Connection between electromagnetic (EM) signals, as quantum systems, which carry the spectroscopic information, and
classical description of EM signals on an input of spectrum analyzers, which action is described in the classical form, is
established. The physical substantiation of classical methods of signal theory (ST) is given. Forming EM of signals in form EM radiation and in the form of the electric current, caused by incident EM radiation, is considered at quantum level. Transition to classical approach of EM signals is carried out.