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The term “frequency domain” is used while describing functions or physical signals with respect to frequency either continuous or discrete. Frequency‐domain representation has been proven to be very useful in innumerous engineering applications while characterizing, interpreting, and identifying signals. Solving differential equations, analyzing circuits, signal analysis in communication systems are few among many others where frequency‐domain representation is much more advantageous than the time‐domain representation. The frequency‐domain signal is traditionally obtained by taking the FT of the time‐domain signal. As briefly explained in Section 1.1, FT is generated by expressing the signal onto a set of basis functions, each of which is a sinusoid with the unique frequency. Displaying the measure of the similarities of the original time‐domain signal to those particular unique‐frequency bases generates the Fourier transformed signal or the frequency‐domain signal. Spectrum analyzers and network analyzers are the common equipments to analyze frequency‐domain signals. These signals are not as quite perceivable when compared to time‐domain signals. In Figure 1.2, the frequency‐domain version of the sound signal in Figure 1.1 is obtained by using the DFT operation. The signal intensity value at each frequency component can be read from the y‐axis. The frequency content of a signal is also called the spectrum of that signal.

Figure 1.1 The time‐domain signal of “prince” spoken by a lady.

Figure 1.2 The frequency‐domain signal (or the spectrum) of “prince.”