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February 2016 • The PCB Design Magazine 41 is closed, are due to this knock-on effect. This explains the observation that a complete circuit is needed for current flow. If one charge carrier cannot cross a gap in the circuit, all the other charge carriers behind cannot move either and current does not flow anywhere in the circuit. This scenario explains DC and low-frequency circuits that have a single point ground refer- ence (as taught in Circuit Theory 101). However, above 100KHz parasitic capaci- tance and inductance become significant al- lowing current to flow in multiple paths. In a multilayer PCB, the electric field charges each section's R-L-C-G elements; in turn, as the ris- ing edge propagates along the transmission line with the return current from each section, flow- ing back to the source. Then as the pulse passes, the falling edge discharges each section's capac- itance. By the time the signal wave reaches the load, it has established multiple paths of return current along the PCB planes. The speed at which the signal travels down the conductor really has nothing to do with the drift speed of the electrons. The signal is an electromagnetic wave that travels at about half the speed of light. The electrons serve to guide the wave down the wire. It is the movement of the electromagnetic field or energy—not volt- age or current that transfers the signal. The volt- age and current exist in the conductor, but only as a consequence of the field being present as it moves past. We have established the fact that signals travel at the same speed, given the same me- dium, but what limits the bandwidth? Let me use a metaphor to explain: The speed of sound is much slower than that of light at 343.2 m/s in air. However, this isn't the speed of the chan- nel—it is its latency. That is, if you are 343 me- ters away, you will hear me one second after I speak. That reveals nothing about how fast I can communicate with you, which is limited by Figure 3: eye diagram displaying jitter and noise in Mentor Graphics' HyperLynx tool. faster than a speeding Bullet

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