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The frequency spectrum from 400 MHz to almost 6 GHz is full of a wide range of wireless and telecommunication allocations. LTE, GSM, CDMA, WiFi, Bluetooth, and many other standards occupy this range of complex frequency resources. To escape the congestion and increase data rates, frequencies around 60 GHz are being planned for close-range, high-speed wireless communications (known as WiGig). The issue of spectrum crowding is compounded when global spectrum usage is considered, as different countries use different spectrum allocations (Fig. 1).

With modern electronics needing to either operate on all of these standards or isolate themselves from them, filter specifications are increasing in complexity. Electronic devices with wireless capabilities also are shrinking in size, requiring clever filter solutions to fit complex impedance networks in compact packages. Often, integration has become a necessity for filter manufacturers as they squeeze filters for nearly 60 wireless channels into a device that easily fits into the hand or pocket.

When asked about the impact of spectrum-density increases on filter design, Kevin Schoenrock, Director of Strategic Marketing & Advanced Development for TriQuint, responded, “New LTE bands with higher filter performance requirements are being squeezed next to existing bands within the crowded RF spectrum. With all the additional bands required for both antennas within a small phone-board area, filter design is now a key consideration and a decisive contributor to phone performance—along with wafer-level packaging for modular integration.” The latest technology in acoustic resonator filters is being put to the test to solve these space and precision constraints. Surface-acoustic-wave (SAW) and bulk-acoustic-wave (BAW) filters are today’s solution for RF and low microwave frequencies.

LTE, WiFi, And WiGig Push Filter Capabilities, Fig. 1

The advantages of acoustic-wave technology include a small footprint, shallow package size, enhanced linear phase, good rejection qualities, and stability over a wide temperature range. The physical structure of acoustic-wave devices also allows for highly reliable and robust designs that exhibit stability in a range of environments/temperatures. In addition, the wafer processing techniques used for these devices enable repeatable fabrication from high- to low-volume production.

The fundamental operation behind both SAW and BAW devices is a piezoelectric material, which converts an electromagnetic (EM) signal into an acoustic signal. This physical effect can be used to enhance the filtering response of a device beyond the capabilities of purely electronic devices for the form factor. Generally, SAW devices operate with good performance in the RF range, whereas BAW devices demonstrate higher performance characteristics in the low microwave range.

SAW filters consist of a piezoelectric substrate that is polished and capped at each end with a bi-directional acoustic-to-electric transducer. The body of the device is constructed of an array of interdigital electrodes with alternating polarities. This arrangement induces the generation of a surface wave when an EM signal voltage is applied across the electrodes (Fig. 2). A SAW filter is designed with specific characteristics by combining the impulse responses of the two transducers—derived from substrate etching of the two impulse responses. This approach differs from the fundamental operation of a BAW device.

LTE, WiFi, And WiGig Push Filter Capabilities, Fig. 2

A BAW filter uses a quartz crystal with metal electrodes on the top and bottom. When electrically excited, the quartz crystal produces acoustic waves that generate a standing acoustic wave within the device (Fig. 3). The acoustic energy is channeled vertically through the device. To achieve high frequency resonance, the bulk of the filter is micrometers thick. These structures are created using thin-film deposition techniques and a micromachined substrate.

Compared to other devices in their size category, BAW filters tend to achieve lower loss and higher Q factors at microwave frequencies. With Q values around 2500 to 2 GHz, the rejection ratio and insertion loss of such devices are very high. Such characteristics help to ensure sharp passband edges. Other variants of the BAW filter are the film bulk acoustic resonator (FBAR), which is fabricated by etching cavities to create suspended membranes with specific resonant qualities.

LTE, WiFi, And WiGig Push Filter Capabilities, Fig. 3

Avago, API, AMTI, Oscilent, and Trilithic all offer variants of acoustic-wave devices in a variety of packages and in common frequencies, which comply with the latest standards. Often, custom filter options are available from these companies for specific applications. To learn more about acoustic-wave filters, Schoenrock recommends Colin Campbell’s book, Surface Acoustic Wave Devices for Mobile and Wireless Communications, as well as RF Bulk Acoustic Wave Filters for Communications (edited by Ken-Ya Hashimoto).

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