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feature article

Amendment 1 Brings Improvements
to CISPR 11

Daniel Hoolihan

Modifications in the international standard for measuring emissions from ISM equipment clarify points and shorten antenna measurement distances.

Amendment 1 to CISPR 11:1997 was released by the International Electrotechnical Commission (IEC) in May 1999. CISPR 11 ("Industrial, Scientific, and Medical (ISM) Radio-Frequency Equipment—Electromagnetic Disturbance Characteristics—
Limits and Methods of Measurement") is the most widely used document for measuring electromagnetic emissions from medical devices. It and its Amendment 1 were developed by technical experts from around the world serving on Subcommittee B of CISPR, the International Special Committee on Radio Interference. The amendment makes several major changes in the document, along with some minor ones. It adds lighting apparatus for ISM equipment to the scope of the standard, and it changes a required antenna distance for radiated emissions for a key class of ISM equipment. Because CISPR documents take the form of recommendations, the new amendment has no impact on regulations around the world until countries adopt its provisions into their laws and directives. This article itemizes key constituents of CISPR 11, Amendment 1.

Change in Scope

Amendment 1 has added requirements for ISM lighting apparatus operating in the protected frequency bands of 915 MHz, 2.45 GHz, and 5.8 GHz. (The 915 MHz frequency is only permitted in Region 2 as defined by the International Telecommunications Union Radio Regulations.) Note: All other types of lighting apparatus are covered in CISPR 15, "Limits and Methods of Measurement of Radio Disturbance Characteristics of Electrical Lighting and Similar Equipment."

Normative References

To the "Normative References" section another standard was added: IEC 60705:1999, "Household Microwave Ovens—Methods for Measuring Performance."

Limits of Electromagnetic Disturbance

The limits for ISM lighting devices within the expanded scope of the document are the same as those for Class B, Group 2 ISM equipment. These limits are given in Tables I and II, for conducted energy and electromagnetic radiated disturbances, respectively. (See the sidebar on page 72 for definitions of ISM equipment categories.)

A major change in testing requirements pertains to Class A, Group 1 equipment as shown in table III. When tested on an open-area test site (OATS), this class of equipment shall now be measured at a 10-m antenna distance for radiated emissions rather than the previous 30-m distance. This has a tremendous impact on testing laboratories that have been having trouble making 30-m measurements because of high ambient signal levels. Also, many labs that have gone to 10-m absorber-lined chambers can now make this measurement at the 10-m distance inside their protected environment.

In addition, table 5 of CISPR 11 has been changed by the amendment to reflect antenna distances of 10 m rather than 30 m. However, in the case of any dispute, Class A, Group 2 equipment shall be measured at a distance of 30 m.

Class B Equipment Limits dB (µV)
Frequency Band
(MHz)
Groups 1 and 2
Quasi Peak
Average
0.15 - 0.50

66
Decreasing linearly
with logarithm
of frequency to
56
56
Decreasing linearly
with logarithm
of frequency to
46
0.50 -5
56
46
5-30
60
50
Note: Care should be taken to comply with leakage current requirements.
Table I. Mains terminal disturbance, voltage limits for Class B equipment measured on a test site.1

Frequency Band
(MHz)
Quasi-Peak Electric Field 10-m Measurement Distance dB (µV/m)
Quasi-Peak Magnetic Field 3-m Measurement Distance dB (µA/m)
0.15-30
–
39
Decreasing linearly with logarithm of frequency to 3
30–80.872
30
–
80.872–81.848
50
–
81.848–134.786
30
–
134.786–136.414
50
–
136.414–230
30
–
230–1000
37
–
Table II. Electromagnetic radiation disturbance limits for Group 2, Class B equipment measured on a test site.1


1–18 GHz Frequency Band

Amendment 1 introduces significant changes in the requirements of CISPR 11 for the frequency range from 1 to 18 GHz. In the 1997 standard, limits were under consideration everywhere within this range except for the narrow frequency band of 11.7–12.7 GHz. For Group 1 ISM equipment, limits are still under consideration (radiated disturbance limits for Group 1 ISM equipment are intended to be identical to the limits currently under consideration for information technology equipment (ITE) above 1 GHz), and with respect to Group 2 ISM equipment, limits are under consideration for Class A equipment. However, a more complicated scenario applies to Class B, Group 2 equipment.

Limits are under consideration for Class B ISM equipment operating at frequencies below 400 MHz. When finalized, these limits will be introduced together with the following conditional testing clause: "If, in the band from 400 MHz to 1 GHz, all emissions are below the Class B limits and the fifth harmonic of the highest internally generated source is lower than 1 GHz (i.e., the highest source is less than 200 MHz), no testing above l GHz is required." For Class B, Group 2 ISM equipment operating above 400 MHz, the limits are specified in new tables 8, 9, and 10 of the amendment. It is also necessary to use a new decision tree, which appears as figure 5 of the amendment (see Tables IV–VI and Figure 1).

Frequency Band (GHz)
Field Strength at a
Measurement Distance of 3 m
dB (µV/m)
1–2.4
70
2.5–5.725
70
5.875–18
70
Note 1: For the protection of radio services, competent national authorities may require lower limits.
Note 2: Peak measurements with a resolution bandwith of 1 MHz and a video signal bandwith equal to or higher than 1 MHZ.
Table IV. Electromagnetic radiation disturbance peak limits for Group 2, Class B ISM equipment producing CW-type disturbances and operating at frequencies above 400 MHz.1


Frequency Band
(GHz)
Field Strength at a
Measurement Distance of 3 m
dB (µV/m)
1–2.3
92
2.3–2.4
110
2.5–5.725
92
5.875–11.7
92
11.7–12.7
73
12.7–18
92
Note 1: For the protection of radio services, competent national authorities may require lower limits.
Note 2: Peak measurements with a resolution bandwith of 1 MHz and a video signal bandwith equal to or higher than 1 MHz.
Note 3: Limits in this table were derived considering fluctuating sources such as magnetron-driven microwave ovens.
Table V. 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.1

Frequency Band (GHz)
Field Strength at a 3-m Measurement Distance dB (µV/m)
1–2.4
60
2.5–5.725
60
5.875–18
60

Note 1: For the protection of radio services, competent national authorities may require lower limits.
Note 2: Peak measurements with a resolution bandwidth of 1 MHz and a video signal bandwidth equal to or higher than 1 MHz.
Note 3: Limits in this table were derived considering fluctuating sources such as magnetron-driven microwave ovens.
Table VI. Electromagnetic radiation disturbance weighted limits for Group 2, Class B ISM equipment operating at frequencies above 400 MHz.1



Figure 1. Decision tree for the measurement of emissions from 1 to 18 GHz of Class B, Group 2 ISM equipment operating at frequencies above 400 MHz.1

Measuring Instrument

Specific details relating to the measuring instrument have been replaced by a note in the amendment. The note simply refers the user to the latest characteristics of a spectrum analyzer as defined in CISPR 16-1, "Specification for Radio Disturbance and Immunity Measuring Apparatus and Methods."

Antennas

This section contains some substituted new words, which read: "For measurements at frequencies above 1 GHz, the antenna used shall be as specified in CISPR 16-1."

Microwave Cooking Appliances

With the amendment, words have been added to this clause and new words substituted for old. The dimensions of the container are referenced to the new standard IEC 60705, and the material of construction may be glass or plastic or some other nonconductive substance.

Specific measurement instructions have been added for peak measurements above 1 GHz. The amendment states that "measurements shall be made with the azimuth of the Equipment Under Test (EUT) varying every 30° (starting position perpendicular to the front door). At each of these 12 positions, a maximum hold shall be made for a period of 20 seconds. Then, at the position where the maximum occurred, a maximum hold for a period of 2 minutes shall be made, and the result compared with the relevant limit."

Radiation Measurements (9 KHz to 1 GHz)

In the original paragraph 7.1.3, the wording suggested that measurements could be made at distances closer to the EUT than specified, but that the limit could not be changed; that is, the limit stayed the same even though the tester was closer to the EUT and the field strength was stronger. Amendment 1 explicitly permits measurements at the closer distance (because of high ambient noise levels or for other reasons), requiring that the test report record the distance and the circumstances of the measurement. Importantly, the new language allows an inverse proportionality factor of 20 dB per decade to be employed to normalize the measured data to the specified distance for determining compliance of the product under test. The amendment
cautions that "care should be taken in measuring a large test unit at 3 meters at a frequency near 30 MHz due to near-field effects."

Regulated Equipment Defined by Category

Group 1 ISM equipment is equipment containing intentionally generated and/or used conductively coupled radio-frequency energy that is necessary for the internal functioning of the equipment itself.

Group 2 ISM equipment is equipment in which radio-frequency energy is intentionally generated and/or used in the form of electromagnetic radiation for the treatment of material, as well as spark-erosion equipment.

Within each group:

Class A equipment is equipment suitable for use in all establishments other than domestic and those directly connected to a low-voltage power supply network which supplies buildings used for domestic purposes.

Class B equipment is equipment suitable for use in
domestic establishments and in establishments directly connected to a low-voltage power supply network that supplies buildings used for domestic purposes.


Receiving Antenna (1–18 GHz)

An added note says that "the distance between the receiving antenna and the EUT shall be 3 meters."

Validation and Calibration of the Test Site (1–18 GHz)

The amendment substitutes new words here to make reference to CISPR 16-2, "Specification for Radio Disturbance and Immunity Measuring Apparatus and Methods," and to specify that "test sites validated for field measurements between 30 MHz and 1 GHz may be used for measurements above 1 GHz, provided that absorbing material is placed on the ground between the EUT and the receiving antenna."

Measuring Procedure (1–18 GHz)

In this area Amendment 1 adds the following recommendation: "The general measuring procedure above 1 GHz specified in CISPR 16-2 should be consulted for guidance."

The amendment also adds this language to Clause 8.4: "Peak measurements above 1 GHz (see table 8 or table 9) shall be the result of a maximum hold on the spectrum analyzer. Weighted measurements above 1 GHz (see table 10) shall be the result of a maximum hold and shall be performed with the spectrum analyzer in logarithmic mode (values displayed in decibels)."

Annex A

This annex presents examples of equipment with their appropriate classifications. Amendment 1 adds microwave lighting apparatus to the list.

Conclusion

The first amendment to CISPR 11:1997 contributes some important clarifications to that document. It adds certain lighting devices to the scope of the standard. It has changed antenna measurement distances for an important class of products, allowing EMC testing laboratories to make measurements at a 10-m distance, either inside an absorber-lined chamber or on an OATS, instead of at 30 m. And it modifies measurements between 1 and 18 GHz in order to reflect changes in test equipment and field experiences with products operating in that range. Overall, the changes are an obvious improvement on the 1997 version of CISPR 11.

References

  1. The tables from CISPR 11 (1999-08) Ed. 3.1 Consolidated Edition are reproduced here with the permission of IEC (Geneva). All IEC publications are available from http://www.iec.ch or any IEC National Committee. IEC takes no responsibility for and will not assume liability for damages resulting from the reader's misinterpretation of the referenced material due to its placement and context in this publication.

Daniel Hoolihan was formerly vice president of Minnesota Operations for TÜV Product Service Inc. (New Brighton, MN). He is the founder of Hoolihan EMC Consulting (Lindstrom, MN) and a member of Compliance Engineering's editorial advisory board. He can be reached at hoolihan@emcxpert.com.

 

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