A complete sine wave in the time domain can be represented by one single spike in the frequency domain.

The
time-domain and frequency-domain plots of a sine wave

A single-frequency sine wave is not useful in data communication. We need to send a composite signal, a signal composed of many simple sine waves.

According to Fourier analysis, any composite signal is a combination of simple sine waves with different frequencies, amplitudes and phases.

A
composite periodic signal

Decomposition
of a composite periodic signal in the time and frequency
domain

Only at Engineer's Knowledge

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