# Problem using variable='z^-1' in Z transform with Z^-1 format

4 views (last 30 days)
Answered: Walter Roberson on 30 Sep 2022
Hi Dears ,
What is the difference between one step and two step to define the 'variable'='z^-1'?
I want Z transform function 1/z^3 in term of z^-1 and the two step is ok not one step.
See the Matlab doc for tf, section: "Specify Polynomial Ordering in Discrete-Time Transfer Function"
num=[1];
den=[1 0 0 0];
% Two step is OK!
X1=tf(num,den,0.1)
X1.Variable='z^-1'
% One step is not OK!
X2=tf(num,den,0.1,'variable','z^-1')
X1 not equal X2
Many thanks to dear @Paul

Walter Roberson on 30 Sep 2022
Setting the Variable property afterwards just changes the Variable property, without reinterpretting the matrices. It is not a conversion operation, it is just rewriting the label.

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