While high-performance test equipment has been used successfully in the laboratory for many years, the trend is moving toward remote testing, which means smaller, lightweight equipment that can be taken into the field. It is crucial that field equipment delivers results with the same quality as those generated in the laboratory environment. Therefore, signal integrity, durability, and long-term reliability of the instrument and its accessories are key factors in ensuring consistent, accurate, and repeatable measurements. Whether you are an instrument designer or a field engineer, you need to consider two components that can directly affect the overall performance of handheld instruments — EMI shielding and long-term cable reliability.

EMI Shielding

Figure 1: Examples of EMI shielding materials that eliminate noise, reflections, and other associated EMI phenomena that can compromise instrument performance.

As an instrument designer, you are challenged to incorporate more functionality into smaller instruments, which increases the complexity of printed circuit board (PCB) designs. These challenges result in the need for high-performance EMI shielding materials that eliminate noise, reflections, and other associated EMI phenomena that can compromise instrument performance. While most engineers are familiar with strategies that reduce EMI issues at the board level — for example, adding extra ground planes, isolating power signals, and carefully locating components — these solutions become more difficult, and in some cases more costly, as new features are added and electronic devices get smaller. By considering your shielding options at the outset of the design process, you can eliminate some of the last-minute EMI issues that often occur during validation testing (see Figure 1).

One consideration is to use thermoformed board-level shields to increase design flexibility. When using traditional shielding technologies, designers usually place noisy components as far away from sensitive electronics as possible, which means that the design is controlled by the shape of the shielding cans rather than by maximizing space and functionality. This process can increase the overall size and weight of the instrument. By using thermoformed board-level shields, you can design the PCB based on circuit and component function without being limited by the shape or size of traditional shielding cans.

Thermoformed shields can be shaped to fit any board, regardless of its geometry. These shields also minimize the amount of space needed to shield individual areas; unlike traditional cans — with each requiring its own trace to connect to the ground plane — thermoformed shields require only a single row of solder spheres to connect individual cavities to the ground plane (see Figure 2). For example, an engineer who was designing a PCB that would have required nine shielding cans significantly reduced the board’s size by using a single, thermoformed board-level shield. The design flexibility gained from thermoformed shields enables components and circuits to be placed in their optimum location without first considering shielding requirements.

A second consideration is to select the best EMI shielding material to be used in conjunction with the instrument housing or closure. Selecting the best shielding option depends upon the materials used for the instrument’s covers and housings as well as the performance of the shielding material. The type and thickness of the housing material can have a significant impact on potential EMI solutions. For example, protective coatings over metallic components (such as chromate used over aluminum) add a nonconductive layer that can prevent the enclosure from engaging with a shielding gasket, resulting in adverse EMI effects. In this case, you need to consider using a gasket material that can pierce the coating. In a recent test equipment application, metallized foam was used to break through the coating and provide improved shielding performance.

Figure 2: Example of a multi-cavity thermoformed shield that fits the custom shape of a PCB.

A third consideration is the housing’s mechanical strength; a flexible plated-plastic enclosure may result in gaps that lead to slot antennas. In this situation, you should evaluate the gasket material’s compressibility, which is the key to consistent electrical and mechanical performance in flexible EMI materials. If the EMI material does not compress and recover at the same rate as surrounding materials, slot antennas can temporarily occur because electromagnetic energy can radiate through the gaps. If you choose surface-mount technology (SMT) grounding pads to protect the instrument at the outset, you have the advantage of providing a uniform enclosure or additional grounding where needed.

Validation testing, a critical step in ensuring the performance of high-reliability test equipment, often highlights EMI-related system failures that are not apparent during testing of an individual component. As components are combined into subsystems or into the final instrument, slot antennas that may have passed before cause the instrument to fail EMI testing. An effective solution to eliminate slot antenna effects without having to redesign the instrument is to add a peel-and-stick shielding material in the housing’s seams or at critical grounding points on the PCB. Adhesive-backed gasket materials provide a flexible EMI solution that can be applied exactly where the problem is located without having to modify current PCB layouts.

Cable Reliability

Figure 3: RF cable assembly that is specifically engineered to perform in a variety of environmental conditions with repeated flexing.

As the user of a high-performance instrument, you need to be confident that it can deliver consistent, repeatable results regardless of the field conditions. RF cable assemblies used between the test instrument and the device under test (DUT) can dramatically affect the measurement accuracy. Therefore, it is essential to use RF cable assemblies that are specifically engineered to perform in a variety of environmental conditions and with repeated flexing (see Figure 3). Otherwise, the cable assembly can cause the instrument’s calibration to drift, resulting in decreased measurement accuracy and repeatability. For example, amplitude and phase distortion in the cable can affect error correction in the instrument’s calibration process, which results in erroneous reflected power values. Or, an unstable cable assembly can result in false measurements, indicating more loss than is actually present.

Cable loss and voltage standing wave ratio (VSWR) can have an impact on an instrument’s measurement accuracy and repeatability. Having confidence in the cable’s VSWR and loss performance over the frequency range being measured is the only way to ensure that your cables will not have an adverse effect on the instrument’s measurement accuracy. Not all RF cable assemblies are created equal. For optimal results, you should only use high-performance assemblies engineered with guaranteed stability in amplitude and phase when flexed (see Figure 4).

Figure 4: Typical amplitude and phase stability of high-performance RF cable assemblies.

Test equipment used in the field requires flexing of the cable assembly during the testing phase, but flexing can also cause performance problems with the cable. As the cable is bent or torqued, errors in its reflection, phase, or loss can occur. Externally ruggedized cables often cannot withstand the torque encountered in the field. Cables constructed with internally ruggedized materials can supply sufficient torque resistance and locking bend radius to resist kinking of the cable and its conductors.

Environmental conditions are another aspect of field testing that can affect a cable’s performance. Unlike the lab environment, conditions such as temperature, humidity, UV exposure, and chemical exposure cannot be controlled in the field. Therefore, you should evaluate the cable’s jacket materials to ensure that they can withstand a wide range of environmental elements. For example, cables made with hydrophobic expanded polytetrafluoroethylene jackets can withstand temperatures ranging from -55 °C through +125 °C, and prevent contaminants such as water, dirt, and chemicals from entering the cable structure.

A cable assembly can compromise measurement accuracy long before it fails completely, so long-term durability is essential for cable assemblies used with field instruments. The best way to ensure that an RF cable assembly can withstand the conditions in the field is for the cable manufacturer to validate its durability and long-term reliability through individual cable assembly testing and qualifying. Insertion loss, VSWR, phase stability, and amplitude stability should be evaluated while the cable is undergoing flex testing to ensure that the assembly’s performance does not degrade during movement. Only with this type of thorough testing can you be confident that the cable will handle mechanical and environmental stress without compromising signal integrity.

Summary

As test instruments become increasingly complex and move into the field, their performance can easily be compromised by the environmental conditions and the operator’s handling of the instrument. The reliability of both EMI shielding materials and RF cable assemblies has a direct impact on the measurement accuracy and repeatability of handheld test instruments. Selecting the right EMI shielding materials during the design process can maximize signal performance within the instrument itself. To ensure measurement accuracy and repeatability, the instrument should be used with high-performance RF cable assemblies engineered specifically to handle the harsh field conditions typically encountered.

This article was written by W. L. Gore & Associates, Inc., Landenberg, PA. For more information, Click Here 


RF & Microwave Technology Magazine

This article first appeared in the October, 2010 issue of RF & Microwave Technology Magazine.

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