The necessity of forming of physical signal theory (ST), whch is adequate to quantum electronics (QE) problems, is substantiated, and its elements are stated. The forming of physical ST is based on ST fundamental postulates, quantum Planck postulate, Einstein hypothesis about light quanta and energy expression of electromagnetic (EM) field in quantum electrodynamics. On the base of this statemet mathematical formulation of quantum Planck postulate is given. This formulation is the basic of exact, quantum description of signal in the form of EM field. A relation between exact, quantum description of signal and its classical approximation is established. Thus fundamental problem of quantum and classical physics join is solved for EM field. Obtained results are applied to spontaneous and monochromatic oscillations description.