In the middle of the 1970s, the United States Federal Communications Commission (FCC) began to look seriously at electromagnetic emissions from electronic data processing (EDP) equipment and office equipment (OE). This growing awareness on the part of the United States telecommunications regulation body was a result of the increasing number of computers being used by society and the increased potential for growth by licensed broadcast services due to the proliferation of electronic-computer sources. The Computer and Business Equipment Manufacturers Association (CBEMA) formed a technical subcommittee to assist in preparing an industry response to the concerns of the FCC. This paper reviews the report developed by that technical subcommittee, made public in May of 1977.
Title of Paper
The title of the published paper was Limits and Methods of Measurement of Electromagnetic Emanations from Electronic Data Processing and Office Equipment.
The paper was published by CBEMA, a trade association dedicated to expanding knowledge in the manufacture, sale, and use of member products. CBEMA was located in Washington, D. C. (CBEMA is now known as the Information Technology Industry Council and is still located in Washington, D.C.).
The report was prepared by Subcommittee 5 on Electromagnetic Interference. SC 5 was organizationally part of the Environment and Safety Committee of CBEMA.
The report generated by the Subcommittee on Electromagnetic Interference (SC5) was a result of several years of technical research by the member companies of CBEMA. It was made public “in the belief that it will be of interest and assistance to the CBEMA membership and to other manufacturers of electronic data processing and office equipment, and also to others having an interest in the general subject matter.”
In 1977, members of CBEMA had experienced very few interference issues relative to the licensed broadcast services in the USA. They attributed this partly to the high standards of engineering and manufacturing in existence for its member companies. At the same time, CBEMA acknowledged the challenge of staying abreast of the fast-moving changes inherent in the transistor-integrated circuit-computer trilogy. CBEMA, therefore, had a strong interest in electromagnetic interference issues and their potential impact on the design and manufacture of EDP and OE.
Membership of Subcommittee 5 – Electromagnetic Interference
Individual members of the Subcommittee and the companies they represented were acknowledged in the report. They are shown in Table 1.
Table of Contents
The report was 183 pages in length. It included a Title Page, Foreword, Table of Contents, Scope, Definitions, Introduction, seven major sections, Conclusion, and one Appendix.
The seven major sections were:
- Section 4 – Electronic Data Processing and Office Equipment as a Source of Electromagnetic Emanations
- Section 5 – Susceptibility of Communication Receivers to Commercial EDP/OE Emanations
- Section 6 – Interference Potential of EDP/OE
- Section 7 – The Commercial EDP/OE Interference Models
- Section 8 – Emanation Limits for EDP/OE Products
- Section 9 – Comparison of Recommended Limits with Others
- Section 10 – Emanation Measurement
Discussion of the Scope of the Report
The report on recommendations from CBEMA on electromagnetic emissions focused on commercial EDP/OE. It addressed both narrowband and broadband emanations from ECP/OE.
The purpose of the report was to disseminate information to manufacturers of EDP/OE, standards organizations (national and international), and regulatory agencies in order to help all parties involved in understanding EDP/OE and its potential for electromagnetic interference. The report suggests general models for establishing the regulatory limits of commercial electronic equipment and presents its results on applying the methodology to products from CBEMA member companies. The limits arrived at through the use of models provides protection of communication services while minimizing emanation control requirements for EDP/OE. Limits and methods of measurement derived from mathematical equations based on the models are also presented.
The limits that were developed are applicable for conducted and radiated emissions from pulsed, periodic, electronic processes occurring in electronic equipment at pulse repetition rates greater than 10 pulses per second and resulting in emanations with a frequency range from 450 kHz to 1000 MHz.
The material in the report was organized in a logical manner based on the basic elements of an EMC interference relationship; that is, the source of the electromagnetic emissions, the propagation means for transferring the energy from the source to the receptor, and the communications receiver that is potentially capable of being interfered with by the sources of emanations.
Definitions covered in the report included: aperiodic waveform, average detector, bandwidth (6 dB), broadband emanation, commercial data processing and office equipment, conducted emanation, electromagnetic interference, emanation, emanation limits, host, identical equipments, interference source, narrowband emanations, peak detector, perception limit, periodic waveform, public power network, pulse repetition rate, quasipeak detector, radiated emanation, radio communication receiver, random waveform, signal-to-noise ratio, spectral occupancy, test unit, and threshold of detectable interference.
In the “Introduction” of the CBEMA Report, it states that “CBEMA is aware of the responsibilities and concerns of the Federal Communications Commission (FCC) and similar government agencies in other countries to ensure compatibility of RF devices and authorized communication services and has established the ESC5 Subcommittee to study this matter with respect to EDP/OE products.”
The Subcommittee studied the technical relationships between electronic products and receivers and their interference relationship. The CBEMA subcommittee admits that the subject is very complex and future studies and reports are anticipated by the subcommittee.
Since very few interference problems were being reported in the mid-1970s, the CBEMA report identified that the reason for controlling emanation characteristics of commercial EDP/OE is to prevent future interference problems.
The Report states that in the USA, the carrier of a continuous wave signal is normally measured with an average detector, while a broadband signal is measured with a peak detector. It points out that in Europe a CISPR quasipeak detector is used to measure all signals. In either case, “measurement of unknown noise source(s) may provide quite different conclusions than observed effects since the measured value depends on instrument characteristics as well as the character of the noise.”
The Report highlights the fact that both theoretical and empirical studies were accomplished over the previous several years to develop a firm technical rationale for the control of EMI. The rationale that was developed implied a quantifiable relationship between emanation sources and communication receivers and is still considered valid for commercial EDP/OE.
The models and resulting limits are considered to be rigorous because:
- They are based on an empirical data base.
- They provide good agreement between the scientific measurement method and the subjective interference effects.
- They are generally conservative.
- They have application to the present USA and International measurement approaches.
- They yield results compatible with environmental experience.
Section 4 – Electronic Data Processing and Office Equipment as a Source of Electromagnetic Emanations
CBEMA members designed and manufactured a large variety of computer and business products. The definition used by CBEMA at that time for their products was:
EDP/OE – electrical/electronic units/systems customarily used in commercial, manufacturing, medical, educational, governmental and similar facilities, and typically not found in home environments.
In the 1970s, EDP/OE were expensive and, therefore, frequently leased by the customer. Whether the equipment was leased or sold, the original manufacturer typically maintained the EDP/OE in the operating environment of the customer (on the customer’s premises). Thus, the original manufacturer was responsible for the equipment for its lifetime and the manufacturer could exercise some control over the installation of the product to assist in the control of electromagnetic interference situations.
Sources of emanation in EDP/OE included:
- low-power, periodic, binary pulsed signals necessary for computation of data
- repetitive electronic processes in EDP/OE that produce electromagnetic emanations,
- which may be transmitted through the air and/or conducted along power lines to the receptors
- authorized communication receivers.
Time-varying signals can be described in terms of their Fourier analysis. The time and frequency spectrums of time varying signals are illustrated in Figure 1.
Figure 1: Time/frequency line spectrum for pulsed periodic waveforms
As part of the 1977 Report, EDP/OE were studied to determine their potential sources of emanations. Product installation and product profiles/emanation patterns (polarization and lobing) were study considerations as well.
The studies resulted in the following conclusions:
- A large number of time-varying processes occur in EDP/OE; some of these signals may have sufficient amplitude to create electromagnetic interference.
- The major sources of electromagnetic emanations are associated with the timing (clock) signals and RF-switching power supplies.
- The periodic sources in EDP/OE occur at pulse repetition rates from 10 to one million pulses per second.
- Higher pulse repetition rates create uniform spectral amplitude distribution over the bandwidth of most communication receivers.
- Spectral line separation increases with pulse repetition rates, and the wide spectral spacing leads to only narrowband receiver responses for most EDP/OE.
- Random sources were found to be few in number and are not considered in the study.
- Aperiodic amplitude-varying sources were also very few in number and also are not considered in the paper.
The following conclusions were reached relative to the propagation of emanations from EDP/OE:
- Emanation profiles of EDP/OE typically decrease rapidly in amplitude above 300 MHz.
- The worst-case emanation profile for radiated emissions is basically the same for different product/system configurations and physical arrangement of the units in the computer system.
- Lobing effects in the 30 to 1000 MHz range are from 8 to 16 dB.
Relative to classification of EDP/OE and their emanations, the report concluded that ”the only necessary interference classification should be in terms of the typical environmental relationships between EDP/OE and receivers.” Thus, the report concentrated on “commercial environments” for its study since the large majority of computer products at that time were all located in companies.
The emanations from EDP/OE were classified into radiated emissions (propagated through air) and conducted emissions (on alternating current power lines).
|Daniel D. Hoolihan
is the Founder and Principal of Hoolihan EMC Consulting. He is a Past-President of the EMC Society of the IEEE and is presently serving on the Board of Directors. He is presently an assessor for the NIST NVLAP EMC and Telecom Lab Accreditation program. Also, he is the Vice-Chair of the ANSI ASC C63® committee on EMC.