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In a previous Beyond Design column, Transmission Lines, I mentioned that a transmission line does not carry the signal itself, but rather guides electromagnetic energy from one point to another. The speed of a computer does not depend intrinsically on the speed of electrons, but rather on the speed of energy transfer between electronic components. Electron flow in a multilayer PCB is extremely slow—about 10 mm per second—so, how does the signal travel so fast, how fast does it actually transfer information and what are the limitations?
In optical communications, electrons don’t carry the signal—photons do. And we all know that photons travel at the speed of light. So surely, optical fibers must transmit information much faster than copper wires or traces on a multilayer PCB? Actually, photons and electrons transmit data at the same speed. The limiting factor is the relative permittivity (dielectric constant) of the medium in which the signal propagates.
An optical fiber is a cylindrical dielectric waveguide made of low-loss materials such as fused silica glass. It has a central core in which light is guided, and embedded in an outer cladding of slightly lower refractive index. The silica glass used has a dielectric constant (Er or Dk) = 3.78 @25GHz. Whereas, for instance, Panasonic’s new Megtron 7, low Dk, glass PCB laminate has an Er = 3.3 at the same frequency.
To read this entire article, which appeared in the February 2016 issue of The PCB Design Magazine, click here.