EMC Analysis 101

Electromagnetic compatibility (EMC) is the ability of systems, equipment, and devices that utilize the electromagnetic spectrum to operate in their intended operational environments without suffering  unacceptable degradation or causing unintentional degradation because of electromagnetic radiation or response. It involves the application of sound electromagnetic spectrum management; system, equipment, and device design configuration that ensures interference-free operation; and clear concepts and doctrines that maximize operational effectiveness.

• EMC encompasses the discipline of controlling/preventing the degrading effects of Electromagnetic Interference (EMI).

Basic Elements Of EMI

An emission source (emitter) and a susceptible receptor are required to make EMI possible. Emitters are natural or man-made sources of noise and interference. Natural sources can be terrestrial such as atmospheric or precipitation static; or extra-terrestrial such as the sun, cosmic noise, or radio stars. Man-made sources can be devices such as communications-electronics (C-E) equipment or ignition systems.

Receptor refers to a class of devices, equipment, and/or systems which, when exposed to conducted and/or radiated electromagnetic energy from emitting sources, will either be degraded or malfunction in performance. Examples of receptors susceptible to EMI include communications receivers, computing devices, electronically guided/triggered ordnance, radars and new systems.

Most electronic devices are susceptible to emissions generated either internally or by other devices and many act as both emitters and receptors of EMI. Dependent upon the emitter and receptor, the resulting effects of EMI can take many forms (e.g., interference on the telephone line due to an electrical storm or radio interference due to an arc welder).

The methods of coupling between emitters and receptors of electrical signals are divided between radiation and conduction. The emissions can be transmitted as electromagnetic radiation (signals radiated through space) or conducted through network cables or wires (signals traveling along interconnecting cables).

The EMI which develops between two or more discrete systems is called antenna-coupled EMI. It occurs as a result of different types of equipment and systems interacting through the electromagnetic radiation that enters or leaves the antenna from another source.

Fundamental Relationships

Degradation occurs in a receptor when an interfering signal (I) exceeds the interference threshold (I_t) set for the receptor or system under analyis. The interference threshold is based on the existence of a signal-to-interference (S/I) ratio or interference-to-noise (I/N) ratio atwhich the performance of a system changes from an acceptable to an unacceptable level.

The effect of interference on a victim receptor depends on several variables which include include the strength of the emitter, transmission medium, distance from the source, coupling mechanisms, antenna gains, and degree of susceptibility. These variables are represented by the terms which define the desired signal power and interference power illustrated by Equations 1-1, 1-2, and 1-3.

The desired signal power (S), in dBm, is represented by the following equation:

S = P_T + G_T + G_R - L_P (1-1)

for a one-way communication system and

S = P_T + G_T + G_R + σ_target - L_P (1-2)

for a two-way radar system where

P_T = transmitter power, in dBm 
L_P = path loss, in dB
G_T = interfering transmitter antenna gain, in dBi
G_R = victim receiver antenna gain, in dBi
σ_target = target cross section, in m²

For communications systems and radar systems, the effective input, on-tune, interference power (I), in dBm, is represented by the interference equation

I = PT + GT + GR - LP - FDR                                                    (1-3)

where

FDR = frequency-dependent rejection, in dB

Noise (N) is represented by the summation of receiver noise and external noise.

Along with noise, Equations 1-1, 1-2, and 1-3 illustrate the elements which make up the S/I and I/N ratios and the interference threshold (I_t) which is expressed as S/I, or I/N. Notice that the transmitter power, transmitter antenna gain, and receiver antenna gain in the interference equation are fixed values. The only elements of the interference equation which may be varied to reach EMC, such that the threshold value is not exceeded, are the path loss and/or frequency-dependent rejection.

EMC Considerations During Development

Factors which must be considered to achieve EMC can best be identified in relation to the life cycle of the device, equipment, or system of interest. The evolutionary life cycle involves the following three phases:  conceptual phase, developmental phase, and operational phase.

During the conceptual phase, selection of the right frequency band for C-E systems is most important. The following factors will have a bearing on the suitability of the choice:

• Mission requirements
• International frequency-allocation regulations
• Present and planned use of the intended band
• Electromagnetic environment in which the equipment will be deployed

During the development phase, system parameters are established, and the following major considerations for EMC of the system are evaluated:

• Adherence to standards and specifications of the design
• Compatibility of the system with other equipment which operates in the same portion of the spectrum and/or in relative physical proximity
• Effectiveness of signal transfer characteristics and processing in eliminating undesired signals
• Design characteristics that may be required to reduce any major interference impact of the equipment on the electromagnetic environment
• Design characteristics that may be required to reduce susceptibility of the equipment to its electromagnetic environment

During the operational phase, the frequency band and operating characteristics are established. The characteristics affecting EMC, which must be considered for specific operational applications, include:

• Selection of operating frequency or development of a frequency plan
• Placement of equipment and antenna to meet operational requirements, avoid radiation hazards, and control potential interactions with C-E environment
• Coordination of operating schedules with other spectrum users where required
• Delineation of operating modes and procedures that promote electromagnetic compatibility with the electromagnetic environment

The Analysis Objective and Major Types of Analyses

After the EMC factors have been considered, theobjective of the EMC analysis must be developed and the appropriate analysis chosen.

The objective of an EMC analysis also depends on the evolutionary life cycle of the device, equipment, or sytem. Generally, the objective will encompass the following areas:

• Selecting a frequency band or determining the suitability of a designated band for the system
• Analyzing the system operating characteristics to determine their adherence to EMC specifications and standards, to identify potential EMI interactions with the operating environment, and to recommend designs or approaches that will minimize the potential for interaction
• Considering the operational application intended for the equipment, determining frequency-distance separation requirements, developing compatible frequency plans, and providing operation configurations and constraints that promote EMC

The objective is applicable to a comprehensive EMC project. Due to time and resource limitations, a subset of the above objective may be more feasibile.

The next step is to review the following analyses types (refer to the following table) and select the analysis type that comes closest to satisfying the analysis requirements.

Cosite and Intersite EMC Analysis

A common EMC analysis investigates potential interference resulting from the introduction of C-E equipment into a previously established electromagnetic environment. Such analyses can be classified into two broad categories; each with its own set of analysis requirements, conditions and objectives: intrasite (i.e., cosite) or intersite. For both types of analyses, the operating environment as seen by the introduced C-E equipment is described by the geographic locations, antenna configurations, and the radio frequency (RF) characteristics of the new equipment and surrounding transmitters and receivers. The calculations for an analysis usually take into consideration antenna coupling gains and losses, propagation path loss, FDR, and other parameters associated with the transmission and reception of RF signals.

In general, an intersite analysis is differentiated from a cosite analysis by the size of the geographic area under examination. Typically, the intersite region is one to two orders of magnitude larger than that of the cosite region. Thus, it is likely that the number of frequency assignments in the intesite region. In most cases, the use of automated tools capable of manipulating large numbers of database records is required to do an intersite analysis.

The distinction made between cosite and intersite EMC analyses is arbitrary, serving only to define the extent of the electromagnetic environment that must be included in the analysis; i.e., effects caused by strong signals are generally caused by environmental emitters located nearby, while those caused by smaller signals may be produced by emitters located considerably further away.

Linear and Nonlinear Interactions

There are numerous environmental factors that degrade the performance of C-E systems, as shown in the following table. Some of these factors are categorized as being linear (having an output that varies in direct proportion to the input) and others as nonlinear (having an output that is neither directly nor inversely proportional to the input).

Linear Processes

A process is defined as linear when the outcome is directly proportional to the process input; that is

A_OUT = G A_IN

A_OUT = amplitude of the output of the process

A_IN = amplitude of the input to the process

G = constant of proportionality (often referred to as the process gain)

In essence, the transfer function of the system element performing the process is a constant.

Nonlinear Processes

In a nonlinear process,