------------------
  s^2 + (1554-200i) s + (3.736e05-200i)
 
Continuous-time transfer function.

opts = bodeoptions;
opts.FreqUnits = 'Hz';
opts.FreqScale = 'linear';
opts.MagUnits = 'abs';

figure
bodeplot(H, {0 600*2*pi}, opts)                         % Plot Transfer  Function
grid

[mag,phase,wout] = bode(H,  {0 600*2*pi});              % Return Numeric Values ('wout' Will Be In rad/sec, So Divide It By (2*pi) To Get Hz)
mag = squeeze(mag);
phase = squeeze(phase);

The units returned by the bode function are absolute for ‘mag’ and degrees for ‘phase’. Use the mag2db and deg2rad functions to convert them to dB and radians to change them if you need to. The ‘wout’ units are radians/time unit, so divide them by (2*pi) to get Hz.

.

[Star Strider]

If my Answer helped you solve your problem, please Accept it!

.

[Bala Sundaram Nauduri]

Once again thank you so much, Now I'm trying plot the root locus using rlocus(), and it says

Error using DynamicSystem/rlocus (line 65)

Cannot compute the root locus of models with complex data.

May be I need to rewrite my transfer function, I will get back to you if I have any questions.

[Star Strider]

My pleasure!

I cannot suggest a work-around for the rlocus problem.