Introduction to RF Emission
Radio frequency emissions tests are one of the basic requirements for electromagnetic compatibility compliance of most electronic and electrical products. Everything from phones, service equipment and modern technological products go through this process.
The purpose of these tests is to ensure that other users are protected from the emissions generated when the product is used in their neighborhood All commercial products will be tested against the standards which are mostly based on CISPR tests.
IEC CISPR publication 16-1: October 1999, “Specification for radio disturbance and immunity measuring apparatus and methods”, specifies the characteristics and 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 known as a Line Impedance Stabilising Network(LISN) as a transducer between the mains port of the Equipment Under Test and the measuring receiver. It has 3 functions as described below.
a) Provides a stable, defined RF impedance equivalent to 50 Ohm in parallel with 50µH (or 50 Ohm/5µH for high-current units) between the point of measurement and the ground reference plane.
b) Couples the RF interference from each of the supply phase lines to the receiver, while blocking the LF mains voltage.
c) Attenuates external interference already present on the incoming mains supply.
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.
Radiated RF Emission Test
The radiated RF measurement according to CISPR 22 and CISPR 11 is usually performed on an open area test site (OATS).
Any open area test site is likely to suffer from ambient signals which are generated in the neighbourhood and received on the site, but not emitted from the EUT. These signals can easily exceed both the EUT's emissions and the limit values at many frequencies.
An emissions plot which contains ambients is hard to interpret and, more importantly, ambients which mask EUT emissions make it impossible to measure the EUT at these frequencies.
Ambient signals can be classified into continuous narrowband, transient narrowband, continuous broadband and transient broadband.
Continuous narrowband signals such as broadcast transmissions can be tabulated for a given site and their frequencies avoided.
A transient narrowband ambient allows a measurement to be made but the test engineer needs to know when the ambient signal is present. The same is true with a transient broadband signal.
Continuous broadband interference such as pulsed noise from an arc welder is more difficult to deal with but a narrowband EUT emission can be extracted by using the average detector with a narrow measuring bandwidth.
One way to avoid ambients altogether is to use a screened room for the radiated measurements.
The tests are typically conducted by test labs which have the facilities to conduct the conducted and radiated emissions. These tests are usually costly to setup and 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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