Stretching immunity test to 6 GHz

New immunity test standards may require changes in test facilities.

Richard A. Quinnell, Contributing Technical Editor -- Test & Measurement World, 9/1/2006

The communications industries are using more and more of the RF spectrum, pushing well into the gigahertz ranges. In response to this trend, the IEC has been rapidly amending its electromagnetic compatibility (EMC) standards. One of the most significant changes comes with the update of the 61000-4-3 radiated immunity test technique standard to Edition 3.0. (For other standards changes, see "2006: A year of change.")

The new edition of this standard increases the frequency range requirement on test setups from 2 GHz to 6 GHz. The specific frequency bands to be tested as well as the power levels used will vary from country to country, so those details will need to wait for the definition of specific product standards. Still, test engineers will need to prepare for the impact of the extended range on their test facilities.

According to AR Worldwide senior applications engineer Jason Smith in the AR Application Note #41
(www.ar-worldwide.com/html/50000.asp?S=2), one of the first things engineers should check is the test chamber. Ferrite-lined chambers in common use will have difficulty above 1 GHz because the ferrite becomes less absorbent and more reflective of the RF energy. As a result, the chamber may not be able to provide the field uniformity that the standard requires. Fortunately, Annex C of the standard provides a detailed explanation of the situation and offers advice and options on correcting the problem. A fully lined anechoic chamber with both ferrite and absorber material may be required.

Another test facility element that will need examination is the test table itself. The standard calls for the test table to be made of a low-permeability material, such as rigid polystyrene. Many test facilities use wooden test tables, which Smith notes may have some unwanted properties at the higher frequencies. As with the ferrite chamber material, wood starts becoming more reflective as frequency increases. The result is a non-uniform field and less test repeatability.

Edition 3.0’s extended frequency range also interacts with its new requirements on harmonic distortion. The standard calls for harmonic distortion to be better than –6 dBc out of the antenna for the test setup. Facilities that use RF amplifiers in compression or that use traveling wave tube (TWT) amplifiers may have trouble meeting this specification at the higher frequencies.

Filters between the amplifier and the antenna may be required in order to reduce the harmonics to acceptable levels. The specific amplifier in use will dictate which and how many filters will be needed. Solid-state amplifiers are much less likely to generate strong harmonics, so using a solid-state replacement of the TWT amplifier may be a simpler option.

Increased harmonic content may arise as a result of running an amplifier close to saturation. In addition, the nonlinear behavior of amplifiers near saturation can cause distortions to the amplitude modulated (AM) signals used in testing, compromising test repeatability. To address these problems, the standard now calls for an additional linearity check of the test setup to locate any 2-dB compression points while connected to the antenna.

The figure shows a representative amplifier with several compression points marked, including the newly required 2-dB point. These points represent the output power at which a 10-dB input increase yields an output power increase that is less than 10 dB.

The revised IEC 61000 standard calls for an additional linearity check on test setups to identify a 2-dB compression point. Courtesy of AR Worldwide.

Unfortunately, because antenna impedances vary slightly, the manufacturer’s amplifier specifications will not be sufficient for determining the 2-dB point for a specific test setup. Engineers can, however, use the
1-dB compression point to gauge whether or not a specific amplifier has enough linear output power to meet test needs.

The new edition of IEC 61000-4-3 is a step forward for test engineers, providing firm guidelines for working at higher frequencies. It will require re-examination of test setups, however, in order to ensure that they are up to the challenge.

Also see:

SIDEBAR
2006—A year of change

TABLE
Additional IEC standards for EMC that change during 2006