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At this point, there are no design tomes and standard filter approaches for many emerging wireless applications. The new WiGig (IEEE 802.11ad) standard, for example, operates closer to millimeter-wave frequencies and requires a different set of filtering solutions. The goal of WiGig technology is to enable very-high-speed data transfer over short ranges in a compact and energy-efficient method. In literature, suggestions for WiGig filter solutions span substrate integrated waveguides, flip-chip bump resonators, micro-machined integrated-cavity approaches, and metal-foil-enhanced non-radiative dielectric waveguides.

Many R&D labs are researching a viable way to implement compact, efficient filters that can be easily integrated into modern IC processes. The propagation of EM radiation at 60 GHz is limited significantly by atmospheric and material absorption. In addition, WiGig can only operate to 10 m at low power levels. As a result, higher-power filters may not be necessary for WiGig applications.  When selecting or designing filters for these complex and often coexisting frequencies, designers are offered many tools to help increase the efficiency of the filter block. Marten Seth, Marketing Director for WiSpry, emphasizes, “We encourage our customers to take advantage of circuit simulation tools, electrical models, and of course the OEM specifications.”

Many companies offer selection guides, modeling tools, and customer support to help designers select, design, or manufacture the most effective device for their application. Among them are TriQuint, WiSpry, API, AMTI, Avago, Oscilent, Reactel, and Trilithic.

Before designing a filter, many performance metrics must be considered. When asked about the key aspects to consider when approaching filter design, Schoenrock responded, “Electrical performance is evaluated based on insertion loss and selectivity, return loss, linearity, and power handling, while temperature stability in various operating conditions is another factor. For today’s thin and lightweight mobile devices, the z-height is just as important as the x-y footprint.” These factors—along with the type of filter—heavily influence the feasibility of certain technologies for a given design.

Some applications, such as military and aviation, have more stringent requirements for filter component specifications and reliability.  Additional design complexity arises when considering a filter operating alongside many other filters to differentiate between the complex mesh of signals present in the modern wireless environment. New methods of solving these complex problems include filters with frequency characteristics that can be actively tuned to multiple frequency bands.

Tunable RF and microwave filters could enable a mobile device to operate with many different wireless standards in an on-demand typology. The root of this technology is the ability to change the fundamental filter component characteristics to alter the input/output characteristics of the filter block. This is often done by adjusting the capacitive values within an integrated environment using voltage control. Microelectromechanical-systems (MEMS) technology is frequently used to provide the highly precise control necessary to deliver accurate parameter adjustment.

When asked about the challenges in modern filter design, WiSpry’s Seth notes, “There are numerous factors to consider including linearity, voltage, performance, etc. But the most challenging issue is often the wide-ranging requirements from OEMs to support growing bands and spectrum. This is why we believe tunable RF is the best path, dramatically reducing design time and board space.” In addition to WiSpry, firms like Cavendish Kinetics offer customizable and tunable RF-technology filtering options specifically for modern wireless standards.

With the constant influx of wireless technologies, meeting the filter requirements to isolate individual signals for telecommunications has become a more advanced design problem. Acoustic-wave technologies have shown promise by providing high-performance filtering characteristics to isolate signals in the highly competitive spectrum. As new wireless standards drive frequency-band densities to upwards of 60 distinct bands, however, fixed-filter solutions may not remain viable in enabling handheld wireless electronics. If tunable filter technologies can prove cost competitive and decrease the size requirements of filters, they may become common in the highly integrated RF ICs in modern devices.

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