digital-brain/content/book/morrison16_groun_shiel.md

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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

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 1 shows the shape of the \(H\) field around a long, straight conductor carrying a direct current \(I\).

{{< figure src="/ox-hugo/morrison16_H_field.svg" caption="Figure 1: The \(H\) field around a current-carrying conductor" >}}

The magnetic field is a force field. This force can only be exerted on another magnetic field. The direction of the force, the direction of the current flow and the direction of the field lines are all perpendicular to each other.

Ampere's law

Ampere's law states that the integral of the \(H\) field intensity in a closed-loop path is equal to the current threading that loop

\begin{equation} \label{eq:ampere_law} \oint H dl = I \end{equation}

The simplest path to use for this integration is the one of the concentric circles in Figure 1, where \(H\) is constant and \(r\) is the distance from the conductor. Solving for \(H\), we obtain

\begin{equation} H = \frac{I}{2 \pi r} \end{equation}

And we see that \(H\) has units of amperes per meter.

The solenoid

The magnetic field of a solenoid is shown in Figure 2. The field intensity inside the solenoid is nearly constant, while outside its intensity falls of rapidly.

Using Ampere's law \eqref{eq:ampere_law}:

\begin{equation} \oint H dl \approx n I l \end{equation}

{{< figure src="/ox-hugo/morrison16_solenoid.svg" caption="Figure 2: The \(H\) field around a solenoid" >}}

Faraday's law and the induction field

When a conducting coil is moved through a magnetic field, a voltage appears at the open ends of the coil. This is illustrated in Figure 3. The voltage depends on the number of turns in the coil and the rate at which the flux is changing.

{{< figure src="/ox-hugo/morrison16_voltage_moving_coil.svg" caption="Figure 3: A voltage induced into a moving coil" >}}

The magnetic field has two measured. The \(H\) or magnetic field that is proportional to current flow. The force field representation that induces voltage is called the \(B\) or induction field. The relation between \(B\) and \(H\) fields is given by:

\begin{equation} \label{eq:relation_B_H} \boxed{B = \mu_R \mu_0 H} \end{equation}

where the factor \(\mu_0\) is the permeability of free space and \(\mu_R\) is the relative permeability of the medium.

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.