The closed-loop transfer function is measured at the output.
Signals may be waveforms, images, or other types of data streams.
An example of a closed-loop block diagram, from which a transfer function may be computed, is shown below:
is called the feedback transfer function, and their product
We define an intermediate signal Z (also known as error signal) shown as follows: Using this figure we write: Now, plug the second equation into the first to eliminate Z(s): Move all the terms with Y(s) to the left hand side, and keep the term with X(s) on the right hand side: Therefore,