We all know the familiar dialogue from the flight attendant: All electronic devices must be turned off now to prevent interference with the aircraft navigation system. After we have reached our cruising altitude, the captain will indicate that it is safe to turn them on again. This includes cell phones, laptop computers, tape recorders, and any other electronic equipment.
We recognize the imperative nature of that announcement. After all, you don’t want to end up in Boston when your appointments are in Philadelphia just because you failed to turn off your laptop computer.
Like the aircraft navigation system, almost any electronic device will be confused and may even fail when subjected to some level and frequency of RF interference. Possibly your cell phone doesn’t output enough power to send an MD-88 to Boston, but are you sure?
At any given time in virtually any populated area of the world, our electronic equipment is surrounded by various sources of electromagnetic interference. Since these radiations are inevitable, each type of product should be tested to determine if the signals that it is likely to encounter in use will cause it to fail.
RF interference can reach the critical area in a product by radiation through free space or by pickup through a conduction path, such as a power input or signal line. Both types of entry must be evaluated in an immunity test sequence.
Europe’s Role in Standards Definitions
European Union (EU) agencies have been the most active in defining acceptable immunity levels for products to be marketed on the Continent. The population density is high in most European countries, and citizens have experienced the results of unwanted radiations for many years. The best-known specifications for susceptibility testing have the EN prefix, come from the European Committee for Electrotechnical Standardization (CENELEC), and are identical to or derived from standards developed by the International Electrotechnical Commission (IEC).
Each product to be certified must be subjected to high levels of radiated electromagnetic energy while the operator verifies proper operation at each frequency in a specified band. If the product has external cables, an interfering signal must be injected into them to see if this conducted interference causes a problem.
Test for Radiated Immunity
The EU directs manufacturers of equipment for sale in Europe to test the products for intrinsic immunity to electromagnetic radiation. Specification IEC 1000-4-3/EN 61000-4-3, Radiated Electromagnetic Field Requirements, calls for monitoring a DUT for continuous proper operation as it is subjected to a high RF radiation level.
Specifically, the test involves generating interference across the 80-MHz to 1-GHz spectrum in specified steps, each defined as a percentage above the current frequency. The signal must have 80% amplitude modulation and produce strong electromagnetic field levels at a 3-m distance from the radiating antenna.
Some programs call for product immunity tests at higher frequencies. These may be new bands, but generally they involve only one or two specific points in the spectrum. Why test at such high frequencies?
“RFI immunity will be required at ever higher frequencies because interfering sources such as mobile phones are exploiting those parts of the spectrum to get greater bandwidth,” according to Vladimir Kraz, president of Credence Technologies. “Also, don’t forget the ubiquitous PCs. Some operate at over 1 GHz already, which puts the important fifth harmonic above 5 GHz.”
In a susceptibility test, consumer equipment is subjected to a field strength of 3 V/m; industrial products get 10 V/m. Generation of such high signal levels dictates that the interference amplifier and antenna can handle a wide range of frequencies and amplitudes. Also, such radiation levels must be contained within a shielded enclosure to comply with FCC regulations and avoid interference with other users. Generally, this is accomplished with a GTEM cell for small products or a shielded room for large ones.
The spot where the DUT is placed for testing must be validated to determine that signal levels from the transmitting antenna are approximately the same across the area. This is a tedious process, and it is impracticable to do it without a PC and calibration software.
To qualify a 1.5-m × 1.5-m plane on which to place the DUT, you divide the area into 16 points on a 0.5-m grid. Choosing a point near the center as the reference, the RF interference source is set to the low end of the test range, such as 80 MHz, and the power into the antenna is adjusted to produce a 3-Vrms level (consumer products) or 10-Vrms level (industrial equipment) as monitored by a field-strength meter.
The frequency is stepped from the low end of the desired spectrum to the high end, and the amplifier output is adjusted at each step to get the same field strength as the reference frequency. The operator then proceeds through the other 15 points, driving the antenna at the same level as before for each frequency step and recording the field strength.
From the results, the four locations that had the greatest variation from the reference are discarded. For the other 12 locations, you verify that all are within the window of allowable variations defined by the specification.
Manufacturers are always looking for ways to streamline the test equipment. For example, “The custom arraying of antennas to achieve the desired pattern for EUT coverage,” noted Wilart Banks, EMC engineer at Antenna Research Associates, “is becoming popular. Arrays offer optimum gain with maximum coverage of the test plane.”
Conducted Immunity Tests
You could determine susceptibility to RF interference solely by the radiated immunity tests. However, radiating antennas exhibit low gain below about 80 MHz, and it is difficult to generate the required field strengths at the lower frequencies. Also, room reflections at the lower frequencies make it hard to repeat measurements.
The accepted solution to the low-frequency testing dilemma is conducted immunity testing. In these tests, interfering signals at 150 kHz to 80 MHz are injected into the system cables in one of four ways. The preferred method is through a coupling/decoupling network (CDN), and the alternatives are via current clamp, electromagnetic clamp, or direct injection.
The EU requirements for conducted interference are spelled out in IEC 1000-4-6/EN 61000-4-6, Immunity to Conducted Disturbances Induced by Radio-Frequency Fields. The DUT must operate continually and properly when the injected interference is 3 Vrms for consumer products or 10 Vrms for industrial equipment. The frequency range is 150 kHz to 80 MHz, with 80% amplitude modulation using a 1,000-Hz sine wave. Characteristics of each coupler type are defined in the specification.
Typical CDNs for these tests present 150-W input and output impedances. The voltage standing-wave ratio (VSWR) can vary significantly from the ideal 1.0:1, and some are even as bad as 50:1. This places a special demand on the driving amplifier, which must generate the appropriate power levels and frequencies into such mismatches without damage, foldback, or oscillation. Insertion of a 6-dB matching pad as recommended by the specification requires more power from the driver but helps with severe mismatches.
In certain cases, the upper frequency of the conducted immunity test can be extended to perhaps 250 MHz. This allows a corresponding increase in the lower-end coverage of the radiated immunity test. Amplifier Research recommends that you adopt a conservative approach, particularly if the DUT is large, unshielded, or safety related. The company suggests performing both radiated and conducted immunity tests from 25 MHz to 250 MHz.
Other Immunity Requirements
In addition to the tests for radiated and conducted immunity, certain products require testing for other interference. Two specifications of timely concern are EN 61000-3-2 relating to harmonics via the AC power line and EN 61000-3-3 limiting flicker on the AC power line. These became effective Jan. 1, 2001.
The harmonics standard specifies current limits up to and including the 40th harmonic of the power frequency and applies to virtually every product that plugs into a power line in Europe. The measurement criteria are quite detailed, such as making an allowance for harmonics that exceed the limit for <10% of the time in 2.9 min.
The flicker test measures the fluctuation of voltage caused by the product under test. The limits are frequency-weighted to correspond to physiological visual perception of lighting flicker, so the test equipment must be able to measure such characteristics.
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April 2001