EMC System Design: A Systematic Methodology

There are three basic phases in the design of a system (Asimow, 1962): 1) feasibility study; 2) preliminary design; and 3) detailed design. The relative content and extent of each of the design phases are determined completely by programmatic considerations which reflect knowledge concerning the market and the cost of the design. The flow and subsequent iterations in the total design of every system will however remain fairly consistent with regard to methodology and order. During the total design effort of all electronic systems, many considerations must be taken into account, one of which is electromagnetic compatibility (EMC).

The economic viability of a system is dependent upon the costs associated with the design and manufacture of the system. The implementation of EMC design considerations and constraints into the design throughout design phases can significantly reduce the manufacturing cost and therefore enhance the economic viability of the system.

The purpose of this article is to provide an order in the design of the electronic system such that EMC considerations can be evaluated throughout the design effort, producing an electromagnetically compatible system at a minimal cost.

The steps that make up the methodology of the EMC design flow diagram are as follows:

1. Compile system electromagnetic interface requirements
2. Define subsystem requirements necessary to meet system requirements
3. Define circuit requirements necessary to meet subsystem requirements
4. Define circuit interfaces
5. Package circuits within subsystems
6. Establish wiring diagram
7. Design circuits
8. Lay out circuit boards (package circuits)
9. Package system

As can be seen Figure 1, many of the steps listed above have a number of arrows entering and leaving them. The flow from one step to another is dependent upon the phase of the design and the results of the analysis performed during the step.

Feasibility Study

The feasibility study culminates in a set of useful solutions which carry with them a probability of the benefits-to-cost ratio being commensurate with the demand of the market. The steps and processes comprised in the feasibility study are as follows:

1. Needs analysis
2. Design problem—systems identification
3. Synthesis—design concept
4. Physical realizability—physical analysis
5. Economic worth—economic analysis
6. Financial feasibility—financial analysis

The design problem identifies and formulates the results of the needs analysis into a series of desired system outputs, and the synthesis fits the separate concepts together to produce an integrated system or series of systems yielding an expected cost of the design of each of the proposed systems. The results of these two steps are analyzed in the three successive steps to obtain a probability of the benefits-to-cost ratio being commensurate with the demand of the market. The study is concluded when this probability is high enough to warrant the expenditure of money and engineering hours necessary to complete the preliminary design phase.

The steps in the design flow during the feasibility study that are pertinent to EMC as illustrated in Figure 1 are: 1) compile the system electromagnetic interface requirements; 2) define subsystem requirements necessary to meet system requirements; 3) define circuit requirements necessary to meet subsystem requirements; 4) design circuits; and 5) package system. Steps 3 and 4 are secondary steps and are dependent upon the state-of-the-art of the design. The investigation to be performed during each of the five steps is as follows:

1. Compile system electromagnetic interface requirements, based on the characteristics of the other systems with which the concerned system must operate. This step is performed during the needs analysis, where the information is used to help determine the system parameters. The electromagnetic interface requirements are:
   1. Tabulate all required sources of RF energy for the proposed system, such as transmitters and computers, including the range of frequencies and the desired levels of energy.
   2. Tabulate the external electromagnetic environment in which the system is expected to operate.
   3. Tabulate all the system requirements for the reception of electromagnetic energy, listing the expected frequency range, dynamic range of power, type of modulation, etc.
2. Define subsystem requirements to meet system requirements. This step is performed during the system identification and design concepts processes, in which the requirements of the market are evaluated in terms of known possible solutions. Since the design of a system is always a compromise between a number of apparently incompatible needs, a number of proposed subsystems are evaluated simultaneously, each of which meets a different subset of the entire set of the market needs at different costs. The subsystems selected to compose the total system are those which possess the maximum benefits-to-cost ratio. The electromagnetic compatibility requirements and subsequent cost form a part of the benefits-to-cost ratio being maximized. The resultant subsystems should be listed along with the following electromagnetic characteristics:
   1. List all high-frequency electromagnetic energy generated by each subsystem. This will include the fundamental and all expected harmonics. If an electromagnetic interference (EMI) specification forms a part of the requirements, the applicable specification requirements are to be listed.
   2. List all receivers or subsystems that receive information in the form of electromagnetic energy, stipulating the frequency or frequencies to which they are to respond and the variance of the strength of the desired signal. List all applicable EMI specification requirements.
3. Define circuit requirements necessary to meet subsystems requirements; Each of the subsystems comprises a number of circuits and thus the output of the subsystem depends on the outputs of the circuits. In maximizing the benefits-to-cost ration of the system, the circuit requirements must often be evaluated to ensure that the circuit can be designed at a cost commensurate with the output.
4. Design circuit. The circuit design step signifies that the success of a given-system in providing the desired outputs is often dependent upon the capability of designing a specific circuit. The design of that circuit must be completed to the point of a) providing assurance that the circuit will function as necessary, and b) obtaining a relatively accurate cost estimate.
5. Package system. A fairly accurate estimate of the size and shape of the final system must be obtained; therefore, an estimate of the size and shape of each of the subsystems must also be considered. Placement of the subsystem in the system should be attempted along with the placement of all antennas that will be needed by the system for communication with the environment. In order to place the subsystems and antennas, the previous history concerning electromagnetic interference with regard to such placement must be considered; this includes coupling of electromagnetic energy between subsystems, grounding and bonding problems, and antenna coupling. All decisions concerning electromagnetic compatibility must be tempered by other compatibility requirements such as thermal, vibration, shock and corrosion considerations.

Preliminary Design

The intent of the preliminary design is to establish an overall concept for the project, which will serve as a guide for the detailed design. The investigations performed are structured to obtain a high enough degree of certainty of having an economically viable product to warrant beginning the detailed design phase as well as portions of production planning. The steps and processes included in the preliminary design (Asimow 1962) are as follows:

1. Selection of design concept
2. Mathematical archetypes
3. Sensitivity analysis
4. Compatibility analysis
5. Stability analysis
6. Optimization
7. Projection into future
8. Prediction of behavior
9. Testing
10. Simplification

The scope of each step is dependent upon the particular system under consideration. The step that we are concerned with is compatibility and, in particular, electromagnetic compatibility. The results of the electromagnetic compatibility analysis are guided by the three previous processes (i.e., selection of design concept, mathematical archetypes, and sensitivity analysis). The mathematical and sensitivity analyses must often be verified by tests.

The preliminary design phase is the most important of the three design phases with regard to EMC, since the system, subsystem and circuit interfaces are defined during this phase. The steps in the design flow during the preliminary design phase that are pertinent to EMC, as illustrated in Figure 1 are: 1) define circuit requirements necessary to meet subsystem requirements; 2) define circuit interfaces; 3) package circuits within subsystems; and 4) establish wiring diagram. Defining subsystem requirements necessary to meet system requirements and packaging the system are within the scope of the preliminary design but they are secondary since the need for the investigations depends on the results of the evaluation performed during the four primary steps. The investigations to be performed by the EMC preliminary design phase are as follows:

1. Define circuit requirements necessary to meet subsystem requirements. Defining the circuit requirements involves tabulating every function that each of the circuits is to perform with regard to the system requirements. All power and signal lines leading to and from each circuit must be listed and accounted for.
2. Define circuit interfaces. In defining the circuit interfaces, every power and signal input for each of the proposed circuits must be investigated in detail, listing:
   1. Each signal pair emanating from the circuit, giving the voltage and current waveforms;
   2. Each signal pair received by the circuit, giving the maximum and minimum voltage and current waveforms necessary for the proper functioning of the circuit; and
   3. Each power line and its return, listing the expected maximum and minimum values of voltage and current necessary for a properly functioning circuit. Maximum tolerable noise and ripple values must be listed. If the internal circuit components are sensitive to any specific frequency, these frequencies, along with the subsequent maximum allowable power, must be stipulated.

Upon completion of the three steps listed above, an iterative process must be performed to ensure that the circuits as previously defined will be compatible with the required interfaces. If they are not, then the circuits must be redefined until compatibility at this point in the design is obtained. If this requires information that is not known for a particular circuit, then the circuit must be evaluated in detail. It is possible at this point that the subsystem requirements will have to be re-evaluated.

3. Package circuits within subsystems. The size and shape of the circuit board or package of every circuit within each of the subsystems should be evaluated. During the evaluation, care should be taken to ensure that the space allocated for each circuit is sufficient for the required function.
4. Establish wiring diagram. A tentative wiring diagram tying all the circuits and subsystems together with the required wire link should be established. The wiring diagram should include the following:
   1. The size and length of every wire
   2. The type of wire handling (i.e., shielded twisted signal pair, shielded twisted power lines, etc.)
   3. The type of grounding to be used, such as single point grounding, multiple point grounding or hybrid system. The expected current waveform flowing through any common wire or chassis return must be stipulated, calculating the expected voltage potential across the common return.
   4. A list of all wires to be carried in each wire bundle
   5. A list of the expected separation of bundles
5. By the use of the information tabulated above, previous experience, mathematical models and laboratory experiments, the voltage and current waveforms at each of the circuit interfaces are to be re-evaluated by assessment of the expected loss and gain of electromagnetic energy to the predetermined waveforms, due to capacitive, inductive and resistive coupling. Because of EMC considerations, the next step in the design flow depends on the results of the investigation just performed. If the expected circuit interfaces are compatible, then the preliminary EMC design phase is complete, with the next step being the design of the circuits. If the expected interfaces are not compatible, then the next step in the EMC design process will be either to re-evaluate the system packaging or to redefine the circuit requirements. An iterative process of re-evaluating the system packaging with respect to wiring considerations is the first choice.
6. Package system. The capability to interface two or more circuits often depends on the placement of the circuits in the entire system. Electromagnetic interference can be minimized through an iterative process that results in an optimum placement of the circuits and subsystems within the subsystems and systems. Upon the completion of the repackaging iteration, if there is still an expected interface problem, then it will be essential to redefine the subsystem requirements in such a way that the expected interference problem is minimized. If the expected interface problem is in the power phase, then additional power supplies, regulators or low-pass filters may have to be added to various subsystems. Any change in the subsystem requirements will automatically necessitate a re-evaluation of all subsequent steps.

Detailed Design

The detailed design phase carries the overall design concept, as developed in the preliminary design, to the final piece of hardware. The point of termination of the preliminary design and the beginning of the detailed design is dependent upon economic considerations and is usually consistent with the ability to define to a very high degree the physical realizability of one or more applicable design concepts. The steps and processes comprised in the detailed design phase (Asimow 1962) are as follows:

1. Preparation for design
2. Description of subsystem
3. Description of components
4. Description of parts
5. Assembly drawings
6. Experimental construction
7. Product test program
8. Analysis and prediction
9. Redesign

The steps comprised in the flow of the design during the detailed electromagnetic compatibility design phase, as illustrated in Figure 1 are: 1) design circuits; 2) lay out circuit boards; 3) establish wiring diagram; and 4) package system. Designing the circuits, establishing the final wiring diagram and packaging the system generally take place concurrently. The final package configuration of the system usually is not completed before the wiring diagram is completed, and likewise the wiring diagram usually is not completed before the layout of the circuit boards. However, this need not be the case, and the order of completion is dependent upon the capability of the system design team to estimate the problem areas prior to the actual design of the system. The investigations to be performed during the detailed EMC design phase are as follows:

1. Design circuits. In designing the circuits, care must be taken by each of the circuit designers that the interface characteristics of the circuit in the final system are used during the design. Low-pass filters that are to be used in conjunction with power or signal lines for the suppression of high frequencies must be designed along with the circuit and used as interfaces during all circuit and subsystem testing. Grounding and bonding techniques to be used in the system should be simulated in the circuit breadboard design and in all circuit, subsystem and system tests. The exact type of solid-state components that will be used in the final design of a circuit should be used during the breadboard design because of the large variance in the listed as well as unlisted component parameters between separate manufacturers. Before the design of any circuit is completed, the response characteristics of the circuits over the expected system power and signal variations should be tested.
2. Lay out the circuit boards (package circuits). In laying out the circuit boards or packaging the individual circuits, it is extremely important to evaluate the electrical as well as the mechanical considerations. Some of the electrical considerations of importance are:
   1. All storage capacitors should be place close to the desired need. If the
      demand is for both high current and high frequency, it may be necessary to use two
      capacitors in parallel, one for high current-low frequency and the other for high
      frequency-low current.
   2. The highly susceptible components should be separated from the relatively noisy components on the circuit boards.
   3. When components are used for the suppression of high-frequency energy generated by the components on the circuit board, the suppression components should be placed as close to the source of interference as possible.
   4. When ground planes are used on circuit boards, the ground planes should be bonded to the chassis if possible; if this is not possible, the chassis ground wire should be as short as applicable.
   5. If low-pass filters are to be used on the circuit boards to reduce high-frequency energy on the incoming power lines, the components should be as close to the incoming lines as possible; the capacitors should be line-to-line, using only small values of line-to-ground capacitors when necessary.
3. Establish wiring diagram. The major requirements for EMC in the establishment of the wiring diagram have been considered. To ensure that the electrical response of each consecutive system is consistent (within the applicable limits), every detail concerning the wiring diagram must be well documented, including how and where every shield on each shielded wire is tied off. Care should be taken to ensure adequate separation between leads going to susceptible circuits and those going to relatively noisy circuits.
4. Package system. The major considerations for the EMC are a) the placement of the subsystems within the system and the circuits within the subsystems for the reduction of electromagnetic coupling, and b) the utilization of the subsystem cases and system chassis to provide shielding between the highly sensitive circuits and the noisy circuits. During the packaging step, extreme care must be taken to ensure that adequate bonding is provided between the subsystem cases and the system chassis.

Summary

The extent and content of each of the three phases of the design of a system are completely dependent upon economic considerations, and each phase is utilized to increase (or decrease) the probability of producing an economically viable system. Assuming that producing the system will be economically viable, regardless of when the certainty of the viability of the system is known, the general flow in the design procedure will remain fairly constant.

With regard to the EMC of the system, the most important single aspect is that the interfaces between the subsystems and the circuits within the system are compatible, where the compatibility is dependent upon the wire treatment and the packaging of the circuits, subsystems and systems. To obtain an electromagnetically compatible system at a minimum cost in terms of time, weight and size of the system, engineering hours and money, it is essential that the expected interface for every power and signal line leading to and from each proposed circuit be evaluated prior to the final design of any of the circuits.

It is recognized that deviations from this sequence are and, for that matter, should be made. However, the sequence described here provides a basis for examination of the problem of systems generation in order to optimize the effectiveness of electromagnetic compatibility activity. Consideration of the degree to which the sequence can be followed in light of programmatic factors (schedules, procurement plans, use of existing hardware, organizational relationships, etc.) can identify hazards and permit the redefinition of EMC activities tailored to the programmatic constraints.

Selected Bibliography

Asimow, M., Introduction to Design. Prentice-Hall, Inc., Englewood Cliffs, N.J., 1962.

Engleman, J., “Guidelines for The Design Review of Circuits,” Electronics Industries. May 1966.

Hall. A. P., A Methodology for Systems Engineering. D. Van Nostrand Company, Inc., Princeton, N. I, 1962.

Kunkel, George M., “Idealized Methodology of EMC System Design,” Aerospace and Electronic Systems Eastcon Technical Convention, October 1967, Washington D.C.

George M. Kunkel is the founder and chief engineer of Spira Manufacturing Corporation. He holds both B.S & M.S. degrees in Engineering from UCLA, and has been active with the IEEE EMC Society for 50 years, serving as a chairman of multiple technical committees and working groups. Kunkel has authored over 100 EMC technical papers and revised several SAE-ARP test standards. He can be reached at George@spira-emi.com.