Digital Circuit Boards: Mach 1 GHz

2.2 Some Common Assumptions

The voltage symbol that we will use in this book implies an unlimited source of current with zero rise time. This perfect voltage source does not exist, but it will be used to avoid adding complications. An instantaneous voltage implies an instant electric field. Since this field stores energy, the generation of a field in zero time requires infinite power. Similarly, the instantaneous flow of current implies a magnetic field and again this field stores energy. The generation of field energy in zero time would again imply infinite power. If we are careful, we can use an ideal step voltage source or a step current, and we will not get into trouble.

N.B.

Connections to a power supply require some length of wiring. This wiring is a transmission line. For this reason alone, there are no sources of voltage with zero source impedance.

N.B.

Every capacitor has lead length associated with its internal conductor geometry. The internal voltage of the capacitor is always in series with some form of transmission line.

The ideal switch does not exist. Even if the switch were mechanical, the capacitance before the moment of contact must get very large. If the switch is solid state, the closure is really a nonlinear dynamic impedance. We will use the ideal switch in our discussions even though it does not exist.¹

A resistor at the end of a transmission line is called a termination. A resistor termination is a complex network because every resistor has a ...