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# Signal velocity

The signal velocity is the speed at which a wave carries information. It describes how quickly a message can be communicated (using any particular method) between two separated parties. Every signal velocity is always slower than (or equal to) the speed of a light pulse in a vacuum (by Special Relativity).

In the vast majority of cases, signal velocity is equal to group velocity (the speed of a short "pulse" or of a wave-packet's middle or "envelope"). However, in a few special cases (e.g., media designed to amplify the front-most parts of a pulse and then attenuate the back section of the pulse), group velocity can exceed even the vacuum speed of light.

Signal velocity is alwaysFact|date=June 2008 the same as front velocity.

For light (electromagnetic waves) in vacuum, the signal velocity is the same as the group velocity and the phase velocity (the speed of any individual crest of the wave).

In electronic circuits, signal velocity is one member of a group of five closely related parameters. In these circuits, signals are usually treated as operating in TEM mode. That is, the fields are perpendicular to the direction of transmission and perpendicular to each other. Given this presumption, the quantities: signal velocity, the product of dielectric constant and magnetic permeability, characteristic impedance, inductance of a structure, and capacitance of that structure, are all related such that if you know any two, you can calculate the rest. In a uniform medium if the permeability is constant, then variation of the signal velocity will be dependent only on variation of the dielectric constant.

In a transmission line, signal velocity is the reciprocal of the square root of the capacitance-inductance product, where inductance and capacitance are typically expressed as per-unit length. In circuit boards made of FR4 material, the signal velocity is typically about six inches per nanosecond. In these boards, permeability is usually constant and dielectric constant often varies from location to location, causing variations in signal velocity. As data rates increase, these variations become a major concern for computer manufacturers.

* Dispersion (optics)
* Front velocity
* Phase velocity

References

* Brillouin, Léon. "Wave propagation and group velocity". Academic Press Inc., New York (1960).
* Clayton R. Paul, "Analysis of Multiconductor Transmission Lines". Johm Wiley & Sons., New York (1994)

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