Constant envelope is achieved when a sinusoidal waveform reaches equilibrium in a specific system.
Steady state, as defined in electrical engineering, occurs after a system becomes settled.
To be more specific, control systems are unstable until they reach a steady state.
Feedback is used to create a feedback signal to control gain, reduce distortion, control output voltage, improve stability or create instability, as in an oscillator.
Some examples of constant envelope modulations are as FSK, GFSK, MSK, GMSK and Feher's IJF - All constant envelope modulations allow power amplifiers to operate at or near saturation levels.