CISPR is the acronym for the French title of the International Special Committee on Radio Interference. It is a committee of the IEC and is responsible for all international radio frequency emissions standards; its publications are also used as European standards to support the EMC, R&TTE and Automotive EMC Directives.

CISPR emissions standards are principally recipes for testing particular types of product, along with limits to apply to the results of those tests. The typical content is:

Scope

Requirements (limits for each test method)

Instructions for test: equipment, method, layout

Conformity assessment (80/80 rule)

Scope

The scope is a critical part of a standard as it defines as clearly as possible to what type of equipment the standard should apply. The scope is given in clause 1 of each standard and is accessible through the "preview" function of the IEC website without having to purchase the standard itself. However, the three most significant standard test methods (in CISPR 11, 14-1 and 22) are applied much more widely than their scope allows, through references in other product and generic standards.

Instructions for test

The test specification will prescribe the frequency ranges and methods of testing, including test layout and instrumentation. There is usually a catch-all requirement that the equipment under test should be operated and configured in a manner which maximizes the disturbance, consistent with typical applications. The implementation of this requirement is mostly left up to the expertise of the test engineer; it is rare, though not unknown, for a CISPR standard to explicitly require a test plan.

Limits

Different standards may have different limits, although the two "Class A" and "Class B" environments are common across a wide variety of product types. Class B is normally for equipment used in the residential (domestic) environment and Class A is for other types of equipment, but you should check the actual standard in use for a proper definition.

Usually, different limits are given for different types of detector, generally the average and quasi-peak detectors, with compliance with both being required for emissions below 30MHz. Graphs for the most common sets of limits are shown below for conducted and radiated tests.

Notes: except for telecom port, measurement is in a 50Ω/50μH LISN; telecom port limits are shown as voltage measurement into a 150Ω ISN. Groups 1 and 2 refer to CISPR 11. Group 1 A is equivalent to other standards' class A. IEC 60945 is for marine equipment. "FCC" refers to US limits.

Notes: except for disturbance power, measurement is of field strength referred to 10m test distance. Disturbance power is of dB(pW). IEC 60945 is for marine equipment. "FCC" refers to US limits.

Notes: measurement is of field strength referred to 3m test distance both above and below 1GHz. The need for tests above 1GHz rests on the highest frequency generated or used in the equipment under test; see the actual standard in use to confirm the required highest frequency.

Conformity assessment

Compliance of mass-produced items with an emissions standard does not imply that all products must emit less than the limit. The actual requirement for series production is that 80% of units will comply with the limit at an 80% confidence level. Although standards allow “for simplicity’s sake” just one product to be tested, this statistical statement can only be proven by testing several samples of a product. The following equation is provided to determine compliance of a group of samples, where k is a constant determined by the number of samples, x is the arithmetic mean of the measured values and Σn is the standard deviation:

x + k · Σn  ≤  limit value

The recommended minimum number of samples is five but in exceptional circumstances a sample of three or four is acceptable. One implication of this rule is that non-compliance of unique (one-off) products cannot be proven. CISPR standards carry the rider that “the banning of sales... shall be considered only after tests have been carried out using the statistical method of evaluation”.

The principal CISPR emission standards