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Can matlab give me difference equation from transfer fucntion

Asked by moonman on 12 Nov 2011
Latest activity Commented on by Ben Le
on 4 Feb 2017
Hi My transfer function is H(z)= (1-z(-1)) / (1-3z(-1)+2z(-2))
How can i calculate its difference equation. I have calculated by hand but i want to know the methods of Matlab as well

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4 Answers

Answer by Wayne King
on 12 Nov 2011
 Accepted Answer

I think I'm trying to say that you see it right away if you have the Z-transform. Do you have the System Identification Toolbox? You can do:
M = idpoly([1 -3 2],[1 -1],'NoiseVariance',0)

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Ok for this equation it is ok
lets suppose we have some complex transfer function
then is there any way matlab gives difference equation
Hi, If I don't have System Identification Toolbox, how can I do it in Matlab? Thanks.

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Answer by Wayne King
on 12 Nov 2011

You have the difference equation:
H(z)= (1-z(-1)) / (1-3z(-1)+2z(-2))
y(n)-3*y(n-1)+2*y(n-2) = x(n)-x(n-1);
zplane([1 -1],[1 -3 2]);
This is not stable, you have a pole outside the unit circle.

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And Thanks for telling me abt new command zplane
I used to use pzmap which does not show overlapped poles and zeros but zplane is showing the digits if multiple poles and zeros are there at same place
Great help

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Answer by moonman on 12 Nov 2011

Thanks king yes i have calculated all this and i know it is unstable systm but i need to know that can matlab give difference equation the way it gives poles and zeros by pole zero command and plots by pzmap

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Answer by Gert Kruger on 29 Jul 2015
Edited by Walter Roberson
on 29 Jul 2015

syms z;
H = (1-z^-1) / (1-3*z^-1+2*z^-2);
[N, D] = numden(H);
Nc = eval(coeffs(N)); %Get coeffs and evaluatle symbolic variable, i.e. make real matrix
Dc = eval(coeffs(D));
Nc = Nc./(Dc(1)); %Turn into proper polynomial, first coeffs of a is 1
Dc = Dc./(Dc(1));
M = idpoly(Dc, Nc, 'NoiseVariance',0)

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