Digital Circuit Boards: Mach 1 GHz

1.15 Moving Energy into Components

To place energy into a capacitor, charges must move. These moving charges are current, and this current implies a magnetic field. To move current into an inductor, a voltage is required. This means both a magnetic and an electric field are needed to move field energy into a capacitor or an inductor. This implies that all electrical activity requires the presence of both an electric and a magnetic field. Analog circuits (below 100 kHz) usually require small currents that change level slowly. For this reason, the magnetic fields associated with low frequency analog signal transport can usually be ignored. For the material in this book where clock and logic signals are changing rapidly, the magnetic field plays a very important role.

All passive and active electronic components require fields to operate. The problem for the circuit designer is to bring these fields to the components over the interconnecting conductors. As we have just stated, these fields carry energy. At low frequencies, there are few problems of field transport. For most analog circuits, the current levels are in the milliamperes. This is not true for digital circuits where amperes may be involved. These currents may need to flow for only a few nanoseconds, but if the energy is not available, the circuit will not function.

The problem we will discuss at great length is supplying the energy that must be carried through logic switches and on traces to other parts of the circuit. ...