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1. INTRODUCTION
Most electronic devices, including digital devices, give rise to electromagnetic emissions, which either by radiation through free space (air) or by conduction through some conducting paths may act as unwanted EMI signals or noise to another electric device in their vicinity. It’s therefore desirable that such emissions be controlled so that their interference effects may be kept at minimum or acceptable levels. For this reason all electronic devices are required to satisfy certain regulations and standards established by national bodies, trade associations, international organizations, and the like. In most cases these EMC standards are similar, but there are also differences in the specific limits and the methods of measurement specified by various organizations. EMC standards may be classified into three broad categories: (1) those mandated by a national government agency, which must be met, (2) those imposed voluntarily by product manufacturers, which are specific to each manufacturer, and (3) those imposed by military agencies of a country. For example, US military agencies require that their own standards must be met by a product for their consideration. US military standards regulate emissions from the product as well as immunity to emissions from other devices, including to ESD. Note that a product that meets these standards may not be immune to interference.
Thus, although complying with a country's EMC standards is essential for the marketing of the product in that country, it should not be taken as a substitute for detailed EMC consideration during the design process.
The subject of EMC standards and their evolution is vast. Brief summaries of standards are given in […] and detailed descriptions of US standards and methods of testing are described in […]. In this section we will concentrate on the US and European civilian standards and regulations. We will also touch upon the international hierarchy set up to create uniform standards and regulations that apply to the European Union (EU) countries and their equivalence with US regulations. Many countries, including the United States, will eventually accept these standards, known as "harmonized" standards. Finally, these standards and regulations are modified and up-dated regularly; hence one should consult the latest versions before venturing to meet these standards.
2. CURRENT US STANDARDS
2.1 Introduction
Any electronic device to be marketed in the US must meet certain requirements or standards established by the Federal Communications Commission (FCC) within the country. Historically the rules for non-licensed use of RF devices were first established by the FCC in 1983 when "the limit applied to the emissions by these early devices was 15 pV/m at a distance equivalent to X/27r, where X is the operating wave length [5]." Part of the FCC's responsibility includes creation (or adoption) of regulations that control electromagnetic interference. These regulations are published in the Code of Federal Regulations, Telecommunications 47 (' Washington, DC, US National Archives and Records Administration). Regulations specifying limits for electromagnetic emissions for radio frequency devices and equipment (both intentional and unintentional radiators) are covered in Part 15 [5]; Part 18 provides similar information for industrial, scientific, and medical (ISM) equipment. The FCC general procedures for measuring emissions are given in [6]; the FCC also advocates and encourages the use of procedures outlined by the American National Standards Institute (ANSI) [7]. The FCC defines an RF device as any "device that in its operation is capable of intentionally or unintentionally emitting radio frequency energy by radiation, conduction or some other means." RF energy defined by the FCC is any electromagnetic energy in the frequency range of 9 kHz to 3000 GHz. The purpose of the standards and regulations set by the FCC is that if a given device meets these standards, then it suffices that it does not cause or create unwanted electromagnetic signals or noises that could otherwise affect the performance of another electromagnetic device located beyond a certain minimum distance from the device.
In this section we will concentrate on digital devices. A digital device is defined by the FCC as "an unintentional radiator (device or system) that generates and uses timing signals or pulses at a rate in excess of 9,000 pulses (cycles) per second and uses digital techniques; inclusive of telephone equipment that uses digital techniques or any device or system that generates and uses radio frequency energy for the purpose of performing data processing functions, such as electronic computations, operations, transformations, recording, filing, sorting, storage, retrieval, or transfer." Note that an intentional radiator "that contains a digital device is not subject to the standards for digital devices, provided the digital device is used only to enable operation of the radio frequency device and the digital device does not control additional functions or capabilities." Transmitters, such as automobile remote door openers are called intentional radiators, and must meet separate FCC rules and regulations for intentional radiators.
Digital devices may be grouped as Class A and Class B. They are defined in as follows: Class A Digital Device "A digital device that is marketed for use in a commercial, industrial or business environment, exclusive of a device which is marketed for use by the general public or is intended to be used in the home." Class B Digital Device "A digital device that is marketed for use in a residential environment notwithstanding use in commercial, business and industrial environments. Examples of such devices include, but are not limited to, personal computers, calculators, and similar electronic devices that are marketed for use by the general public."
2.2 FCC Radiated Emission Limits for Digital Devices
The FCC requires that over certain frequency bands the amplitude of the radiated electric field from a device at a specific distance must be equal to or less than certain limiting values and measured following the procedure described in [6]. As an alternative to the FCC's radiated emission limits for digital devices, in an effort to approach the ideal of a harmonized standard, "digital devices may be shown to comply with the standards contained in Third Edition of the International Special Committee on Radio Interference (CISPR), Pub. 22, "Information Technology Equipment-Radio Disturbance Characteristics-Limits and Methods of Measurement," known as CISPR 22.
Measurements must be carried out with an antenna horizontally and vertically polarized, meaning both the horizontal and vertical components of the electric field must be measured. Tables 1 and 2 give the pertinent information for the radiated emissions from Class A and Class B electronic devices.
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TBL. 1 FCC and CISPR 22 radiated emission limits for Class A digital devices Frequency Fie Id Strength Field Strength Distance (MW (CcVlm) (dB ,uV/m) (m)
FCC 30-88 90 39.0 10 88-2 16 150 43.5 10 2 16-960 210 46.0 10
>960 3 00 49.5 10 CISPR 30-230 31.6 30 30 230-1 000 70.8 37 30
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TBL. 2 FCC and CISPR 22 radiated emission limits for Class B digital devices Frequency Field Strength Field Strength Distance (MW bV/m) (dB ,uV/m) (m)
FCC 30-88 100 40.0 3 88-2 16 150 43.5 3 2 16-960 200 46.0 3
>960 500 54 3 CISPR 30-230 31.6 30 10 230-1 000 70.8 37 10
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It’s necessary to compare these standards; however, the field limits given above apply to differing distances. To compare them, it’s required to express all of them at the same measurement distance, for example, for the Class B distance of 3 m. Assuming that the far zone fields of the source vary as 1/~, where T is the distance from the source, it can be shown that the relationship between the fields El, E2 at distances dl , d2, respectively, from the source is ....
When the fields are expressed in dBpVim, the relation is
El = 20 log10 (2) + E2. ( 1 2.1 b)
Thus, using (1b), we add 2010g10 (10/3) to the Class A fields given in TBL. 1 to obtain the equivalent fields at a distance of 3 m and then compare them with the corresponding Class B fields given in TBL. 2. Following this procedure, we present in FIG. 1 Class A and Class B standards at a distance of 3 m.
FIG. 1 FCC and CISPR 22 radiated emission limits extrapolated to a 3
m measurement range.
The results now demonstrate that the Class B standards are more stringent.
The above is only approximate. The assumption of far zone behavior of the fields may not be valid at the frequency of interest. For a point radiator ( Section 6) the far zone distance d 5 A/27r 'v X/6. However, in most cases the radiator may be extended; that is, it may have a maximum linear dimension D. In such cases ( Section 6) the far zone distance is d 2 2D2/X. Note that for a X/2-dipole we have D 2 0.5 A. Thus the field strength standards may not be in the far field of the device, and, moreover, the measurement antennas and the radiating device may not be in the far zone of each other at the measurement distance for all frequencies tested. This can have significant implications for measurements intended to demonstrate compliance.
2.3 FCC Conducted Emission Limits for Digital Devices
The FCC requires that for a "digital device that is designed to be connected to the public utility (AC) power line, the radio frequency voltage that is conducted back onto the AC power line on any frequency or frequencies within the band 150 kHz to 30 MHz shall not exceed the limits in the following table, as measured using a 50 pH/50 ohm line impedance stabilization network (LISN). Compliance with this provision shall be based on the measurement of the radio frequency voltage between the power line and ground at the power terminals [5-71." Note that the CISPR 22 conducted emission limits have been directly adopted by the FCC.
For quick reference, the conducted emission limits are shown graphically in FIG. 2.
TBL. 3 FCC and CISPR 22 limits on conducted emission from digital devices
3. EMI/EMC STANDARDS: NON-US COUNTRIES
As mentioned at the beginning of this section, most countries, with the exception of the US and Canada, are in the process of adopting European Standards that also include mandatory immunity standards. In the following we give some brief comments about foreign standards; more details and references pertinent to this topic are given in [3].
3.1 CISPR Standards
The European-based CISPR (Comite Internationale Speciale des Perturbations Radio-electroniques) since its founding in 1934 has been developing international standards on EMI/EMC methods of measurements to ensure product compliance. Among the first agreements in CISPR at its founding, a signal-to-noise ratio of 40 dB was provided as a tolerable limit of interference for a reference field strength of 1 mV/m modulated to a depth of 20 percent [3]. The standards have been published by the IEC (International Electrotechnical Commission). The CISPWIEC effort is not only on the part of European nations but also other advanced nations such as Australia, Canada, India, Japan, Korea and the United States. CISPR has no regulatory authority, but its standards, when adopted by governments, become national standards. In 1985 CISPR adopted a new set of emission standards (Publication 22) for Information Technology Equipment (ITE) for digital electronics. Many European countries have adopted those requirements as their national standard, and more are expected to do so in the future. The United States, as a voting member of CISPR, voted in favor of the new standards, and is working to merge those standards with its own [2]. The limits given in CISPR Publication 22 with slight modifications are likely to become the international standard. CISPR specifications are described in detail in [ 1-3].
FIG. 2 FCC and CISPR 22 conducted emission limits for digital devices
3.2 European Norms
After the formation of the European Common Market and the removal of trade and tariff barriers, a group of countries took the initiative to evolve common EMI/EMC standards in the form of Euro Norms (EN) or European standards [3]. CENELEC (Comite European de Normalization Electrotechnique), representing all concerned European countries, agreed to harmonize and integrate their national standards as Euro Norms that are derived from the related international standards published by CISPWIEC. Once the Euro Norms are published, the agencies responsible for standardization and regulations in different member countries produce their national standards, which are harmonized with EN standards. Thus identical standards are used in EU member countries. Euro Norms cover not only the emission limits but also the minimum immunity levels for different equipment. More detailed descriptions and guidelines for international aspects of EMI/EMC are given in [3].
Equipment that has been tested and complies with subject European Norms is identified by the letters "CE".
REFERENCES:
1. C. R. Paul, Introduction to Electromagnetic Compatibility, Wiley, New York, 1992.
2. H. W. Ott, Noise Reduction Techniques in Electronic Systems, 2nd ed., Wiley, New York.
3. V. P. Kodali, Engineering Electromagnetic Compatibility, IEEE Press, New York, 1996.
4. C. Christopoulos, Principles and Techniques of Electromagnetic Compatibility, CRC.
5. FCC General Docket No. 87-389 First Report and Order (Released, April 1987). Revision of part 15 of the Rules Regarding the Operations of Radio Frequency Devices within License, April 18, 1984.
6. FCC, General Docket No. 8944, Procedure for Measuring Electromagnetic Emissions from Digital Devices, March 7, 1989.
7. ANSI, Methods of Measurement of Radio Noise Emissions from Low-Voltage Electrical and Electronic Equipment in the Range 9 kHz to 40 GHz, ANSI. C 63.4-2003, IEEE, New York, 2004.
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