Frequency response is the analysis of the response of systemswhen subjected to a sinusoidal change in input. When a linear system is subjected to a sinusoidal input, its ultimate response is also a sustained sinusoidal wave, with the same frequency. The figure below compares the output response of a system (solid line) with a sinusoidal input (dashed line) disturbing the system
System compensation is the process of designing a controller that will produce an acceptable transient response while maintaining a desired steady-state accuracy .These two design objectives are conflicting in most systems ,since small errors imply high gains reduce system stability and may even drive the system unstable .Compensation may be thought of as the process of increasing the stability of a system without reducing its accuracy below minimum acceptable standards
