Linear Time Invariant System

Definition: LTI system is a system that its output, Y, to its input, X, relationship is linear (i.e Y to X follows a straight line) and its output response Y does not vary with time, T, (i.e the system out only dependent on system input variable and not time variable).

In practice:

• Nonlinear system is common and often desired such as a RF mixer
• Linear system is generally wanted such as a straight line conversion of voltage to digital scale in Analog to Digital converter. Any deviation or non-linearity is considered an error in conversion.
• Time invariant system in short term generally holds true. For example, a freshly built electronic filter when measured consecutively, its frequency response is the same, however, as time goes by component aging happens for inductor and capacitor used in the design of the analog filter design, then filter's frequency response will change.