Filter Blocks

Make sure that the Simulink window is foremost and create a New Model from the menubar. Then drag and drop, in the "Continuous" blockset of Simulink the "Transfer Fcn" to your new model. This is adequate since our Butterworth filters are under transfer form. The model will look like this:

If you double click on the block, it will display a dialog which prompts you to manually enter the values for the numerator and denominator. This not very practical, since we wish to experiment with the order and cutting frequency of the filter. Give num as a parameter for the numerator, and den as the parameter for the denominator. These two will be computed by the mask we will build later, so it is important that they have the proper names, as these names are the names of the variables that will be initialized by the mask.
We will customize our block in two steps
1) embed the transfer function into a subsystem using the "Edit>Create Subsystem" submenu in the model menubar. Your model will now look like this

If you double click on the new block, you will see the transfer function inside

Observe that the block displays the name of the variables for the numerator and denominator.

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