Constant envelope is achieved when a
sinusoidal waveform reaches
equilibrium in a specific system. This happens when negative
feedback
Feedback occurs when outputs of a system are routed back as inputs as part of a chain of cause and effect that forms a circuit or loop. The system can then be said to ''feed back'' into itself. The notion of cause-and-effect has to be handle ...
in a
control system, such as in radio
automatic gain control or when an
amplifier
An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). It is a two-port electronic circuit that uses electric power from a power su ...
reaches steady state.
Steady state, as defined in
electrical engineering
Electrical engineering is an engineering discipline concerned with the study, design, and application of equipment, devices, and systems that use electricity, electronics, and electromagnetism. It emerged as an identifiable occupation in the l ...
, occurs after a system becomes settled. To be more specific, control systems are
unstable until they reach a steady state. Constant envelope needs to occur for the system to be stable, where there is the least amount of
noise and feedback gain has rendered the system steady.
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. Although, the
power spectrum efficiency of a non-constant amplitude envelope is always higher than that of a constant envelope modulation.
See also
*
Constant-weight code
*
Envelope (waves)
References
{{reflist
Waveforms