4.20 Glossary
A/D Converter (Section 4.14): An integrated circuit that converts analog signal levels to digital signals.
Analog/Digital interface (Section 4.14): On circuits boards that contain analog and digital circuits, the interface occurs at the IC that handles both signal types. If the fields for the analog and digital signals use different volumes of space there will be no cross talk. This includes traces, components, connectors, and ground planes. Separate ground planes are never needed.
Coaxial (Section 4.5): A round outer cylindrical conductor geometry with a round center conductor. The outer conductor can be braid or a thin hollow metal tube. A coaxial cable is usually built to have a specific characteristic impedance. A low characteristic impedance requires that the conductor spacing be very small compared to the cable diameter.
Circuit board resonance (Section 4.6):
Decoupling transmission line (Section 4.4): A four-terminal capacitor where the electrical length of the transmission path is defined and the characteristic impedance at the load terminals is controlled in value.
Dielectric losses (Section 4.12): The heating of the dielectric that results when fields in the dielectric change amplitude.
Four-terminal device (Section 4.4): A capacitor with four terminals uses two to introduce energy and two to draw energy out the conductor geometry. Resistors can be built four terminals where two terminals connect to the source of current and two inner terminals are used to measure the voltage drop across the exact resistance specified.
Ideal voltage source (Section 4.2): A source of voltage that does not sag when any load is applied or removed. A zero impedance source.
Interposer board (Section 4.5): An intermediate connecting surface between a die and its mounting hardware.
Loop area (Section 4.2): The area between connecting leads that carry current. The area between a trace and a ground plane is a loop area. The area formed by the power supply leads to a die is a loop area. The connecting leads of a capacitor form a loop area.
Loss tangent (Section 4.12): When a sine wave field propagates in a dielectric, there is a phase shift that results from losses that is not associated with wave position. The tangent of this loss phase shift angle in 1 in of transmission is used as a measure of dielectric loss. The loss in decibel per inch is given by Equation 4.6.
Maxwell's equations (Section 4.11): A set of differential equations that describe all electrical activity in terms of fields.
Natural frequency (Section 4.7): The frequency where the reactance of the capacitance equals the reactance of a series or parallel inductance. The reciprocal of 2π(LC)1/2.
Port (Section 4.3): A terminal that carries a voltage or a signal into or out of a component or device. In theory, the port also includes the return conductor.
Power time constant (Section 4.2): The length of time it takes to increase the energy flow in a section of transmission line to within 37% of a final value.
Resonant circuit (Section 4.6): A parallel or series capacitor and inductor. Any active circuit that responds like a series or parallel inductor and capacitor.
Service entrance (Section 4.1): The utility power entry point in a facility.
Skin depth (Section 4.11): The depth where a sinusoidal current that flows in a conductor is reduced to 37% of its surface value. Skin effect has a complex effect on the character of the leading edge of a step function. In general, the rise time increases over distance.
Split ground (Section 4.13): When a conducting plane is not continuous under a trace, the plane is said to be split. The split is often used to allow one or more traces to reach an inner point or the split can involve islands of ground or power. These splits cause return current to move along the edges of the split. This pattern of current flow is classic to slot antennas. The slot causes wave reflections and radiation.
Wave (Section 4.1): The electromagnetic energy that flows along a transmission line after a switch closure connects a voltage to the line. Energy is reflected when a wave reaches a transition in characteristic impedance.
Notes
1 The logic traces that are connected to the power leads inside an IC must sag when the power voltage sags. All of these connected logic conductors will then supply some energy to any new loads switched on to the IC. Reflections on these logic lines will return energy to the switched load.
2 The idea of “bringing back energy” is explained as follows: When a load is first connected to a transmission line, a wave carries energy into the load from the electric field in the transmission line. The wave that leaves the point of connection reflects from every discontinuity. When this discontinuity is a branch circuit, the reflected wave usually carries energy back to the load. When this wave reaches the load, another reflection occurs. A fraction of this wave propagates (transmits) into the load increasing the voltage. The capacitor is the dominant transmission line of the system. In a sense the reflection of waves brings energy back to the capacitor and the load.
3 This assumes that the waves travel in a dielectric with a relative dielectric constant of 4.