17 KiB
+++ title = "Grounding and shielding: circuits and interference" author = ["Thomas Dehaeze"] draft = false +++
Tags :
- Reference
- (Morrison 2016)
- Author(s)
- Morrison, R.
- Year
- 2016
Voltage and Capacitors
This first chapter described the electric field that is basic to all electrical activity. The electric or \(E\) field represents forces between charges. The basic charge is the electron. When charges are placed on conductive surfaces, these forces move the charges to positions that store the least potential energy. This energy is stored in an electric field. The work required to move a unit of charge between two points in this field is the voltage between those two points.
Capacitors are conductor geometries used to store electric field energy. The ability to store energy is enhanced by using dielectrics. It is convenient to use two measures of the electric field. The field that is created by charges is called the \(D\) field and the field that results in forces is the \(E\) field. A changing \(D\) field represents a displacement current in space. This changing current has an associated magnetic field. This displacement current flows when charges are added or removed from the plates of a capacitor.
Introduction
{{< figure src="/ox-hugo/morrison16_field_conf.png" caption="Figure 1: Field configurations around a shieded conductor" >}}
Charges and Electrons
The electric force field
Field representation
The definition of voltage
Equipotential surfaces
The force field or \(E\) field between two conducting plates
Electric field patterns
The energy stored in an electric field
Dielectrics
The \(D\) field
Capacitance
Mutual capacitance
Displacement current
Energy stored in a capacitor
Forces in the electric field
Capacitors
Dielectric absorption
Resistance of plane conductors
Magnetics
This chapter discusses magnetic fields. As in the electric field, there are two measures of the same magnetic field. The \(H\) field is the direct result of current flow. The \(B\) field is the force of induction field that operates motors and transformers. As in the electric field, the magnetic field is represented by field lines. The \(B\) field lines are continuous and form closed curves. The \(H\) field flux lines follow the \(B\) field lines but change intensity depending on the permeability of the material in the magnetic path.
In this chapter, the movement of electrical energy into inductors or across transformers is discussed. This extends the ideas that both fields are need to move energy. Both electric and magnetic fields are need in transformers action or to place energy into an inductor. It will be shown that iron cores in transformers reduce the magnetizing current so that transformer action is practical at power frequencies. The idea that a changing electric field creates both a displacement current and a magnetic field discussed in Chapter 1. In this chapter, it is shown that a changing magnetic field produces both an electric field and voltages. Both fields must be in transition before an electrical energy can be moved.
Magnetic Fields
In a few elements, the atomic structure is such that atoms align to generate a net magnetic field (neodymium, iron, cobalt, ...).
The flow of electrons is another way to generate a magnetic field.
The letter \(H\) is reserved for the magnetic field generated by a current.
{{< figure src="/ox-hugo/morrison16_H_field.svg" caption="Figure 2: The \(H\) field around a current-carrying conductor" >}}
Ampere's law
The solenoid
Faraday's law and the induction field
The definition of inductance
The energy stored in an inductance
Magnetic field energy in space
Electron drift
Digital Electronics
This chapter shows that both electric and magnetic field are needed to move energy over pairs of conductors. The idea of transporting electrical energy in field is extended to traces and conducting planes on printed circuit boards. Logic signals are waves that carry field energy between points on the board. These waves are reflected and transmitted when different transmission lines are interfaces. There are several sources of first energy that play a role in circuit performance. These sources are connected logic, the ground/power plane structure, and decoupling capacitors. Decoupling capacitors are actually short stub transmission lines that supply energy.
The use of vias in the transmission paths is discussed in detail. The fact that energy cannot pass through a conducting plane is stressed. Limiting interference coupling in an A/D converter is a problem in keeping analog and logic fields separated. Terminating balanced transmission lines is also discussed.
The concept of displacement current and its associated magnetic field is important. These ideas show how field energy flows into a transmission line and is placed into capacitance at the leading edge of the wave. Radiation occurs at the leading edge of a wave as it moves down the transmission line.
Introduction
The Transport of Electrical Energy
Transmission Lines–Introduction
Transmission Line Operations
Transmission line field patterns
A terminated transmission line
The unterminated transmission line
A short circuit termination
The real world
Sine waves versus step voltages
A bit of history
Ideal conditions
Reflection and tramission coefficients
Taking energy from an ideal energy source
A capacitor as a transmission line
Decoupling capacitors and natural frequencies
Printed circuit boards
Two-layer logic boards
Vars
The termination of transmission lines
Energy in the ground/power plane capacitance
Poynting's vector
Skin effect
Measurement problems: ground bounce
Balance transmission
Ribbon cable and connectors
Interfacing analog and digital circuits
Analog Circuits
This chapter treats the general problem of analog instrumentation. The signals of interest are often generated while testing functioning hardware. Tests can take place over time, in a harsh environment, during an explosion, during a flight, or in a collision. The signals of interest usually have dc content and can be generating from floating, grounded, balanced or unbalanced transducers. These transducers may require external balancing, calibration, or excitation. Accuracy is an important consideration. Where data must be sampled, the signals may require filtering to avoid aliasing errors. The general two-ground system is examined. Protecting signals using guard shields, transformer shields, and cable shields is described. The use of feedback and tests for stability in circuit design is considered. Strain-gauge configuration, thermocouple grounding, and charge amplifiers are discussed.
Introduction
Instrumentation
History
The basic shield enclosure
The enclosure and utility power
The two-ground problem
Instrumentation and the two-ground problem
Strain-gauge instrumentation
The floating strain-gauge
The thermocouple
The basic low-gain differential amplifier (forward referencing amplifier)
Shielding in power transformers
Calibration and interference
The guard shield above 100kHz
Signal flow paths in analog circuits
Parallel active components
Feedback stability - Introduction
Feedback theory
Output loads and circuit stability
Feedback around a power stage
Constant current loops
Filters and aliasing errors
Isolation and DC-to-DC converters
Charge converter basics
DC power supplies
Guard rings
Thermocouple effects
Some thoughts on instrumentation
Utility Power and Facility Grounding
This chapter discusses the relationship between utility power and the performance of electrical circuits. Utility installations in facilities are controller by the NEC (National Electrical Code). Safety and lighting protection requires that facilities connect their systems to earth. Designers of electric hardware use utility power and also make electrical connections to earthed conductors. This sharing of the earth connection creates many problems that are considered in this chapter.
Ground planes and isolation transformers can be used to limit interference. The role of line filters, equipment grounds, and ground planes in facilities is explained. The problems associated with using isolated ground conductors are discussed. Lighting protection in facilities and for watercraft is a big safety issue. The fact that current cannot enter the water below the water line is considered. The battery action that causes the metal on boats to corrode is discussed. The grounding methods in the Pacific Intertie are unique. Solar winds can disrupt power distribution and damage oil pipelines.
Introduction
Semantics
Utility power
The earth as a conductor
The neutral conneciton to earth
Group potential differences
Field coupling to power conductors
Neutral conductors
\(k\) factor in transformers
Power factor correction
Ungrounded power
A request for power
Earth power currents
Line filters
Isolated grounds
Facility ground - Some history
Ground planes in facilities
Other ground planes
Ground planes at remote sites
Extending ground planes
Lightning
Lightning and facilities
Lightning protection for boats and ships
Grounding of boats and ships at dock
Aircraft grounding (fueling)
Ground Fault Interruption (GFI)
Isolation transformers
Radiation
This chapter discusses radiation from circuit boards, transmission lines, conductor loops, and antennas. The frequency spectrum of square waves and pulses is presented. Matching of impedances is required to move energy from a transmission line to an antenna so that it can radiate this energy into free space. Common-mode and normal-mode coupling of fields to conductors is considered. The concept of wave impedance and its relation to shielding is considered. Interference can be analyzed by using a rise-time frequency to represent pulses or step functions.
Effective radiated power from various transmitters is presented. The field intensities for lightning and electrostatic discharge are given. Loops generate low-impedance fields that are often difficult to shield. Simple tools for locating sources of radiation are suggested.
Handling radiation and susceptibility
Radiation
Sine waves and transmission lines
Approximations for pulses and square waves
Radiation from components
The dipole antenna
Wave impedance
Field strength and antenna gain
Radiation from loops
E-field coupling to a loop
Radiation from printed circuit boards
The sniffer and the antenna
Microwave ovens
Shielding from Radiation
Cable shields are often made of aluminum foil or tinned copper braid. Drain wires make it practical to connect to the foil. Coaxial cables have a smooth inner surface that allows for the circulation of current and provide control of characteristic impedance. Transfer impedance is a measure of shielding effectivity. Multiple shields, low-noise cable, and conduit each have merits that are discussed.
The penetration of fields into enclosures is considered. This includes independent and dependent apertures, the wave penetration of conducting surfaces, and waveguides. The use of gaskets, honeycombs, and backshell connectors are described. Handling utility power, line filters, and signal lines at a hardware interface are discussed. Methods for limiting field penetration into and out of a screen are offered.
Cables with shields
Low-noise cables
Transfer impedance
Waveguides
Electromagnetic fields over a ground plane
Fields and conductors
Conductive enclosures - Introduction
Coupling through enclosure walls by an induction fields
Reflection and absorption of field energy at a conducting surface
Independent apertures
Dependent apertures
Honeycombs
Summing field penetrations
Power line filters
Backshell connectors
H-field coupling
Gaskets
Finger stock
Glass apertures
Guarding large transistors
Mounting components on surfaces
Zappers
Shielded and screen rooms
Bibliography
Morrison, Ralph. 2016. Grounding and Shielding: Circuits and Interference. John Wiley & Sons.