Key points you have learned

A signal is merely a function of time, representing a physical quantity like voltage in a circuit or pressure wave of sound in air

A sinusoid is an especially important example of a signal. It has three parameters -- amplitude, frequency, and phase -- that can themselves vary with time, in a process known as modulation.

A sinusoid is an example of the larger class of periodic signals.

A superposition of sinusoids of the same frequency is also a sinusoid at that same frequency.

A convenient representation of a signal consisting of a superposition of sinusoids is in the frequency domain, where the amplitude and phase of the sinusoids is plotted against their frequency.

A superposition of sinusoids of different frequencies is a more complicated-looking signal. If the frequencies are rationally related, it is periodic.

Any periodic signal can be represented by a superposition of sinusoids all harmonically related to a fundamental frequency that is the reciprocal of the period. This is the Fourier series.