# Substitute s for jw in a transfer function

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Inés Bodoque el 11 de En. de 2021
Comentada: Inés Bodoque el 11 de En. de 2021
Hi, I have a transfer function define by
G = ([1 2],[3 4 5]) (as an example)
I want to change the 's' for 'jw'. Does anyone know how to do it?
I've tried subs(G,{s},{1j*omega}) but it didn't work.
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### Respuesta aceptada

Jon el 11 de En. de 2021
Hi,
I think you mean that you define your transfer function using (you forgot the tf() in your example)
G = tf([1 2],[3 4 5]) %(as an example))
Then if you want to evaluate it at a particular frequency, that is a specific value of jw, you can use
w = 3; % for example 3 radians/sec
val = evalfr(G,j*w)
You can also use the freqresp function to evaluate it for multiple values along the jw axis
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Jon el 11 de En. de 2021
If as Pat suggest you don't have the Control System Toolbox, you can do this a little more neatly using
num = [1,2]
den = [3,4,5]
w = 3; % for example 3 rad/s
val = polyval(num,j*w)/polyval(den,j*w)
You can also use i*w MATLAB defines by default i and j as sqrt(-1)
Inés Bodoque el 11 de En. de 2021
It worked! Thank you very much

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### Más respuestas (1)

Pat Gipper el 11 de En. de 2021
Matlab uses the reserved constant "i" which is set equal to sqrt(-1). Using your transfer function as defined try the following which will result in the variable "G" which will be a complex number.
num=1*i*w+2;den=3*(i*w)^2+4*i*w+5;G=num/den;
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Pat Gipper el 11 de En. de 2021
Editada: Pat Gipper el 11 de En. de 2021
Based on some of your other questions it looks like you don't have access to the Control Systems Toolbox. So you need to do this arithmetic explicitly.
Inés Bodoque el 11 de En. de 2021
Thank you for your answer! I need this for a project from one of my subjects and I am not allowed to use the Control Systems Toolbox, that's why I have to do it this way

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