We can interpret the waveforms we see in terms of components at different frequencies. For example, we might consider a voltage *V*(*t*) that is the sum of three cosine components:

We can include a DC term as a special case if we assume that one of the frequencies is zero. Representing a function in terms of its frequency components is helpful in understanding how the filter in a Class-C or Class-D amplifier operates. We also use the frequency components to define the relationship between DC and AC currents in oscillators and to predict mixer output frequencies.

If a function is periodic, it has a ...

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