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When very high standards of performance and frequency response are required, it is often difficult to achieve the desired specifications by adding a filter component to a pre-existing design. This is the case with military, space, telecommunications, and public-safety systems. All of these applications tend to require reliable performance under a wide range of conditions. A solution to these challenges is to design filter components into a combined assembly, which is known as an integrated filter assembly (IFA; Fig. 1). Considering the demanding specifications of modern applications due to size, weight, and power (SWaP) constraints, many other components are incorporated into the filter assemblies. In fact, an integrated multi-function assembly (IMA) can house amplifiers, mixers, couplers, and other active/passive components.

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Filter components tend to be highly reflective circuit elements. As a result, their electrical behavior is very sensitive to the impedances and parasitics at the filter’s internal and external ports. Jon Scoglio, engineering manager with API Technologies, comments, “If you tune a filter component considering an ideal system, and then drop it into the next-level assembly, you hope that everything plays well from an impedance standpoint. But that is never the case.” This scenario requires the careful design of the filter components and IFAs, so that each individual block can be tuned throughout the assembly process for optimized performance. Many factors play into the critical aspects of IFA/IMA designs including materials, interconnect, filter type, added functions, tuning, and application.

The design of IFAs, like any system, is divided into functional blocks and modeled around the most limiting structures. The goal of modeling the filter electrically is to gauge the interaction between the filter elements and the electrical components. This approach enables the up-front analysis of the design, so that some of the adjustment of the filter structure into the surrounding circuitry can be performed preemptively. “The more you can do on the computer, from a modeling perspective, the less you will have to do on the bench,” notes Scoglio. “The better you can model the filter components, the less time—and possibly material waste—you will have downstream when you have physical hardware and prototypes manufactured.” The mechanical portions of the filter are designed using advanced 3D modeling techniques to model the complete structure. Even the filter elements are modeled to scale, so that the IFA can be made as compact as possible if needed. Other desired properties can be optimized using these design techniques as well. The IFA or IMA application heavily influences the design criteria of the assembly.

Filters for military and space applications in particular center around broadband, powerful, and reliable performance under an extreme range of temperature and environmental conditions. An IFA enables a filter to meet such performance criteria by being accurately tuned and compensated for environmental conditions. In addition, the assembly can be equipped with a ruggedized housing that meets environmental criteria. Weight and size can be more readily controlled with an IFA, as there are techniques that can bend filter stages into small and complex shapes. To further enhance the durability and stability of IFAs, laser sealing, low-dielectric-material stabilizers, and temperature-stabilizing elastomers can be added to key areas of the design.

For their part, public-safety radio systems generally differ from military requirements in that they need to have very precise and narrow filter responses around set frequencies. They also have slightly less stringent ruggedness requirements. The narrowband response requirement is caused by the intense spectrum competition around the frequencies in which public-safety radio systems operate. Achieving a high-Q filter response in a small and light footprint is a difficult task with discrete designs, considering the reliability and ruggedness requirements of public-safety systems.

In contrast, the filter requirements for telecommunications applications focus on high-power and extremely high-Q designs. Here, an individual frequency channel—only a few megahertz wide—may need to be filtered. Scoglio comments, “In a lot of the cases, with the crowding of the spectrum, the telecom filters are some of the most complex you can find.” The higher Q or power requirements generally increase the size of the individual filter components. These complex and bulky designs often must fit into a compressed rack-mount profile with high electromagnetic-interference (EMI) specifications. Custom filter assemblies using multi-technology filters and integrated amplifiers/mixers can reduce the part counts needed as well as the overall size of the systems.

Together with other factors, this compacting method makes filter components very dependent upon their mechanical structure and the way they are physically housed. Most IFAs are designed and built in machined metal housings. Naturally, there are tradeoffs between the different metals used for the housing of the assembly. With cavity filters, for example, the cavity is formed by the surrounding housing of the chassis. The behavior of a cavity filter is characterized by the type of metal used, its finish, and plating (if used). Another example is a combline cavity-type filter. Here, the temperature performance of the filter is highly dependent upon the thermal expansion of the metal used in its construction.

The primary metal of choice for IFAs is aluminum, as it is low in both cost and weight. Brass would be the second-most common choice. More specialized metals like stainless steel, Invar, and Kovar follow. For extreme-temperature purposes, it may be necessary to use lower-thermal-coefficient-of-expansion metals like Invar or Kovar. Compared to aluminum, brass offers a slightly better thermal coefficient of expansion and is still fairly cheap. But brass is a much heavier metal than aluminum. For its part, aluminum is lossy. It also is difficult to solder to and must be plated, which adds cost and quality-control challenges. Although brass is solderable, it still needs to be plated for loss or corrosion purposes. Scoglio states, “We are building highly integrated assemblies, where the filters are mechanically laid into the assemblies in a compact way—even bending around corners as you do in the game of Tetris and putting these assemblies in as small a footprint as possible.”

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