Fig. 2 The most popular way to generate DC power from the AC power mains

 

After obtaining the DC voltage, at the first look the ground structure seems to be useful only to provide a voltage reference to the circuits which are powered and are processing the signals. This is totally wrong! For example, one can consider a single-ended signal being transmitted between two gates, as shown in figure 3. The current travels forward from gate A to gate B. A large number of PCB designers do not understand that always it is of the highest concern to have into account that there is a return current which travels backward, from gate B back to gate A through the ground connection (or any other convenient connection, if ground is very poor).