Contents
What is a PCB signal?
A PCB transmission line is a type of interconnection used for moving signals from their transmitters to their receivers on a printed circuit board. A PCB transmission line is composed of two conductors: a signal trace and a return path which is usually a ground plane.
What is velocity factor of transmission line?
The velocity factor (VF), of a transmission medium is the speed at which a wavefront of a signal passes through the medium, relative to the speed of light. The speed of radio signals in a vacuum, for example, is the speed of light, and so the velocity factor of a radio wave in a vacuum is unity, or 100%.
Why should PCB tracks be short?
Using short traces that take direct paths will help reduce EMI risk from both antennas and trace proximity. A smaller antenna will always radiate or receive less than a larger antenna. Traces themselves aren’t the only things on the high speed PCB design that can radiate EMI.
What is propagation delay in PCB?
The propagation delay on a PCB trace is the one-way (source to load) time required by a signal to travel on that trace. It is expressed in time per unit length. The propagation delay is the function of the dielectric constant (Er) and the trace geometry/structure.
What affects the velocity factor in transmission lines?
In electrical cables, the velocity factor mainly depends on the insulating material (see table below). The use of the terms velocity of propagation and wave propagation speed to mean a ratio of speeds is confined to the computer networking and cable industries.
How is the signal speed on a PCB determined?
A signal travels on a PCB transmission line at a slower speed, affected by the dielectric constant (Er) of the PCB material – the relations for calculating the signal speed on a PCB are given below: Vc is the velocity of light in vacuum or air Er is the dielectric constant of the PCB material
When to use PCB trace length matching in high speed design?
As computer peripherals and other digital systems require successively faster operating speeds, the propagation delay in a computer network places tight tolerances on the allowed trace length in a conductor carrying digital signals. Here are some best practices for applying PCB trace length matching in different types of systems.
What is the characteristic impedance of a PCB trace?
If you consider the PCB trace as a lossless transmission line, the characteristic impedance Z 0 = L C but the velocity factor is inversely proportional to L ⋅ C (where L & C are per unit length).
How is the propagation delay of a PCB calculated?
It is expressed in time per unit length. The propagation delay is the function of the dielectric constant (Er) and the trace geometry/structure. For a given PCB laminate and a given dielectric constant, the time delay is fixed for various impedance lines.