(Conducted) Immunity Testing Overview

Introduction

The following discusses the concept of electromagnetic immunity testing by means of injecting current into the input and output cables of a system under test. The primary supporting reference document is the IEC 1000-4-6 specification titled "Immunity to conducted disturbances induced by radio-frequency fields." The actual test methodology and detailed setup is beyond the scope of this overview, but is thoroughly covered in the specification. Specific equipment recommendations are provided which address the three test levels called out in the specification.

Background

Electrical and electronic equipment are susceptible, to varying degrees, to radiated electromagnetic disturbances from various man-made and natural sources. Close proximity to an RF transmitter, or even a small hand-held transceiver, can potentially create havoc with circuitry not properly designed with electromagnetic immunity in mind.

Since capability problems are caused by close proximity to a source of electromagnetic energy, the method generally used to test for immunity is to immerse the test object, or equipment under test (EUT), in a given electromagnetic field. The EUT is then powered-up and monitored to verify proper operation when exposed. Amplifier Research is a leader in supplying equipment for this purpose (referred to as radiated immunity testing) and our catalogs and application literature address the issue of equipment selection in great detail. See typical (radiated) rf susceptibility test configuration for additional information on this topic.

One can determine the susceptibility of an EUT by radiated-immunity testing alone. However, radiating antennas exhibit poor gain at lower frequencies, up to approximately 80 MHz, thus, greatly increasing power requirements.

Also, room reflections at lower frequencies are difficult to minimize, making repeatability of test results a problem.

One potential solution to the low-frequency testing dilemma is conducted immunity testing. Here, various schemes are employed to directly induce current into cables of the EUT. The dimensions of the EUT are generally small compared to the wavelength of these lower frequencies, but the total length of the signal, control, and power cables can be several wavelengths long. Thus, the EUT cabling acts like a passive receiving antenna, which accounts for most of the electromagnetic vulnerability effects at these lower frequencies.

In January 1995, the International Electrotechnical Commission (IEC) updated the IEC 801-3 specification titled "Radiated electromagnetic field requirements." The new IEC 1000-4-3 specification increases the lower-frequency testing limit from 27 MHz to 80 MHz, with the understanding that the difficult region below 80 MHz will eventually be addressed by means of conducted immunity testing.

The operative specification that deals with this frequency range is IEC 1000-4-6. While this specification is not part of a published European Norm at this time, it will most likely be used to complement the IEC 1000-4-3, and as such, will address the frequency range from 150 kHz to 80 MHz.

Salient Details from IEC 1000-4-6

The specification contains four test levels; 1 volt, 3 volts, 10 volts, and an X level in volts to cover special situations. The frequency range covered is 150 kHz to 80 MHz, and the RF is 80% AM modulated at a frequency of 1 kHz.

Currently there are four accepted methods of injecting current into system cables. The preferred method involves the use of coupling and decoupling networks (CDNs). If CDNs are either not available or not appropriate, then either clamp injection via a current clamp or an electromagnetic clamp [EM-clamp], or, possibly, direct injection can be used.

IEC 1000-4-6 details the procedure one must follow to select the appropriate means of signal injection, and the signal method of application. In addition to a section on definitions and terminology, there are five informative annexes that provide background information pertaining to conducted susceptibility testing.

A block diagram of a generic conducted immunity test set-up appears in the informative section of the IEC 1000-4-6 specification. Amplifier Research broadband amplifiers suitable for testing at the required levels are recommended below.

The test level is determined by the electromagnetic environment that the EUT will be exposed to. The following general guidelines are taken from Annex B of the specification:

• Class 1 (one volt)-Low level electromagnetic radiation environment.
  Levels typical of local radio/television stations located at a distance of more than 1 km, and levels typical for low-power transceivers.
• Class 2 (three volts)-Moderate electromagnetic radiation environment.
  Low-power portable transceivers (typically less than 1 watt rating) are in use, but with restrictions on use in close proximity to the equipment. This class constitutes a typical commercial environment.
• Class 3 (ten volts)-Severe electromagnetic radiation environment.
  Portable transceivers (2 watts and more) are in use relatively close to the equipment, but at a distance not less than 1 meter. Higher power broadcast transmitters are in close proximity to the equipment and ISM equipment may be located close by. This class constitutes a typical industrial environment.

Amplifier Research Recommendations

The AR Model 25A250A power amplifier is appropriate for the 1 V/m and 3 V/m test levels, and the AR Model 75A250 is recommended for 10 V/m testing.

In general, CDNs are preferred. If a CDN is not available or suitable, then one of the other allowed methods can be used. The specification provides guidance in selecting the appropriate method of signal injection.

The amplifiers specified are capable of driving severe mismatches characterized by very poor VSWRs. Furthermore, these amplifiers provide sufficient power to allow for insertion of a 6dB matching pad between the amplifier and the coupler, as recommended by the specification.

Summary

A method is available to test for system immunity at frequencies where the power required for radiated immunity testing becomes fairly large. Conducted immunity testing has been used in both military and avionics testing applications for quite some time, but due to the European EMC directive, will become a major factor in commercial testing applications. Equipment is available to generate the test levels called out in the specification.

The VSWRs of the injection devices typically vary greatly from an ideal VSWR of 1.0:1. It is not uncommon to experience VSWR variations ranging from a low of 2:1 to as much as 65:1. It is critical that the power amplifiers specified are capable of driving the load mismatches normally encountered in conducted immunity testing without damage, foldback, or oscillation. Only a properly designed Class A power amplifier can deliver the required power for this application. (See The Importance of Load Tolerance.) While the normal testing frequency range is 150 kHz to 80 MHz, there is a possibility that small EUTs (less than (1/4 wavelength) may be tested per a dedicated product standard to a maximum of 230 MHz. Both of the amplifiers recommended provide sufficient power at 230 MHz to accommodate this contingency.

Finally, since the immunity of an EUT is determined by a host of conditions, not the least of which is the impedance of the susceptible circuits (regardless of their physical size) and the effectiveness of cable filters and system shielding, Amplifier Research recommends that both radiated and conducted immunity testing be performed from 25 MHz to 250 MHz. This conservative testing philosophy is especially germane when the EUT is large, unshielded, or a safety-related product.