If a monochromatic signal is fed into a nonlinear system, the output is composed of the input frequency (fundamental) and harmonics that are integer multiples of the fundamental frequency, as depicted in Figure 2.48. By only considering second- and third-order nonlinearities of the system, the output of a nonlinear block can be modeled as a simple polynomial function:
where α1 is the small-signal gain and α2 and α3 are positive factors related to the second- and third-order nonlinearities, respectively.
The negative term in front of α3 is due to the fact that the third-order term compresses the gain of the fundamental and to avoid any confusion arising from the assumption α3 < 0.
If x(t) = A cos(ω1t), we can develop Equation 2.152:
in which we find a DC component, the input frequency called the fundamental with amplitude H1, and the second- and third-order terms called the “harmonics” with amplitudes H2 and H3, respectively.
As the input signal amplitude increases, the output level of the fundamental ...