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Signal Integrity Issues and Printed Circuit Board Design by Douglas Brooks

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Chapter 2. Propagation Times

Propagation Speed

Electrical signals traveling along a wire or trace in the air travel at the speed of light, 186,280 miles per second (which, if you do the arithmetic, works out to 11.8 in/ns). That speed is equivalent to 7.5 trips around the Earth’s equator in one second. Yet we need to worry about how fast these signals travel along traces on our boards! That is pretty amazing when you think about it.

Consider this experiment: String a copper wire in the air across a lake and measure the time it takes a signal to travel from one end of the wire to the other end. Now lower the wire into the lake and measure the signal propagation time again. What do you think happens to the propagation time?

When a current flows along ...

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