Conduct EMC Testing Economically In-House

To succeed in industry these days, most manufacturers have to maintain a strong position in the international marketplace. In Europe, that means products must qualify for the CE mark, which ensures compliance with all applicable European Union directives on electromagnetic compatibility (EMC). Designing for this stringent specification, which has been mandatory since January 1, 1996, requires knowledge about both design practices and test methods.

This article discusses equipment and methods electronics manufacturers can use to qualify their products. Included is an example of how Symbol Technologies, Holtsville, N.Y., tests its products, along with the practices the company uses to solve emissions problems. Several problems with test setup and configuration are also discussed. Finally, we will cover some possible surprises that engineers should be aware of when designing a product for EMI compliance.

From gun-shaped bar code scanners to hand-held PCs and wireless LANs, Symbol's products all have one thing in common: electronics. More specifically, they all incorporate embedded digital systems surrounded by analog circuitry. Thus, the products fall into the category of information technology equipment (ITE). The applicable standards include EN 55022 for radiated emissions and EN 61000-4-3 for radiated susceptibility. EN 55022 requires the use of an open area test site (OATS), a gigahertz transelectromagnetic (GTEM) cell, an anechoic chamber, or other alternate test setup to perform electromagnetic interference (EMI) testing.

For a number of reasons, Symbol chose to purchase a GTEM cell rather than build an OATS. A GTEM cell is immune to ambient noise conditions, and tests take up to 90% less time to run, making the system easier and faster to use. GTEM cells also perform susceptibility testing, something an OATS cannot accomplish. The equipment under test (EUT) fits inside the cell, a convenience not all companies have. Finally, the cost of GTEM cells versus an OATS is comparable, around $225,000, including the test equipment. Therefore, there's much more value for the dollar with a GTEM cell.

Despite all the advantages of a GTEM cell, Symbol is still considering the purchase of an OATS. One large disadvantage of a GTEM cell is its frequency limit. An OATS can measure beyond 25 GHz, while a GTEM cell peaks at 5 GHz. For most companies, this limit is fine. However, Symbol manufactures radio products that operate in the 2.4 GHz band, and approval of these radios requires testing up to the 10th harmonic.

The company's GTEM setup consists of an HP 8593E spectrum analyzer with a quasi-peak detector card, an Emco Boss manipulator, an HP 8648A RF signal generator, an IFI SMX-100 RF amplifier, and a PC. The entire setup is controlled by GTEM software running on the PC via the IEEE-488 (GPIB) interface (Fig. 1). The signal generator and amplifier are used for susceptibility testing and the spectrum analyzer is used for radiated emissions testing.

To perform an emissions test properly, the product (usually in prototype plastics) is securely mounted on the manipulator table with all the appropriate cables attached (Fig. 2). The test is run with the unit operated in a user-intended mode. For a bar code scanner, this means continuously scanning a bar code and transmitting the data to a host. To accomplish this, a pneumatic actuator pushes the trigger on the scanner, whose exit window aims at a bar code mounted a specified distance away. The power-supply cable plugs into an EMI-filtered ac outlet inside the chamber, and the data cable attaches to a host through a filtered or isolated connector.

Before the emissions test begins, the engineer enters specific standards information into the system's test software. This software is typically purchased from the manufacturer of the GTEM cell.

At the start of the testing, the manipulator rotates the unit to a 45° azimuth, 120° orthogonal angle to the ground plane inside the GTEM. The spectrum analyzer then sweeps the frequency range. When completed, the manipulator moves to a 45° azimuth, 0° orthogonal angle, and repeats the test. Finally, the manipulator rotates to a 45° azimuth, -120° orthogonal angle for the final radiated measurements. These positions, which are controlled by the software, are devised by the manufacturer of the GTEM cell to produce the desired test results.

The GTEM software then performs calculations that correlate the data to produce a plot of radiated emissions. The software uses many parameters during its correlation: the height of the EUT's center from the ground plane, the distance from the GTEM antenna to the EUT, the separation between the ground plane and the septum, and the distance between the EUT and the septum, just to name a few. By locating the EUT in these three positions and performing the calculations, Symbol has consistently shown a correlation between its GTEM and an OATS. The company can and does certify non-RF products in-house with this setup.

A good example of EMC design and test procedures is a project currently in the works: a cordless scanner that uses a base station for charging and data transfer. Regulations insist that a product be tested under worst-case conditions. One such configuration consists of the scanner mounted in the base, charging and transferring data. Running an emissions test on this system checks two interconnected microprocessors that transfer data between each other and a host. Charging adds to the emissions because the charging circuitry inside the product uses a switch-mode power supply. In order to obtain the worst case, a completely dead battery is used to draw the most charging current.

Symbol wants to certify the new scanner to the CISPR B standard in Europe. Although things could have been worse, a graph of the results for the initial prototypes of this product shows that the system did fail to meet the CISPR B limit at a few frequencies.

To succeed in industry these days, most manufacturers have to maintain a strong position in the international marketplace. In Europe, that means products must qualify for the CE mark, which ensures compliance with all applicable European Union directives on electromagnetic compatibility (EMC). Designing for this stringent specification, which has been mandatory since January 1, 1996, requires knowledge about both design practices and test methods.

This article discusses equipment and methods electronics manufacturers can use to qualify their products. Included is an example of how Symbol Technologies, Holtsville, N.Y., tests its products, along with the practices the company uses to solve emissions problems. Several problems with test setup and configuration are also discussed. Finally, we will cover some possible surprises that engineers should be aware of when designing a product for EMI compliance.

From gun-shaped bar code scanners to hand-held PCs and wireless LANs, Symbol's products all have one thing in common: electronics. More specifically, they all incorporate embedded digital systems surrounded by analog circuitry. Thus, the products fall into the category of information technology equipment (ITE). The applicable standards include EN 55022 for radiated emissions and EN 61000-4-3 for radiated susceptibility. EN 55022 requires the use of an open area test site (OATS), a gigahertz transelectromagnetic (GTEM) cell, an anechoic chamber, or other alternate test setup to perform electromagnetic interference (EMI) testing.

For a number of reasons, Symbol chose to purchase a GTEM cell rather than build an OATS. A GTEM cell is immune to ambient noise conditions, and tests take up to 90% less time to run, making the system easier and faster to use. GTEM cells also perform susceptibility testing, something an OATS cannot accomplish. The equipment under test (EUT) fits inside the cell, a convenience not all companies have. Finally, the cost of GTEM cells versus an OATS is comparable, around $225,000, including the test equipment. Therefore, there's much more value for the dollar with a GTEM cell.

Despite all the advantages of a GTEM cell, Symbol is still considering the purchase of an OATS. One large disadvantage of a GTEM cell is its frequency limit. An OATS can measure beyond 25 GHz, while a GTEM cell peaks at 5 GHz. For most companies, this limit is fine. However, Symbol manufactures radio products that operate in the 2.4 GHz band, and approval of these radios requires testing up to the 10th harmonic.

The company's GTEM setup consists of an HP 8593E spectrum analyzer with a quasi-peak detector card, an Emco Boss manipulator, an HP 8648A RF signal generator, an IFI SMX-100 RF amplifier, and a PC. The entire setup is controlled by GTEM software running on the PC via the IEEE-488 (GPIB) interface (Fig. 1). The signal generator and amplifier are used for susceptibility testing and the spectrum analyzer is used for radiated emissions testing.

To perform an emissions test properly, the product (usually in prototype plastics) is securely mounted on the manipulator table with all the appropriate cables attached (Fig. 2). The test is run with the unit operated in a user-intended mode. For a bar code scanner, this means continuously scanning a bar code and transmitting the data to a host. To accomplish this, a pneumatic actuator pushes the trigger on the scanner, whose exit window aims at a bar code mounted a specified distance away. The power-supply cable plugs into an EMI-filtered ac outlet inside the chamber, and the data cable attaches to a host through a filtered or isolated connector.

Before the emissions test begins, the engineer enters specific standards information into the system's test software. This software is typically purchased from the manufacturer of the GTEM cell.

At the start of the testing, the manipulator rotates the unit to a 45° azimuth, 120° orthogonal angle to the ground plane inside the GTEM. The spectrum analyzer then sweeps the frequency range. When completed, the manipulator moves to a 45° azimuth, 0° orthogonal angle, and repeats the test. Finally, the manipulator rotates to a 45° azimuth, -120° orthogonal angle for the final radiated measurements. These positions, which are controlled by the software, are devised by the manufacturer of the GTEM cell to produce the desired test results.

The GTEM software then performs calculations that correlate the data to produce a plot of radiated emissions. The software uses many parameters during its correlation: the height of the EUT's center from the ground plane, the distance from the GTEM antenna to the EUT, the separation between the ground plane and the septum, and the distance between the EUT and the septum, just to name a few. By locating the EUT in these three positions and performing the calculations, Symbol has consistently shown a correlation between its GTEM and an OATS. The company can and does certify non-RF products in-house with this setup.

A good example of EMC design and test procedures is a project currently in the works: a cordless scanner that uses a base station for charging and data transfer. Regulations insist that a product be tested under worst-case conditions. One such configuration consists of the scanner mounted in the base, charging and transferring data. Running an emissions test on this system checks two interconnected microprocessors that transfer data between each other and a host. Charging adds to the emissions because the charging circuitry inside the product uses a switch-mode power supply. In order to obtain the worst case, a completely dead battery is used to draw the most charging current.

Symbol wants to certify the new scanner to the CISPR B standard in Europe. Although things could have been worse, a graph of the results for the initial prototypes of this product shows that the system did fail to meet the CISPR B limit at a few frequencies.