bode plot discrepancy

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aaa
aaa 2012-6-26
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

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回答（2 个）

Arkadiy Turevskiy
Arkadiy Turevskiy 2012-6-26
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
  2 个评论
aaa
aaa 2012-6-27
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
aaa 2012-6-27
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.

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Maryam Ghadrdan
Maryam Ghadrdan 2012-6-26
what is the alternative way to get the phase plot?

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