The sinusoidal signal may be written as the real part of the rotating phasor, i.e., . A as the function of frequency f , and the other its phase angle q as a function of f . These plots are referred to as single-sided spectra . More complex functions can be built up of sums or products of singularity functions. A power signal is a signal with nonzero power and infinite energy.
All periodic signals are power signals.
The system equation must involve only linear operators. The system must contain no internal independent sources. When one or more coefficients are function of time, the system is time-varying. A system is causal if its present response depends on the present or past values of the input signal. The system equation contains no derivative, and every term in x and y has identical arguments.