Now all this works fine in theory, but few systems of interest are perfectly linear. If the output of a system is found to have frequencies not present in the input, or if the output at one frequency is affected by inputs at other frequencies, then the system is said to be nonlinear. The response of a nonlinear system can be quite unpredictable. Our method of obtaining the transfer function does not apply to nonlinear systems. Below are two examples of common nonlinearities:
Note that in the case of intermodulation distortion the higher input frequency is attenuated in level at the output. This is refered to as suppression. We will have more to say about this effect later.
