HDMI Layout Notes for EOMA68 Cards by Richard Wilbur

Speed of Propagation

First let's calculate the speed of electromagnetic wave propagation in a microstrip differential pair on our circuit board. Then we can use that speed to convert the timing tolerances of the HDMI standard into distance tolerances which will guide our priorities in routing the differential pairs.

Electromagnetic waves travel at the speed of light. But since the electromagnetic fields of these signals extend between the conductors into both air and the FR-4 substrate of our printed circuit board (PCB), they will not travel at the same speed as if they were travelling through a vacuum.

Speed of light in a vacuum: c = 3.00e8 m/s Speed of light in some other substance: v(wave travelling in substance) = c / sqrt(relative permittivity of substance)

Since the electromagnetic waves in question propagate partly in air and partly in the PCB substrate (FR-4), we estimate their governing permittivity to be between that of air and FR-4.

Speed of propagation for signals conducted in differential microstrip on the PCB: relative permittivity of FR-4 = 4.4 [1] relative permittivity of air = 1 v(in FR-4) = 3.00e8 / sqrt(4.4) = 1.43e8 m/s v(partly in FR-4, partly in air) ~= 3.00e8 / sqrt(4) = 1.5e8 m/s = 150um/ps[2]

References: [1] https://en.wikipedia.org/wiki/FR-4 [2] Toradex, page 21

Bibliography: http://docs.toradex.com/102492-layout-design-guide.pdf