Referencing FRD Models Through Frequencies :: Operations on LTI Models (Creating and Manipulating Models)

Creating and Manipulating Models

Referencing FRD Models Through Frequencies

You can extract subsystems from FRD models, as you do with other LTI model types, by indexing into input and output (I/O) dimensions. You can also extract subsystems by indexing into the frequencies of an FRD model.

To index into the frequencies of an FRD model, use the string 'Frequency' (or any abbreviation, such as, 'freq', as long as it does not conflict with existing I/O channel or group names) as a keyword. There are two ways you can specify FRD models using frequencies:

Using integers to index into the frequency vector of the FRD model
Using a Boolean (logical) expression to specify desired frequency points in an FRD model

For example, if sys is an FRD model with five frequencies, (e.g., sys.Frequency=[1 1.1 1.2 1.3 1.4]), then you can create a new FRD model sys2 by indexing into the frequencies of sys as follows.

sys2 = sys('frequency', 2:3);
sys2.Frequency
ans =
    1.1000
    1.2000

displays the second and third entries in the frequency vector.

Similarly, you can use logical indexing into the frequencies.

sys2 = sys('frequency',sys.Frequency >1.0 & sys.Frequency <1.15);
sys2.freq
ans =
    1.1000

You can also combine model extraction through frequencies with indexing into the I/O dimensions. For example, if sys is an FRD model with two inputs, two outputs, and frequency vector [2.1 4.2 5.3], with sys.Units specified in rad/s, then

```
sys2 = sys(1,2,'freq',1)
```

specifies sys2 as a SISO FRD model, with one frequency data point, 2.1 rad/s.

Basic Subsystem Concepts Referencing Channels by Name