3.16 Glossary
Common mode (Section 3.5): Refers to an average interfering signal that appears between all conductors in a cable and the local ground. The term finds use in working with balanced signal lines. It is often the ground potential difference between two pieces of hardware. A common-mode signal is not always interference. There can be many common-mode signals present at the same time. In logic, common mode is called even mode.
Cross coupling (Sections 3.4 and 3.8): The unwanted coupling of signals between traces on a circuit board.
Culprit (Section 3.8): The signal causing coupling or interference.
Critical length (Section 3.12): Half the distance traveled by a wave in its rise time.
Dipole (Section 3.2): A conductor geometry used as an antenna. Two conductors in the shape of outstretched arms driven at the midpoint by a balanced signal.
Fall time (Section 3.4): The time for a step function to change from 80% to 20% of final value.
Far field (Section 3.4): The electromagnetic field far from a radiating source.
Forward coupling (Section 3.8 and 3.11): The cross coupling that moves in the same direction as the culprit wave. Forward coupling is a pulse that lasts as long as the culprit rise time. Forward coupling increases with time and is most severe on long lines.
Ground bounce (Section 3.14): The IR drop along the ground plane or on conductors between a ground pad and the die.
Induction field (Section 3.4): The field near a loop where the H field dominates; a low impedance field; a magnetic field.
Impedance of free space (Section 3.4): The ratio of E- to H field intensity in the far field or 377 ohm.
Interference (Section 3.1): Any unwanted signal that couples to a circuit. All coupling is field related. Coupling associated with current flow is called conductive coupling. Sometimes called common-impedance coupling.
Near field (Section 3.4): The electromagnetic field near a radiating source.
Near-field/far-field interface distance (Section 3.4): The distance from a radiator where the field can be considered a far field. Beyond this distance, the wave impedance is 377 ohm. These waves are called plane waves.
Noise budget (Section 3.1): In logic, the signal level fluctuations that are permitted for normal logic operations.
Normal mode (Section 3.5): The normal signal. In balanced logic, the signal is called odd mode. In analog systems, it is the signal of interest.
Plane waves (Section 3.4): Electromagnetic waves beyond the near-field/far-field interface distance. This definition is applied to sinusoidal radiation.
Radiation (Section 3.3): The field energy that leaves a circuit and does not return.
Reverse coupling (Section 3.8): The wave that is cross coupled that moves in the direction opposite to the culprit wave.
Rise time (Section 3.4): The time for a wave to change from 20% to 80% of final value.
Rise-time frequency (Section 3.4): The frequency determined by rise or fall time in the equation 1/πτr. A signal at this frequency can be used in an analysis to determine the magnitude of response to a transient input.
Transitional electrical length or TEL (Section 3.12): The distance a wave travels in one rise time.
Victim (Section 3.8): The trace influenced by cross coupling or interference.
Wave: A general term implying the movement of electromagnetic energy.
Wave impedance (Section 3.4): The ratio of E field intensity to H field intensity at a point in space. This ratio is the characteristic impedance of a transmission line. Near a radiating source the ratio changes depending on the nature of the radiator. In the far file the ratio is 377 ohm.
Notes
1 The characteristic impedance is the ratio of E to H field intensities at all points in the space between the transmission line conductors. Each wave that is in motion must be considered separately.
2 Both rise and fall times must be considered. Use the smaller number. Rise time is usually specified as the time between 10% and 90% points. In digital circuit analysis, it is safer to consider the time between the 20% and 80% points.
3 Trace impedances are about 50 ohm compared to a free space of 377 ohm. This would imply that the coupling between adjacent traces is dominated by induction. It turns out that for stacked traces, the cross coupling is mainly capacitive (Section 3.8).
4 This assumes that G1 and G2 are not tied together near the point of measure.