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Multiplying transfer functions gives different bode plots

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IBa
IBa 2019-2-13
回答： Santhosh A V 2019-2-19
I have two transfer functions (L and GS_d, see tf.mat) and try to multiply them. They are both of type tf and in discrete time. But GS*L gives another solution in the bode diagram than L*GS.
Here is my code
clear
load('tf.mat')
A = L*GS_d;
B = GS_d*L;
C = ss(L)*(GS_d);
bodemag(A);
hold on
bodemag(B);
bodemag(C);
legend('A','B','C')
Bode.png
Although all of the plots don't seem to be right.
Can anyone give an explanation?

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回答（1 个）

Santhosh A V
Santhosh A V 2019-2-19
I am able to reproduce the issue mentioned by you only when I use your data.
I have tried multiplying two custom discrete transfer functions h1 and h2 in the same fashion as specified by you.
The code is shown below.
clear
h1 = filt([1 3],[21 2 10 40],5e-05);
h2 = filt([2 1],[1 -2 8 14 6.269e-12],5e-05);
A = h1*h2;
B = h2*h1;
subplot(211);
bodemag(B);
hold on;
subplot(212);
bodemag(A);
bodemagissue.png
I have even tested with “tf” function and everything seems fine .
If you are facing this weird behavior with any other data kindly share it. Hope this helps.

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