bode plot discrepancy

i am plotting some bode plots, and i found the following discrepancy, and I'm not quite sure what is the root cause of it. My code is

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 expected that the semilogx plot of the magnitude should be identical to the plot of bode(Cu4), however, it doesnt seem to be the case. Where might I be going wrong with this?

TIA

Answer 1

[Arkadiy Turevskiy]

The very slight difference between two magnitude curves is due to the different frequencies you are computing the bode magnitude at. In your semilogx you are using wout, which you specified as logspace (-2,7,300). When you call bode with no frequency input argument, it determines the frequencies to compute the bode plot at automatically, and those frequencies are different from wout.

To better see that do this and zoom in on magnitude plot.

    close;
    semilogx(wout,20*log10(magCu4),'x'),grid;
    hold on
    bode(Cu4,'ro')

To have two identical magnitude curves, modify your code:

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

Arkadiy

[aaa]

Hi Arkadiy.

When i use

bode(Cu4,wout)

I still do not see the same plot. I'm seeing the same problem with the phase. Have I perhaps misread your suggestion?

[aaa]

Hi, I seem to have figured out the problem. wout needs to be rewritten as a frequency i.e. wout/2pi for semilogx!!! small mistake, but huge difference. Thanks.

Answer 2

[Maryam Ghadrdan]

what is the alternative way to get the phase plot?