Constant envelope is achieved when a
sinusoidal waveform
A sine wave, sinusoidal wave, or just sinusoid is a mathematical curve defined in terms of the '' sine'' trigonometric function, of which it is the graph. It is a type of continuous wave and also a smooth periodic function. It occurs often in ...
reaches
equilibrium in a specific system. This happens when negative
feedback 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 may increase the power significantly, or its main effect may be to boost t ...
reaches steady state.
Steady state
In systems theory, a system or a process is in a steady state if the variables (called state variables) which define the behavior of the system or the process are unchanging in time. In continuous time, this means that for those properties ''p' ...
, as defined in
electrical engineering, occurs after a system becomes settled. To be more specific, control systems are
unstable
In numerous fields of study, the component of instability within a system is generally characterized by some of the outputs or internal states growing without bounds. Not all systems that are not stable are unstable; systems can also be mar ...
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
Noise is unwanted sound considered unpleasant, loud or disruptive to hearing. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrations through a medium, such as air or water. The difference aris ...
and feedback gain has rendered the system steady.
Feedback is used to create a feedback signal to control gain, reduce
distortion
In signal processing, distortion is the alteration of the original shape (or other characteristic) of a signal. In communications and electronics it means the alteration of the waveform of an information-bearing signal, such as an audio signa ...
, control output voltage, improve
stability
Stability may refer to:
Mathematics
*Stability theory, the study of the stability of solutions to differential equations and dynamical systems
** Asymptotic stability
** Linear stability
** Lyapunov stability
** Orbital stability
** Structural sta ...
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
The power spectrum S_(f) of a time series x(t) describes the distribution of power into frequency components composing that signal. According to Fourier analysis, any physical signal can be decomposed into a number of discrete frequencies, ...
efficiency of a non-constant amplitude envelope is always higher than that of a constant envelope modulation.
See also
*
Constant-weight code
In coding theory, a constant-weight code, also called an ''m''-of-''n'' code, is an error detection and correction code where all codewords share the same Hamming weight.
The one-hot code and the balanced code are two widely used kinds of constan ...
*
Envelope (waves)
In physics and engineering, the envelope of an oscillating signal is a smooth curve outlining its extremes. The envelope thus generalizes the concept of a constant amplitude into an instantaneous amplitude. The figure illustrates a modulated sin ...
References
{{reflist
Waveforms