bode plot discrepancy

Question

I am plotting the following

Cu4 = tf([1 2], [1 2 6]);

[magCu4 phaseCu4 wout] = bode(Cu4,logspace(-2,7,300));
magCu4 = squeeze(magCu4);
phaseCu4 = squeeze(phaseCu4);
semilogx(wout,20*log10(magCu4)),grid;
hold on
bode(Cu4,'r')

I would expect that the semilogx plot would return an identical plot as 'bode'. however, this doesn't seem to be the case. Does anyone know what is going wrong here?

Answer 1

The difference is that you do not specify a frequency vector in your second call to bode so MATLAB chooses a default vector (in your code it had length of 46).

Instead you could try:

bode(Cu4,'r',logspace(-2,7,300))

Compare the plots made by the following code

[magCu4 phaseCu4 wout] = bode(Cu4,logspace(-2,7,300));
magCu4 = squeeze(magCu4);

figure(1);
semilogx(wout,20*log10(magCu4))
hold on; 
bode(Cu4,'r') 
hold off;

figure(2);
semilogx(wout,20*log10(magCu4))
hold on; 
bode(Cu4,'r',logspace(-2,7,300)) 
hold off;