Signal Integrity TUTORIAL

Impedance and Reflection

An eletromagnetic wave travels down a transmission line with little or no loss if the impedance of the transmission line stays constant. However, if the impedance of the transmission line changes for any reason, a part of the electromagnetic wave is reflected back towrds the source.

The amount of the voltage reflected back is given by the following equation

p = (Zi - Zo) / (Zi + Zo)

Let us consider the case Zi = Zo. In that case there is no reflection. The greater the mismatch between the Zi and the Zo, the more is the signal reflection. From a PCB designer point of view, we must ensure that the impedance stays constant and should be equal to the source impedance.

Some more details will be added later. <