RF Emission Testing - Effect, Standard and Methodology

Introduction to RF Emission

Radio frequency emissions tests are one of the basic requirements forelectromagnetic compatibility compliance of most electronic and electricalproducts. Everything from phones, serviceequipment and modern technological products go through this process. Thepurpose of these tests is to ensure that other users are protected from theemissions generated when the product is used in their neighbourhood. Allcommercial products will be tested against the standards which are mostly basedon CISPR tests.

IEC CISPR publication 16-1: October 1999, “Specification for radio disturbanceand immunity measuring apparatus and methods”, specifies the characteristicsand performance of equipment for measuring electromagnetic immunity in the frequency range of 9kHzto 18GHz. It includes specifications for

a) The quasi-peak, peak, average and rms measuring receivers

b) Artificial mains networks

c) Current and voltage probes

d) Absorbing clamps

e) Antennas and test sites

There are basically 2 tests for RF Emissions. They are conducted and radiated emission tests.

Conducted RF Emission Test

Conducted emissions tests use an artificial mains network which is also knownas a Line Impedance Stabilising Network(LISN) as a transducer between themains port of the Equipment Under Test and the measuring receiver. It has 3 functionsas described below.

a) Provides a stable, defined RF impedance equivalent to 50 Ohm in parallelwith 50µH (or 50 Ohm/5µH for high-current units) between the point ofmeasurement and the ground reference plane.

b) Couples the RF interference from each of the supply phase lines to thereceiver, while blocking the LF mains voltage.

c) Attenuates external interference already present on the incoming mainssupply.

The internal circuit of the standard 50 Ohm/50µH LISN is shown below.

Although the LISN will reduce both the noise on the mains supply and variations in the supply impedance, it does not do this perfectly and a permanently installed RF filter at the mains supply to the test environment is advisable. Ambient radiated signals should also be attenuated and it is usual to perform the measurements inside a screened room, with the walls and floor of the room forming the ground reference plane.

A typical setup for conducted emission is shown in the figure below.

The radiated RF measurement according to CISPR 22 and CISPR 11 is usually performed on an open area test site (OATS). An OATS setup is shown in figure below.

Ambient signals can be classified into continuous narrowband, transient narrowband, continuous broadband and transient broadband.

Continuous narrowband signals such as broadcast transmissions can betabulated for a given site and their frequencies avoided.

A transient narrowband ambient allows a measurement to be made but the testengineer needs to know when the ambient signal is present. The same is truewith a transient broadband signal.

Continuous broadband interference such as pulsed noise from, say, an arcwelder is more difficult to deal with but a narrowband EUT emission can beextracted by using the average detector with a narrow measuring bandwidth.

One way to avoid ambients altogether is to use a screened room for theradiated measurements.

The types of antennas used for the measurement and their frequencies is as below.

The above tests are typically conducted by test labs which have the facilities to conduct the conducted and radiated emissions. These tests are usually costly to setupand the choice of whether to invest in these facilities will depend on the need and frequency of testing the products. Usually they are used for research and development purposes.

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