The major changes in the latest edition of this EMC standard concern both manufacturers and test facilities.
For many years, IEC 61000-4-3 Electromagnetic compatibility (EMC)-Part 4-3: Testing and measurement techniques-Radiated, radio-frequency, electromagnetic field immunity test has been one of the basic standards that controls the testing of many products.1 It establishes test levels and the required test procedures for the immunity requirements of electrical and electronic equipment to radiated electromagnetic energy.
The third and newest edition, published in February 2006, impacts product manufacturers, test equipment manufacturers, and test facilities.2 Compared to the second edition, the two major differences are in the areas of frequency range and harmonic requirement.
The test frequency range has been increased from 2 GHz to 6 GHz. Although it is not intended for a test to be applied continuously over the entire frequency range from 1.4 GHz to 6 GHz, test laboratories must update their facilities to handle up to 6 GHz. Product manufacturers must review product design to ensure compatibility with the new requirement.
Many existing immunity semi-anechoic chambers are lined with ferrite tile or grid tile absorbers on the four walls, ceiling, and a floor section between the front of the uniform field area and the antenna, namely the 3-meter path length. They work fine up to 1 GHz or 1.5 GHz, and then the performance falls dramatically.
With the extended frequency range, additional absorbing materials such as foam absorbers are needed to fulfill the field uniformity requirements. Each chamber manufacturer offers different types and sizes of absorbers. These additional absorbing materials take about 12″ to 18″ of space from the walls, ceiling, and floor, decreasing the available inside space.
To accommodate this loss of space, an absorber manufactured from a fibrous composite material that looks like white cardboard has been introduced to the market. Several chambers use this type of absorber.
The upgrade only requires a partial treatment of hybrid absorbers in specular regions of the chamber. Hybrid absorbers on the end wall behind the EUT are important to absorb extraneous signals. Hybrid absorbers on the end wall behind the antenna contain any backscatter from the antenna, and absorbers on the ceiling should mirror the sidewalls.
Hybrid absorbers on the floor are important between 1 GHz and 6 GHz. Once the frequencies go higher, the floor no longer is within the beam width of most antennas.
If the chamber is very small, there may not be enough space to meet the separation distances required by the specification between the end walls, EUT, path length, and antenna. Normally, at least 1 meter from the back wall to the antenna and 1 meter from the opposite back wall to the EUT. However, it may not be necessary to replace these small chambers with new ones. They can be modified easily by adding length, width, and height if the chamber is a modular shield construction.
If a chamber is a welded shield construction, the modification is more difficult and expensive. Also, for small EUT, these chambers may be useable with a smaller uniform field area (UFA). Remember that the test results from chambers with a 3-meter test distance take precedence over chambers with a smaller test distance.
If the existing chamber is a 3-meter, full-compliance emission chamber with only ferrite tile absorbers, it also can be upgraded to a compliant radiated immunity chamber per EN 61000-4-3 by adding hybrids on the walls, ceiling, and floor section similar to the specular treatment. However, the wall treatments would be slightly larger.
RF signal generators, power amplifiers, and antennas are needed to generate the required field strength. This is frequency-dependent equipment and must cover frequencies up to 6 GHz.
Many RF signal generators currently on the market cover the frequency ranges of interest. The modulation requirement is the same for both editions of the standard: amplitude modulated by a 1-kHz sine wave with a modulation depth of 80%. Some product family standards require pulse modulation above 1 GHz.
Power amplifiers have much less variety than RF signal generators. Many commercially available amplifiers operate in octave frequency ranges such as from 2 GHz to 4 GHz and 4 GHz to 8 GHz. Newer solid-state type amplifiers are available that operate over a frequency range of 2.5 GHz to 6 GHz. With the current selection of amplifiers on the market, it may take two additional amplifiers to extend the frequency range from 2 GHz to 6 GHz.
Any linearly polarized antenna can be used if it satisfies the frequency range and power requirements. Most BiLog antennas have an upper frequency limit of 2 GHz or 3 GHz while some cover up to 6 GHz or 7 GHz. A double-ridged waveguide horn antenna typically ranges from 1 GHz to 18 GHz.
Field strength can be monitored through field probes and field monitors. Forward power can be monitored through dual-directional couplers and power meters with sensors. This equipment also is frequency dependent. If the existing test equipment does not operate up to 6 GHz, additional equipment will be needed.
Dual-directional couplers normally do not cover wide frequency ranges. New dual-directional couplers may be needed.
Tables and Supports
For years, wooden tables and supports have been widely used in the test setup. They are affordable and easy to make. However, as frequencies increase above 1 GHz, tables made from wood can be reflective.
A low-permittivity material is required to satisfy the field uniformity requirements for frequencies greater than 1 GHz. Rigid polystyrene is an example of a low-permittivity material. For reference, Table 1 lists relative permittivity of different materials.3
Product manufacturers must review product design, both electrical and mechanical, to ensure products are still in compliance. As frequencies increase, circuits behave differently. For example, a piece of copper trace can become a good antenna, and a seam or an opening of an enclosure can be a good slot antenna.
The second edition of the standard requires that the harmonics and distortion produced by the power amplifier must be at a level £ 15 dB below the carrier level. The third edition states that the level of any harmonic frequency generated by the power amplifier and measured in the UFA must be at least 6 dB below that of the fundamental frequency.
If a power amplifier has a harmonic rating of -15 dBc to -20 dBc and is not operating into compression, it should meet 6-dB requirements within the UFA. Solid-state amplifiers should meet this requirement easily.4 Traveling wave tube amplifiers may require filtering.
Linearity Check Requirements
The third edition of the standard requires that amplifiers must handle the required modulation without saturating during testing. Procedures to check the linearity of power amplifiers are added to the calibration methods.
The saturation of the test system is checked to verify that the amplifier is below its 2-dB compression point at each harmonic frequency. The 1-dB compression point corresponds to about a 20% reduction in gain.5 At the 2-dB compression point, gain has reduced by 36%. Figure 1 shows 1-dB, 2-dB, and 3-dB compression.
ï¿½ Reference documents are undated, and the latest version of the references applies.
ï¿½ Stripline circuits and TEM cells no longer are referred to as alternative methods of generating EM fields. The combination of an RF signal generator, power amplifier, and an antenna becomes the standard method to generate EM fields.
ï¿½ Partially lined screened rooms and open area test sites no longer are referred to as alternative test facilities. A modified semi-anechoic chamber has become a standard facility to accommodate EUT.
ï¿½ A combination antenna such as BiConiLog, BiLog, and Bilogical and a horn antenna are included, reflecting the acceptance of the latest developed antennas.
ï¿½ Circularly polarized antennas no longer are appropriate because they are not linearly polarized.
The authors would like to thank the members of EMC-PSTC, Jason Smith of AR Worldwide, Kevin Baldwin and Zhong Chen of ETS-Lindgren, Peggy Girard of Panashield, Roland Gubisch of Intertek-ETL Semko, and Wayne Owens of Crestron Electronics for their contributions to and support of this article.
1. IEC 61000-4-3 Electromagnetic compatibility (EMC)-Part 4-3: Testing and measurement techniques-Radiated, radio-frequency, electromagnetic field immunity test, Edition 2.1, September 2002.
2. IEC 61000-4-3 Electromagnetic compatibility (EMC)-Part 4-3: Testing and measurement techniques-Radiated, radio-frequency, electromagnetic field immunity test, Third Edition, February 2006.
3. ï¿½Lessons in Electric Circuits,ï¿½ http://www.allaboutcircuits.com/vol_1/chpt_13/3.html
4. Smith, J., ï¿½Updates on the new release of IEC 61000-4-3 Edition 3,ï¿½ AR Worldwide, Application Note #41, http://www.arww-rfmicro.com/pdfs/appNotes/AppNote41.pdf
5. ï¿½Rating Power Amplifiers for RF Immunity Testing,ï¿½ http://www.electroline.com.au/feature_article/article.asp?item=289
About the Authors
Grace Lin is senior compliance engineer at Crestron Electronics. She has worked in the EMC field for many years, including nine with Intertek-ETL Semko. Crestron Electronics, 6 Volvo Dr., Rockleigh, NJ 07647, 201-750-7004, e-mail: [email protected]
David Schramm is a NARTE certified engineer for Intertek-ETL Semko. He has more than 11 years of experience in the EMC field. Intertek ETL Semko, 1950 Evergreen Blvd., Suite 100, Duluth, GA 30096, 678-775-2400, e-mail:[email protected].