# Using tfdata

#### cloud

Joined Dec 15, 2006
11
Does anyone know know to use the function, [num,den]=tfdata(sys)? My transfer function(tf) is a double(complex). When I type [num,den]=tfdata(tf), it states that the function 'tfdata' is not defined for values of class 'double'.

Thank you.

#### Dave

Joined Nov 17, 2003
6,969
Does anyone know know to use the function, [num,den]=tfdata(sys)? My transfer function(tf) is a double(complex). When I type [num,den]=tfdata(tf), it states that the function 'tfdata' is not defined for values of class 'double'.

Thank you.
Correct, because tf (or sys in the original syntax) must be expressed in polynomial form, hence the only data-type it can accept is type int (as far as I can see both signed/unsigned and variable-bit).

The context in which tfdata is used is:

Rich (BB code):
H = tf([1 2],[3 0 4]); % H = (s + 2)/(3s^2 + 4)

[num,den] = tfdata(H)
num =
1 2
den =
3 0 4
The tfdata function is only really useful when you have a transfer function expressed polynomially excapsulated within a variable, like H, and wish to extract the numerator and denominator coefficients.

Dave

Dave