Multiplying transfer functions gives different bode plots

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')

Although all of the plots don't seem to be right.

Can anyone give an explanation?

Answer 1

[Santhosh A V]

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);

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.