EMC Technologies - Independant EMI/EMC Test Laboratory

Commercial Emissions Limits - The Design Challenge

(Note: This Section of the Site is Under Construction)

This is the first page in the EMISSIONS section of the site and lists the Commercial Emissions Limits, their origins and rationale, and some typical emissions profiles.

CONTENTS

The Source of Conducted and Radiated Limits

With the exception of North America, most countries adopt standards and their limits produced by the IEC (International Electrotechnical Commission) and its Technical Committees as they stand. A committee of particular importance to our discussion is CISPR which has been around since 1932 and is itself a Technical Committee of the IEC. There are over 200 Technical Committees and Sub-Committees and 700 working groups that cover topics well beyond the emissions of ITE (Information Technology Equipment) and ISM (Industrial, Scientific and Medical) equipment which will mainly concern us.

The standards of most importance to us are CISPR22 (AS/NZS3548), CISPR11(AS/NZS2064) which cover the emissions of ITE and ISM equipment respectively in Europe and Australia/New Zealand, and FCC part15 for North America.

Australian/New Zealand and European Emissions Limits

The following diagram displays the Class A conducted emissions limits for CISPR22 - AS/NZS3548.

The following diagram displays the 10m Class A radiated emissions limits for CISPR22 - AS/NZS3548.

The more stringent Class B limits are 10dB lower than the Class A limits and will generally apply where a device will be used in a domestic environment.

North American Emissions Limits

The following diagram displays the Class A conducted emissions limits
for FCC part 15.

The following diagram displays the 10m Class A radiated emissions limits
for FCC part 15.

Some typical Emissions Profiles

The similarity of sources of EMI and mechanisms of propagation of EMI result in certain typical emissions profiles.

For instance, a typical conducted emissions profile for a sample digital equipment with a switched mode power supply and a 10MHz crystal clocking a microcontroller will display power supply switching harmonics dominating from the start of the applicable limit (eg.150kHz for CISPR22). These emissions can roll off up to about 6MHz where digital clock and other digital system harmonics begin to dominate and may persist up to the limit boundary at 30MHz.

A typical radiated emissions profile for the same device can display cable-radiated harmonics up to about 300MHz with a broad peak corresponding to cable quarter wave lengths (maximum radiation efficiency) and then enclosure and circuit board radiated harmonics dominating thereafter and sometimes persisting in amplitudes threatening marginal compliance right up to the upper limit boundary at 1000MHz.

The following diagram displays a typical conducted emissions profile for a device with a SMPS and microcontroller.

The following diagram displays a typical radiated emissions profile for a device with a SMPS and microcontroller.

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