Can someone answer or point to a link explaining why a nonlinear system causes harmonics.
According to Wikipedia
... non-linear systems generate harmonics, meaning that if the input of a non-linear system is a signal of a single frequency, then the output is a signal which includes a number of integer multiples of the input frequency.
The only explanation that I can come up with is the following.
Let's say we have a frequency fc going through a NLTI system with a span. The upper and lower part of the span, as well as all the other intermediate frequencies will be affected differently than the it's surrounding frequencies due to the NLTI system causing mixing between all different frequencies thus causing harmonics.
If this is true then sending a delta function through a NLTI system will appear to be a LTI system.