Signal velocity
The signal velocity is the speed at which a wave
In physics, a wave is a disturbance that travels through space and time, accompanied by the transfer of energy.Waves travel and the wave motion transfers energy from one point to another, often with no permanent displacement of the particles of the medium—that is, with little or no associated mass...

 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
Special relativity
Special relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...


Signal velocity is usually equal to group velocity
Group velocity
The group velocity of a wave is the velocity with which the overall shape of the wave's amplitudes — known as the modulation or envelope of the wave — propagates through space....

 (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 the speed of light in vacuum, while the signal velocity will still be less than or equal to the speed of light in vacuum.

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
Transverse mode
A transverse mode of a beam of electromagnetic radiation is a particular electromagnetic field pattern of radiation measured in a plane perpendicular to the propagation direction of the beam...

(Transverse ElectroMagnetic) 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 (15 cm) 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.
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