Determining signal characteristics

In many cases, the information conveyed by a signal is carried by its spectral content. In optics, for example, the spectral analysis of light provides a wealth of information on the physical state of the source and its chemical composition. A signal can be described both by its time evolution x (t ) and by its Fourier transform X ( j2πf ); the spectrum analyzer plays the same role in the frequency domain as the oscilloscope does in the time domain. In reality, most spectrum analyzers give only the power spectrum and not the two components, modulus and phase, of the Fourier transform.