5.18 Glossary
Apertures (Section 5.16): Any opening that allows electromagnetic energy to enter a conductive enclosure. There is no attenuation of an external field (worst case) when the aperture's maximum dimension is one-half the wave length. The field attenuation for a smaller opening is the ratio of half wave length to aperture opening.
Apertures that are spaced so that surface currents can circulate freely around the aperture are independent. The field penetration for several independent apertures is additive. Dependent apertures such as screens, seams, or ventilation arrays act as one aperture.
Coax: See shielded cable.
Common (Section 5.11): A term used to mean ground or the conductor at zero volts. This conductor is sometimes called the reference conductor. Terms such as signal common, output common, digital common, and power supply common are often used.
Cross coupling/cross talk (Section 5.4): The coupling of wave energy from one trace to a nearby trace. The coupling involves both forward and reverse traveling waves. Coupling only occurs where the wave amplitude is in transition. Reverse wave coupling causes most interference problems. The amplitude depends on rise time. The reverse wave lasts twice as long as the coupling time. The forward coupled wave is a pulse that increases in amplitude with time. Forward coupling is apt to be a problem on long outer traces.
Embedded microstrip (Section 5.6): The outer layer traces are covered by a dielectric. These traces are not plated with solder so the skin effect resistance is that of copper and not solder. Forward wave cross coupling is reduced, because the E field is reduced in the dielectric.
Envelope (Section 5.2): A graph of permitted values. The graph usually shows the maximum and minimum permitted values as a function of time or of position.
Equipment ground (Section 5.12): An NEC term. Any conductor that could come in contact with a “hot” or ungrounded power conductor. All equipment grounds must be bonded together to provide a short and immediate low inductance return path for a fault current so that a breaker will interrupt the power. Equipment grounds can be multiply connected to earth.
Electrostatic discharge (Section 5.17): The arcing that results when an insulated object accumulates a charge large enough to ionize the air. This charge results in an electric field that surrounds the object. When this field builds up on a human and the human gets near a grounded metal object a pulse results that discharges the accumulated charge.
ESD: See electrostatic discharge.
Ground (Section 5.12): In power terminology, this word means an earth connection or its equivalent. In circuit terminology, it often means the low or common side of a power supply.
Grounding (Section 5.12): The connection between a circuit common and a larger external conducting surface such as a metal housing, a rack, or the earth.
Grounding rod (Section 5.12): A conductor buried in the earth to make an electrical connection to the earth. Typical connections to earth are about 10 ohm at low frequencies. In the power industry, this conductor is called a grounding conductor. It is used to provide a lightning path for the neutral or grounded conductor at the service entrance to a facility.
Grounding rods do not control electromagnetic interference. They are required by code to provide lightning protection and electrical safety.
Guard shield (Section 5.13): A shielding method used in analog instrumentation to protect very small analog signals carried over long cables. This shield is connected to signal ground at the signal source. It is brought into the instrumentation to shield the signal, but it is not connected to the input circuitry. The common-mode signal that is rejected is derived from the signal pair. The guard shield is terminated on output ground external to the instrumentation at frequencies above 100 kHz.
NEC (Section 5.13): The National Electrical Code. This code is law in most cities in the United States. It represents permitted practice in power wiring for facilities. The code protects against electrical shock, fire, and lightning. There are many ways to build a facility and have it satisfy the code. All interference problems can be resolved without violating the code. The words used in the code are defined in the code. Power engineers use these definitions.
Net list (Section 5.3): A list of traces on a circuit board that have similar characteristics.
Noise budget (Section 5.4): The range of values a signal can take and still allow a logic circuit to perform correctly. The budget includes items such as cross talk, errors caused by variation trace characteristic impedance, variations in terminating resistors, line driver limitations, and temperature rise.
Ohms per square (Section 5.6): The ratio of current to voltage drop in a square of thin conducting material when the current flow is uniform between opposite edges on one surface. The resistance is independent of the size of the square.
Resistor network (Section 5.11): A group of resistors packaged for simple mounting on a circuit board. The packaging can be SIP or DIP. In high speed logic, the lead lengths and spacing of these structures can be a problem.
Separately derived (Section 5.12): A power term used to describe a power source that has its own neutral. Examples are a distribution transformer or an auxiliary power source. A computer power center or CPC has its own distribution transformer with a grounded neutral. In all cases, the added neutral is connected once to the one grounding electrode system of the facility. One advantage to using a separately derived system is that it allows a second neutral conductor not used by other hardware in a facility. Neutral voltage drops can be a source of common-mode interference that enters electronics through the power transformers.
Shielded cable (Section 5.14): Shielded cables are conductors routed inside of a conducting sheath. Cables that are used to transport signals at frequencies below 100 kHz are often covered by a conductive braid. To control characteristic impedance the outer conductor must maintain shape and the conductor spacing and dielectric constant must be controlled. Single conductor cable intended for high frequency work is called coax.
Shielding effectivity: The ratio of performance before and after the shield is included in the design.
Test strip (Section 5.10): A trace added to a panel of circuit boards used to test characteristic impedance.
Transfer impedance (Section 5.13): The transfer of external field energy into a cable. It is measured as the ratio of external surface current to voltage measured at the output terminations for a unit length of cable. Since transferred energy travels in both directions, the voltage used in this ratio is double the signal measured at one termination.
The shielding effectivity of braid can be negligible at frequencies above 100 MHz. Two shields or very dense braid can be an improvement. The lowest transfer impedance is found in thin wall solid conductor shielding. This tubing can be corrugated to improve flexibility. The wall thickness can be just a few mils to be effective electrically.
Wave guide (Section 5.17): A hollow cylindrical conductor that can support wave transmission without a center conductor. The lowest frequency that can be transported has a half wavelength equal to the aperture opening. For frequencies lower than this value, the wave guide attenuates electromagnetic field energy exponentially. The attenuation in decibels is 30d/h, where d is the depth and h is the dimension of the maximum opening. This attenuation is in additional to the aperture attenuation. See aperture.
Wave guide construction (Section 5.17): Apertures or seams that have depth. An example might be a folded conducting lip or a honeycomb.
Zapper (Section 5.17): A test device that generates ESD pulses.