CISPR 11 is the International Standard for electromagnetic emissions (disturbances) from Industrial, Scientific and Medical (ISM) Equipment. The official title of the standard is “Industrial, Scientific, and Medical Equipment – Radio-Frequency Disturbance Characteristics – Limits and Methods of Measurement.” The premiere edition of the standard was released in 1975 and the latest edition (Edition 5.0) was released in 2009. The standard includes both limits and methods of measurement for conducted phenomena and radiated phenomena. This article traces the history and development of the content of the standard over the last 35 years.
First Edition – 1975
The title of the Premiere Edition was “Limits and Methods of Measurement of Radio Interference Characteristics of Industrial, Scientific, and Medical (ISM) Radio-Frequency Equipment (excluding surgical diathermy apparatus).” The publication was prepared by CISPR Subcommittee B (Interference from Industrial, Scientific, and Medical Apparatus). It summarized the technical content of a number of CISPR Publications, Recommendations and Reports over a period of 8 years (1967-1975).
NOTE – CISPR is the International Committee on Radio Frequency Interference; it has a number of Subcommittees that address various areas of electromagnetic interference.
The frequency range covered by the First Edition was 150 kHz to 18 GHz. The terminal voltage limits were quoted in millivolts and covered the frequency range 150 kHz to 30 MHz. Table 1(Terminal Voltage Limits) from the First Edition is reproduced in Figure 1.
The radiated limits were quoted for the frequency range 0.150 MHz to 1000 MHz and they were quoted in microvolts per meter. They were quoted at antenna-measurement distances of 30, 100, and 300 meters from the equipment or 30 meters or 100 meters from the boundary of the users’ premises.
Table II [Limits of Radiation in microvolts/meter and decibels (uV/m)] from the First Edition is recreated in Figure 2.
There was a special limit for radiation from microwave equipment used for heating and medical purposes in the frequency range from 1-18 GHz; it was 57 dB above a picowatt Effective Radiated Power (ERP) referred to a half-wave dipole.
Methods of measurement quoted CISPR Publications 1, 2, and 4 for quasi-peak measuring sets. Measurement of the radio-frequency voltage on supply mains (AC voltage lines) was done with a V-network with an intrinsic impedance of 150 ohms.
Magnetic field measurements are made with a balanced loop antenna below 30 MHz. Above 30 MHz, an “electric aerial” would be used as per CISPR Publications 2 and 4. The center of the “aerial” would be 3 meters above the ground.
Above 1 GHz, the “receiving aerial” shall be made with a directive aerial of small aperture capable of making separate measurements of the vertical and horizontal components of the radiated field. The height of the aerial had to be the same as the height of the approximate radiation center of the equipment under test.
SECOND EDITION – 1990
The Second Edition was released in 1990 and it contained changes over the previous 15 years including two amendments.
In this Edition, ISM Equipment was divided into two Groups and two Classes. The Groups were (1) Group 1 which included all ISM equipment that used RF energy only for internal functioning of the equipment and (2) Group 2 equipment which included ISM equipment that was used for external treatment of material and similar processes. Class A equipment is equipment suitable for use in all establishments other than domestic buildings. Class B equipment is that equipment that is suitable for use in domestic surroundings.
The frequency bands for conducted emissions were stated as covering 150 kHz to 30 MHz. The second edition included NEW separate limits for Class A and Class B equipment.
The Class A limits in dBuV are shown in Figure 3.
The Class B Limits in dBuV are shown in Figure 4.
Electromagnetic radiation disturbance limits for Group 1 equipment in Edition 2 are shown in Figure 5.
There were additional limits for radiated emissions for Group 2 equipment.
In the frequency range, 1 GHz to 18 GHz, the limit for radiation disturbance power was 57 dB above a picowatt (effective radiated power) referred to a half-wave dipole in the narrow frequency range 11.7 GHz to 12.7 GHz.
The standard used statistics for compliance conclusions. Par. 6.1 stated “It cannot be shown that equipment in series production fails to meet the requirements of this publication without a statistical assessment of compliance being carried out.”
In the General Measurements Requirements clause, the standard allowed Class A equipment to be measured either on a test site or in situ as determined by the manufacturer. However, the standard was firm that Class B ISM equipment SHALL be measured at a test laboratory.
The measuring equipment used by the testing lab had to comply with CISPR 16. Receivers needed both average and quasi-peak capability. An Artificial Mains Network (LISN) was needed for conducted emissions and it was a 50 ohm-50 microhenry network.
Antennas used included a loop antenna below 30 MHz and a balanced dipole antenna from 30 MHz to 1000 MHz. Measurements were made in both horizontal and vertical polarizations. Class A equipment was measured with the center of the antenna 3 meters above ground while there was a change for Class B equipment in that the center of the antenna had to be adjusted between 1 and 4 meters.
The test lab had to meet special provisions for measuring radiated emissions including having a minimum-sized ground plane, an area free of reflecting structures, and large enough to allow appropriate separation of the equipment under test and the receiving antenna.
Amendment 1 to the second edition was released in March of 1996. It changed some conducted emission limits especially for Class A equipment. Amendment 2 was also released in March of 1996 and it released limits for Induction Cooking Appliances for both conducted limits and radiated magnetic field limits. Also, radiation limits for Group 2 equipment were changed in Amendment 2.
THIRD EDITION – 1997
The Third Edition of CISPR 11 was also developed by CISPR Subcommittee B and was released in 1997. It replaced the second edition and its two amendments.
The Main content of CISPR 11 standards are based on the original CISPR Recommendation No. 39/2 which was titled “Limits and Methods of Measurement of Electromagnetic Disturbance Characteristics of Industrial, Scientific, and Medical (ISM) Radio-Frequency (RF) Equipment.” It says “The CISPR considering a) that ISM RF equipment is an important source of disturbance; b) that methods of measuring such disturbances have been prescribed by the CISPR;
c) that certain frequencies are designated by the International Telecommunication Union (ITU) for unrestricted radiation from ISM equipment RECOMMENDS that the latest edition of CISPR 11 be used for the application of limits and methods of measurement of ISM equipment.”
The Third Edition reorganized the first Clause; it was changed from “Scope and Object” to “General” and was comprised of two sub-clauses; (1) Scope and Object and (2) Normative References.
Clause 6 of the second edition was renumbered as Clause 11 in the third edition and the sub-clause 6.1 (Equipment in series production) was replaced with sub-clause 11.2 “Equipment in small scale production.”
A new sub-clause was added in Clause 5 (Limits of Electromagnetic Disturbance); it was 5.4 (Provisions for Protection of Specific Sensitive Radio Services.)
Clause 7 in the second edition became Clause 6 in the third edition; Clause 8 became Clause 7, Clause 9 became Clause 8, Clause 10 became Clause 9, and Clause 11 became Clause 10.
Annexes A – D remained the same in the Third Edition. Two new annexes were added; Annex E (Safety-Related Service Bands) and Annex F (Sensitive Service Bands).
The classification of equipment remained the same from Edition 2 to Edition 3; Group 1 and Group 2 plus Class A and Class B.
With respect to the Limits of electromagnetic disturbance, Class A equipment could still be tested either at a test lab or in situ while Class B equipment had to be measured in a testing laboratory.
The limits for conducted emissions on the power leads were measured from 150 kHz to 30 MHz using a 50ohm/50uH network. The limits remained the same for Class A and Class B equipment from Edition 2 except that another category was added for Class A-Group 2 equipment for mains supply currents in excess of 100 amps per phase when using the CISPR voltage probe. The limits are shown in Figure 6 for this special case.
However, new limits were added in Table 2c (Mains terminal disturbance voltage for inductive cooking appliances) for Group 2-Class B equipment for both domestic and commercial cooking appliances.
Table 3 (Electromagnetic radiation disturbance limits for group 1 equipment) had a major change as the measurement distance for Group 1-Class A equipment was changed from 30 meters to 10 meters with a corresponding increase in limits of 10 dB (assuming an inverse distance fall-off of the radiated electromagnetic field.)
Par. 5.2.2 of this edition also introduced the concept of measuring products at shorter distances than the specified measurement distances for radiated disturbances. For example, it allowed Group 2 Class A equipment to be measured at a distance of between 10 and 30 meters instead of 30 meters. Also, it allowed Group 1 and 2, Class B, equipment to be measured at antenna distances between 3 and 10 meters. However, it stated that “In case of dispute, Class A-Group 2 equipment shall be measured at a distance of 30 meters; Class B-Group 1, Class B- Group 2, and Class A-Group 1 equipment SHALL be measured at a distance of 10 meters.”
Tables 3a and 3b were added to cover Group 2 induction cooking appliances for Class B and Class A, respectively. Table 3a (Limits of the magnetic field induced current in a 2 m loop antenna around the device under test) was intended to use the “Van Veen Loop Method” measurement method as per CISPR 16-2. Table 3b (Limits of the magnetic field strength) is measured at a 3-meter antenna distance with a 0.6 meter loop antenna as described in CISPR 16-1.
Table 4 (Electromagnetic radiation disturbance limits for Group 2 – Class B equipment measured on a test site) added a new column of requirements which was simply: the quasi-peak magnetic field (measured at 3 meters) will not exceed 39 dBuA/meter decreasing linearly with the logarithm of the frequency to 3 dBuA/meter from 150 kHz to 30 MHz.
Table 5 changed the measurement distance from 30 meters to 10 meters and increased the limits by 10 dB from Edition 2 limits.
Table 6 was added to the Third Edition. It was titled “Electromagnetic radiation disturbance peak limits for Group 2- Class B ISM equipment producing CW-type disturbances and operating at frequencies above 400 MHz.” Table 7 (Electromagnetic radiation disturbance peak limits for Group 2- Class B ISM equipment producing fluctuating disturbances other than CW and operating at frequencies above 400 MHz) and Table 8 (Electromagnetic radiation disturbance weighted limits for Group 2 – Class B ISM equipment operating at frequencies above 400 MHz) were also added to the Third Edition.
Clause 5.4 (Provisions for protection of specific sensitive radio services) was added to the fourth edition and it referenced the new Annex F which gave examples of bands to be protected.
The same general measurement conditions existed as in the previous edition which is that Class A equipment could be measured at a test lab or in situ. Class B equipment had to be measured on a test site (in a test lab).
For equipment on a turntable, the distance to the antenna was measured from the center of the turntable. For equipment not on a turntable, the distance to the antenna was measured from the edge of the equipment.
Paragraph 6.5.6 (Single and multiple-zone induction cooking appliances) was added to the third edition.
Amendment 1 to the Third Edition added requirements for ISM lighting apparatus operating in the frequency bands of 915 MHz, 2.45 GHz, and 5.8 GHz. It also added IEC 60705:1999 (Household microwave ovens – methods for measuring performance) to the normative standards. It also added new words in Clause 5.2.2 (discussed earlier) and it added Table 5 (Electromagnetic Radiation disturbance limits for Group 2 – Class A equipment). All new wording was added to Clause 5.2.3 by Amendment 1. In Clause 6.2.1, it added the requirement that “for measurements at frequencies above 1 GHz, a spectrum analyzer with characteristics as defined in CISPR 16-1 shall be used.” Additionally, in clause 6.2.4, it added the words “for measurements at frequencies above 1 GHz, the antenna used shall be as specified in CISPR 16-1.” Also, Clause 6.5.4 (Microwave cooking appliances) was added by Amendment 1. An important (and somewhat controversial) sub-clause was added by Amendment 1 in 7.1.3 (Radiation measurements [9 kHz to 1 GHz]). It added two sentences that impacted Edition 3 and future editions. The first sentence said “For the test site measurements, an inverse proportionality factor of 20 dB per decade shall be used to normalize the measured data to the specified distance for determining compliance.” Also, it added the parenthetical sentence, “Care should be taken in measuring a large test unit at 3 meters at a frequency near 30 MHz due to near-field effects.” It deleted a key sentence from Edition 2 that said “At the closer measurement distance the electromagnetic disturbances measured shall not exceed the limit values specified in Clause 5.” In sub-clause 8.2 it added the sentence “The distance between the receiving antenna and the EUT shall be 3 meters.” Sub-clauses 8.3 (Validation and calibration of test site) and 8.4 (Measuring Procedure) were completely rewritten. Finally, Amendment 1 added Figure 5 (Decision tree for the measurement of emissions from 1 GHz to 18 GHz of Class B- Group 2 ISM equipment operating at frequencies above 400 MHz).
Amendment 2 replaced “spark erosion equipment” with “electro-discharge machining (EDM) and arc welding equipment.” It also made additional editing changes to a number of sub-clauses.
Fourth Edition – 2003
The Fourth Edition of CISPR 11 was published in March of 2003. The Fourth Edition superseded the Third Edition (1997), its first amendment (1999) and its second amendment (2002).
There were a limited number of changes in going from the Third Edition to the Fourth Edition. The first two sentences in Clause 4 were changed to read “The manufacturer and/or supplier of ISM equipment shall ensure that the user is informed about the class and group of the equipment, either by labeling or by the accompanying documentation. In both cases, the manufacturer/supplier shall explain the meaning of both the class and the group in the documentation accompanying the equipment.”
Clauses 7.1 and 7.2 were interchanged from the Third Edition.
Clause 6.2.5 (Artificial Hand) was added in Edition 4 as well as Figure 6 (Artificial Hand, RC Element). The concept of an artificial hand was introduced to simulate the effects of the user’s hand during the conducted emission measurements
The definitions of Group 1 ISM equipment, Group 2 ISM equipment, Class A equipment, and Class B equipment remained basically the same as the Third Edition.
With respect to limits of electromagnetic disturbance, Class A equipment could, again, be measured either in a testing laboratory or in situ (as preferred by the manufacturer). However, again, Class B equipment must be measured in a testing laboratory.
The limits of terminal disturbance voltage (conducted emissions) gives the manufacturer two choices: (1) meet the average limit with an average detector and the quasi-peak limit with a QP detector or (2) meet the average limit when using a QP detector. This was the same as stated in Edition Three.
For radiated disturbances from 150 kHz to 1000 MHz, the limits stayed basically the same as the third edition. Measurements were allowed at closer distances than the specified distances, under certain considerations. In case of dispute, however, Class B (Group 1 and Group 2) and Class A (Group 1) shall be measured at a distance of 10 meters and Class A (Group 2) shall be measured at a distance of 30 meters.
Receivers use for the measurements shall meet the criteria of CISPR 16-1. The artificial mains network (LISN) remained the same as the third edition; a 50 ohm/50 microhenry V-Network as specified in CISPR 16-1. The antennas used for measuring CISPR 11 products must meet CISPR 16-1. In a testing laboratory, the antenna must be raised and lowered from 1 to 4 meters in the frequency range 30 MHz to 1000 MHz. For measuring products in situ, the antenna’s center must be fixed at 2 meters above the ground.
Amendment 1 to Edition 4 was released in 2004; it primarily replaced Table 6 in Edition 4 with a new table that addresses Group 2 (Class A and Class B) ISM equipment producing CW type disturbances and operating at frequencies above 400 MHz.
Amendment 2 added CISPR 16-4-2:2003 to the Normative References. It also added a new Table 2c for Mains Terminal disturbance voltage for induction cooking appliances. It also modified Clause 6.5.4 (Microwave Cooking Appliances) and 6.5.6 (Single and multiple-zone induction cooking appliances) to more closely match the IEC Product Standard. Amendment 2 also added clauses 6.6 (Recording of test-site measurement results), 6.6.1 (Conducted Emissions), and 6.6.2 (Radiated Emissions). Also, Clause 11.4 (Measurement Uncertainty) was added and it said “Determining compliance with the limits in this standard shall be based on the results of the compliance measurement, not taking into account measurement instrumentation uncertainty.” However, results of measurements of emissions from ISM equipment were supposed to reference the measurement uncertainty considerations contained in CISPR 16-4-2.
Fifth Edition – 2009
The Fifth Edition of CISPR 11. the current edition, was released in 2009.
The Fifth Edition continued the long-standing practice of Group 1 and Group 2, Class A and Class B equipment classifications. The limits stated in the 2009 edition were similar to the limits in the Fourth Edition.
A comparison of the table of contents of the First Edition and the Fifth Edition indicates the growth in length and complexity of the standard over a period of 35 years
Edition 5’s first Clause is titled “Scope” versus the previous Edition’s title of “General.” Clause 2 has a new title; “Normative References.” Clause 3 (Terms and Definitions) is also new. Clause 4 (Frequencies Designated for ISM Use) was the previous Ediiton’s Clause 3. Clause 5 was the same as Edition 4’s Clause 4 except Edition 5 added sub-clause 5.1 (Information for the user) to the main body of the standard.
Clause 6 underwent a major overhaul from Edition 4. Its Main Clause and Sub-Clause headings are as follows:
Clause 6 – Limits of Electromagnetic Disturbance
6.1 – General
6.2 – Group 1 Measured on a Test Site
6.3 – Group 2 Measured on a Test Site
6.4 – Group 1 and Group 2 Class A Measured in situ
Clause 7 added a new sub-clause 7.1 (General) and a new sub-clause 7.7 (Recording of Test Site Measurement Results).
Clause 12 (Assessment of Conformity of Equipment) added a new sub-clause 12.1 (General) and then the next three sub-clauses were the same as sub-clauses 11.1 – 11.3 in Edition 4.
Clause 13 is new and titled “Figures and Flowcharts.” Annex E is also new for Edition 5.
The entire Edition was written to have a more transparent structure.
Table 17 was added with a title of “Electromagnetic Radiation Disturbance Limits for Class A Group 1 Equipment Measured in situ.” It especially addressed equipment with input power greater than 20 KVA.
An Amendment 1 to Edition 5 was released in 2010. It created a new subset of equipment, Small Equipment. Small Equipment was defined to be “equipment, either positioned on a table top or standing on the floor which, including its cables, fits in a cylindrical test volume of 1.2 meters in diameter and 1.5 meters above the ground plane.”
Using this definition, Tables 4, 5, 9, 10, and 11 were modified to allow testing of Class A and B products meeting the “Small Equipment” definition to be tested at a 3-meter test distance. The limit at three meters would be extrapolated from the typical test distance of 10 meters using an inverse-distance fall-off assumption (free-field).
SUMMARY AND CONCLUSIONS
The CISPR 11 standard for measuring disturbances (emissions) from Industrial, Scientific, and Medical Equipment has been in existence for over 35 years. It has grown from a simple document to a complex document involving a number of types of products.
It has grown from measuring products at a larger distance (100 meters and 30 meters) for Class A Equipment to measuring them at 3 meters. Class B equipment measurement distances have seen an equivalent degradation of “far-field” radiated emission measurements. This steady erosion of the “laws of physics” is worrisome and a trend to reverse this erosion is overdue in the engineering field of EMC and the EMC Standards arena.
Dan Hoolihan is the president of Hoolihan EMC Consulting and can be reached by e-mail at Danhoolihanemc @aol.com.