Without question, selecting an immunity test system is a tough job. With so many standards and test instruments to choose from, the process can be mind-boggling. To help make your job easier, here are some tips to get you up and testing without any hitches.
The immunity test system should be designed holistically with each part configured to be complementary with a guaranteed performance of the final system, and at the lowest possible cost, said Joe Heins, senior EMC applications engineer at Chase EMC. The guaranteed performance should have a specification that is clear, concise, and warranted. And when the system is pre-engineered, assembly and delivery time and costs are saved.
According to Mike Hopkins, director of sales and marketing at KeyTek, to determine which test instruments are needed to perform RF immunity testing, you should answer several key questions:
What is the budget?
Is if for compliance or precompliance testing?
Will the equipment have other uses, such as for circuit design?
What standards must the test instruments meet?
Will the instruments need to perform beyond compliance levels?
What are the size and power requirements of the product under test?
The functions of the instruments depend on the test you need to perform, said Mr. Hopkins. For example, questions about the features of a surge simulator are very different than those for an RF enclosure.
Amplifier Research recommends that amplifiers feature high-level voltage standing wave ratio (VSWR) tolerance, Class A operation, the capability to accommodate high power and broad bandwidth, and a power and frequency match of the antennas and amplifier. A high VSWR, or load tolerance, keeps the amplifier from shutting down. Class A operation helps ensure linearity and minimize harmonics. And with the high-power, broad bandwidth features, the tests can continue uninterrupted, and more test requirements can be met.
When amplifiers and antennas operate within the same bandwidth and power ranges, RF tests continue uninterrupted for longer periods of time. The need to stop a test to change an antenna whose power and frequency specifications do not match the amplifier is avoided.
Immunity Test Components
Figure 1 is a block diagram of an immunity test system like one used for IEC 1000-4-3 testing. The test setup is composed of a signal generator, an amplifier, a forward- and reverse-power coupler, a radiating antenna, and an omnidirectional E-field probe system.
The signal generator should have adequate output resolution to precisely set the reference level of the E-field to 1% of the desired level. It must provide the desired 80% amplitude modulation with a 1-kHz sine wave. Figure 1a shows a typical signal-generator output.
The amplifier provides the desired E-field levels when applied to the antenna. EMC test amplifiers are specified with a minimum gain. Due to the wide bandwidth, the amplifier can show ripple of several dBs in the passband. The amplifier must be operated in a linear mode to assure repeatability. Figure 1b shows a typical amplifier response.
The forward/reverse power coupler is placed as near to the antenna as practical. The difference in the forward and the reverse power is recorded to determine the input level necessary for developing the desired test signal at the input to the antenna. Figure 1c shows the output.
The antenna generates the desired E-field. The E-field performance of the antenna is given by the transmit antenna factor shown in Figure 1d.
An omnidirectional probe measures the antenna field strength. Figure 1e is a typical representation of the antenna E-field. The E-field test level is calculated by adding the signal generator output, amplifier gain, and transmit antenna factor (TAF):
E(dBµV/m) = SGout(dBµV) + AG(dB) + TAF(dBm-1)
where: E(dBµV/m) = E-field output test level
SGout(dBµV) = signal generator output
AG(dB) = amplifier gain
TAF(dBm-1) = transmit antenna factor
The variables and terms in the expression for the E-field output level are used for calibration test setups. They demonstrate how instrumentation and facility factors contribute to meet the E-field uniformity values of -0.0 dB and +6.0 dB typically required. The testing to demonstrate that the equipment-under-test (EUT) does not malfunction when exposed to the desired level requires the addition of 80% amplitude modulation with a 1-kHz sine wave to the test signal.1
Table 1 shows the features recommended for equipment performing radiated immunity testing per standards such as EN 50082-1, EN 50082-2, EN 61000-4-3, and IEC 801-3.
Effect of Standards Changes
Test-equipment needs change as the radiated and conducted immunity standards are refined. To remain current, test engineers must look out for any early warnings on changing information.
Fortunately, the IEC 1000-4-x standards for surge, ESD, EFT, dip and interrupt, and magnetic fields are mostly stable, said KeyTek’s Mr. Hopkins. Some less significant issues in the ESD standard include changes to how the discharge is made to the horizontal coupling plane, clarification of what the operator-accessible points are on an EUT, and a method for testing double-insulated products.
Changes are in the works for RF immunity testing such as requirements for a uniform field- test area allowing fewer test points for smaller products, continued Mr. Hopkins. There also is a normative annex to specify the requirements for the use of the transverse electromagnetic (TEM) cell, including the gigahertz-TEM.
Recent changes in the radiated immunity testing primarily are associated with the frequency range of the test and the method used to measure and calibrate the field strength, said Chase’s Mr. Heins. The test-frequency range soon will exceed 1 GHz once preliminary standards are passed.
The test-methodology differences will affect antennas, amplifiers, and software, said Mr. Heins. Although most EMC engineers are familiar with the active field-strength leveling procedures detailed in IEC 801-3-1984 immunity standard, EN 61000-4-3-1996 requires a hypothetical vertical plane to be calibrated. The IEC 801-3 levels the field strength with the EUT, and the EN 61000-4-3 calibrates the plane without the EUT. Because of the difference in calibration, the EUT pass/fail results will be different.
The selection of immunity test equipment also is affected by the Medical Device Directive and demands of the automobile industry. Interference generators are used to test the electronics in medical and automotive instruments that must comply with immunity regulations. For example, the electronic subsystems in automobiles must be tested individually and with other subsystems to ensure they work in real-world situations. This validation needs test equipment that lets you run conducted immunity checks to industry specifications and your own requirements.
Reference
1. “Essential Equipment for EMC Testing,” Antenna Catalog, EMC Test Systems, p. 69, 1997.
EMC Immunity Products
Generators Test for Compliance
To Six Immunity Specifications
The UCS500M/2 and the UCS500M/4 Interference Generators test for full compliance to the immunity specifications of IEC 1000-4-2, -4, -5, -8, -9, and -11. The instruments integrate electrostatic discharge, burst, surge, voltage dips, power frequency, and magnetic-field waveforms using a built-in coupling and decoupling network. The M/2 tests to 2,500 V and the M/4 to 4,400 V. The M/2 provides an air-discharge voltage to 8 kV, the M/4 supplies up to 15 kV. An 8-kV test voltage for contact discharge also is furnished. Amplifier Research, (215) 723-8181.
Antenna Combines Bilog and
Low-Frequency Technology
The CBL6140 X-Wing Bilog Antenna allows low-frequency power transmission without affecting high-frequency elements. It performs RF immunity testing per the requirements of IEC 1000-4-3. The antenna uses a matching network, has an antenna factor of 3 dB @ 26 MHz, and handles power up to 500 W. Chase EMC, (973) 252-8001.
EMC Chamber Complies
With IEC 1000-4-3 Requirements
The Model 25 SpaceSaver™ Chamber is lined with ferrite tile and includes Rantec’s FerroSorb™ FS-400 Absorber in select regions of the chamber to increase the available working space. The chamber complies with the radiated immunity test requirements of IEC 1000-4-3 and MIL-STD-461D/462D. The chamber provides ±6-dB deviation from the normalized site attenuation per ANSI C63.4 procedures and ±3 dB with a site correction factor over a 1.5-m dia test volume. For radiated immunity testing, it offers a -0 and +6-dB field uniformity from 26 MHz to 18 GHz, and safely handles up to 200 V/m. EMC Test Systems/Rantec, (800) 253-3761.
System Generates EFT
Tests to 2 MHz
The PEFT 4010 Immunity Test System generates spike frequencies from 1 Hz to 2 MHz, produces five burst patterns, and provides a test voltage from 0.10 V to 4.5 kV. The system contains an electrical fast transient generator, a single-phase mains coupler, and control functions for the company’s ESD generator. The test voltage, spike frequency, synchronization angle, and signal polarity are programmable during testing. Haefely Trench, (703) 494-1900.
Immunity Test System Meets
Requirements for CE Marking
The EMCPro™ EMC Immunity Test System meets the requirements for the CE Marking and other international standards. It performs tests for ESD, EFT, surge, dips, interrupts, and magnetic-field immunity for IEC and EN standards. The system also checks surge levels up to 6.6 kV to meet ANSI, CCITT, and UL requirements. In addition to the 1.2/50-µs surge waveforms required by the IEC standard, a 10/700-µs telecom waveform or a 100-kHz ring wave can be added as options. KeyTek, (508) 275–0800.
GTEM Cell Used for
Testing to IEC 1000-4-3
The GTEM Cell is used for radiated immunity testing including IEC 1000-4-3 and MIL-STD-461/462. The unit provides repeatable results for 3-, 10-, and 30-meter open- air test-site measurements or for anechoic chambers to within 4 dB from 30 MHz to 1,000 MHz. Lindgren RF Enclosures, (630) 307-7200.
RF Simulator Features
Automated Calibration
The NSG 2070 High-Frequency Generator supports testing for susceptibility to induced RF signals per the requirements of IEC 1000-4-6. It features an automated self-calibration function and an interrupt mode to pause the test at any point. The unit consists of a 100-kHz to 250-MHz frequency synthesizer and an 85-W power amplifier. A keypad on the front panel accesses the calibration and test functions. The instrument also can be controlled remotely from a PC using the company’s software. Schaffner EMC, (800) 367-5566.
Anechoic Chamber Available
For 3-Meter Testing
An anechoic chamber for 3-meter testing meets the requirements of the IEC 1000-4-3 immunity standard. It uses a ferrite tile in a double-layer configuration and has an upper frequency limit of 18 GHz. The chamber offers a vertical plane test area ranging from 0.8 m to 2.3 m above the ground plane and 1.5-m wide. TDK of America, (847) 803-6100.
Compact System Performs
Compliance Testing
The TEMCell is a compact test system that monitors equipment up to 19″ × 19″ × 19″ in size to the immunity requirements of EN 61000-4-3. A user-supplied PC controls the system through a GPIB interface. Up to 24 signal channels from the EUT, both digital and analog, can be recorded and displayed. Pass/fail limits can be software established. Sweep and spot frequencies are programmable over a 26-MHz to 1-GHz range. Field strengths from 1 V/m to 20 V/m or greater also are programmable. Wayne Kerr, (800) 933-9319.
Conducted Immunity Generator
Tests at 15 mW to 25 W
The CWS500 Transient Generator helps meet the requirements for the IEC 1000-4-6 and EN 50082-1 and -2 specifications for conducted immunity. The generator performs tests at 15 mW to 25 W from 9 kHz to 240 MHz. The voltage standing wave ratio is 1:1 at maximum power and at all phase angles. Calibration typically takes less than 3 min. Up to four coupling/decoupling network calibration values are stored in memory. Serial and parallel interfaces are standard. Amplifier Research, (215) 723-8181.
Table 1
|
Test Instrument |
Features |
|
Signal Generator |
10 kHz to 2.4 GHz frequency range, IEEE port free of transients while switching internal oscillators during sweep functions, internal amplitude modulation with a programmable modulation oscillator, internal pulse modulator performing tests at 900 MHz and 1.8 GHz or with external modulator. |
|
Power Amplifier |
Output power should be rated at 1-dB compression point (not at the maximum), and 3.24× greater than required power for continuous-wave power to allow linear amplification of the amplitude modulated carrier. Harmonic distortion <-15 to -20 dBc at full power. Class A or Class AB operation for linear interpretation between 3 V/m and 10 V/m, to determine pass/fail thresholds. |
|
Directional Coupler |
30 dB or less for 3 V/m testing; if >30 dB, the signal received at the power meter will be very close to the internal noise floor of the meter and will yield invalid and unrepeatable measurements. Voltage standing wave ratio <1.5:1. |
|
Power Meter |
Sensor diode input >+20 dBm; >300 meas/s; dual channel for monitoring forward and reverse power with capability to obtain absolute power. |
|
Immunity Antenna |
20 MHz to 2 GHz to eliminate the need to move the antenna during band switches, reduce measurement uncertainty. Voltage standing wave ratio from 20 MHz to 80 MHz should be <12:1, from 80 MHz to 1 GHz <2:1. Antenna size matched to shielded enclosure size, reducing standing waves and waveguide effects developed in enclosure. Mounting fixture to allow movement between vertical, horizontal polarization without changing the X, Y position with respect to the enclosure. |
|
Isotropic Probe |
10 kHz to 3 GHz, remote control fiber-optic links, 1 to 800 V/m measurement range. |
|
Immunity Software |
Perform radiated and conducted immunity testing for commercial, military needs; automated test with pass/fail analysis, threshold determination, post measurement interaction; control of any manufacturer’s equipment with option to edit drivers. |
Copyright 1998 Nelson Publishing Inc.
April 1998
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