Harmonics
Definition
The spectrum of a periodically varying voltages, except single-frequency sine and cosine wave voltages, contains a fundamental frequency component plus harmonic frequency components (higher frequency sinusoids.) A time signal having very abrupt ampltitude changes, like a square wave, contains higher-frequency spectral content than a single-frequency sinusoidal signal that has more gradual ampltitude changes.
Example
Consider a high-amplitude 2Hz sine wave along with lower-amplitude 6Hz and 10Hz sine waves as shown in Figure 1 (a). The summation of the three waves yields to the square wave as shown in (b). The more of its odd harmonics we add to a 2Hz sine wave, the closer the summation waveform will look like a 2Hz square wave. Hence, we say that a square wave compromises a fundamental 2Hz sine wave plus that sine wave's odd harmonics.
The spectrum of the square wave is shown in (c).
