The core function of a spectrum analyzer is to decompose a complex signal into its constituent frequency components. This process allows users to identify the frequencies present in a signal, their relative amplitudes, and any spurious signals or distortions. Chapter 2 defines many of the specified performance parameters of diffraction-g rating-based optical spectrum analyzers and discusses the relative merits of the single monochromator, double monochromator, and double-pass-monochromator- ased optical spectrum. At the most basic level, a spectrum analyzer can be described as a frequency-selective, peak-responding voltmeter calibrated to display the rms value of a sine wave.