Specifications

2 LTI Models
2-20
By convention, the sample time of continuous-time models is Ts = 0.Setting
Ts = –1 leaves the sample time of a discrete-time model unspecified. For
example,
h = tf([1 –0.2],[1 0.3],–1)
produces
Transfer function:
z – 0.2
-------
z + 0.3
Sampling time: unspecified
Note Do not simply omit Ts in this case. This would make h a
continuous-time transfer function.
If you forget to specify the sample time when creating your model, you can still
set it to the correct value by reassigning the LTI property
Ts.See“Sample
Time” on page 2-33 for more information on setting this property.
Discrete-Time TF and ZPK Models
Youcanspecifydiscrete-timeTF andZPKmodelsusingtf andzpk as indicated
above. Alternatively, it is often convenient to specify such models by:
1 Defining the variable z as a particular discrete-time TF or ZPK model with
the appropriate sample time
2 Entering your TF or ZPK model directly as a rational expression in z.
This approach parallels the procedure for specifying continuous-time TF or
ZPK models using rational expressions. This procedure is described in “SISO
Transfer Function Models” on page 2-8 and “SISO Zero-Pole-Gain Models” on
page 2-12.
For example,
z = tf('z', 0.1);
H = (z+2)/(z^2 + 0.6*z + 0.9);