synchronous signals

For synchronous signals with fast edges traces on printed circuit boards are transmission lines, not just conductor connections. If the length of the PCB trace exceeds certain values, it is necessary to match its impedance with the signal source and/or receiver. Impedance mismatches would bring signal reflections from the source to the receiver and back through the trans- mission line. Such reflection or distortion of the signal is called "echo" or "ringing". It is shown in emissions and irregularities of the waveform, especially the rise and fall edge.