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March 2015: The Alchemy of RF Filtering for Smartphones

The mobile industry is being fuelled by tremendous advancements in device capabilities, which can be largely attributed to Moore’s Law performance gains in semiconductor technology. This creates challenges for RF filter designs, which languish under a shroud of complexity, and rely on design techniques that have not changed much since the 1920’s.

In many respects, RF filter design is as much art as it is science. While there are a variety of RF filter technologies available, the filters used in mobile phones generally fall into one of two categories, those that are based on Surface Acoustic Waves (SAW), which are approximately half the price of higher performance Bulk Acoustic Wave (BAW) filters. These filters use tuned resonators to create the necessary RF response, which is complicated because of the compound resonance that the filter elements generate with the simultaneous presence of open and closed circuit conditions. As a consequence of these complications, discrete filters are normally tuned to a single frequency, and in many cases the more expensive BAW filters are needed to ensure adequate filter performance. This creates challenges for module vendors like Avago, Murata, Qorvo and Skyworks, who have to cope with smartphone designs that commonly require between 30 and 35 RF filters.

As the RF requirements in smartphone devices increase, the cost, complexity and physical size of the RF filter and switching fabric will increase. This creates tremendous opportunities for RF filter design innovation, such as improvements in SAW filter performance, so that they can replace the more expensive BAW technology and advanced filter designs to enable multiplexing and filter tune-ability. These are challenges which innovative start-up companies like Resonant Inc. are tackling. In the case of Resonant, its designers have adapted optimization techniques originally developed for superconductor filters that were used in radio base stations.

The mobile industry continues to evolve at breath-taking speed. As this occurs, industry pundits commonly place much of their confidence in continued technology advancement and tout the virtues of Moore’s Law. However Moore’s Law is underpinned by the miniaturization and densification of digital semi-conductor technology. Unfortunately, the same gains cannot be achieved with discrete technologies such as RF filters. As a consequence, we believe that it is crucial for the industry to invest in RF technology innovations, such as with RF filters, to ensure that they do not create a bottleneck for the continued advancement of smartphone devices.

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