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Bulk Acoustic Wave Filters for 2.4 GHz Applications

WLAN access points and routers based on the new 802.11n draft 2.0 standard will have the potential to deliver data throughput at up to five times and twice the range of previous-generation Wi-Fi equipment. In doing so, consumers will have home-wide access to content-rich applications, including streaming high definition video, rapid transfer of data (music, photo, etc.), single network/multiple user online gaming and more. Meanwhile, new 802.11n products will allow enterprise users to increase their network capacity and improve robustness.

The speed and range specifications outlined in the new WLAN standard translate into a demanding set of requirements for individual RF components, particularly calling for small footprint, low profile and high performance. Located directly in the high frequency signal path with the responsibility of blocking unwanted frequencies, RF filters are among the more critical components in the radio front-end. The SKY33100-360LF family of bulk acoustic wave (BAW) bandpass filters from Skyworks, which are now available in volume shipments, deliver the high performance called for in the design of these next generation WLAN, cellular phones, PCIe or mini PCI cards and other 2.4 GHz applications.

As an increasing number of communication systems use RF frequency bands, very high selectivity filters have become more important in ensuring that systems with close frequency bands do not interfere with each other. The technical challenge for RF filters is to reduce the insertion loss in the desired transmission band, and yet provide sufficient rejection at nearby frequencies, such as those just outside the passband. BAW filter technology achieves these two important characteristics by making use of very low loss electromechanical resonators to perform very highly selective RF filtering. The typical configuration for a BAW filter is a ladder network containing series and shunt resonators with slightly differing resonant frequencies to form a bandpass filter with steep near-band roll-off.

BAW filters transduce electrical signals into very low loss resonant acoustic vibrations in piezoelectric structures, similar to what happens in quartz crystals used as electromechanical circuit elements in timing references. BAW resonators consist of a thin film layer of piezoelectric material sandwiched between two metal electrodes. These layers are formed on top of an acoustic mirror, comprised of several alternating layers of high and low acoustic impedance materials, the composition and thickness of which are both very tightly controlled in order to produce the desired filtering.

Electrical signals are coupled into the piezoelectric layer via the electrodes and due to the piezoelectricity, that layer changes dimension, primarily thickness. The longitudinal acoustic wave that is generated is resonant in the layered structure at a frequency of operation dictated by the thickness of the various layers and the acoustic properties of the layers used. The large currents that result at the acoustic resonance produce the sizeable impedance changes that govern the RF filter characteristics.

The SKY33100-360LF bandpass filters are available in a space saving QFN 8L 2 x 2 mm lead (Pb)-free, RoHS-compliant package. The filter is intended for use in the 2.4 GHz WLAN and ISM bands and allows designers to meet all filtering requirements with a single component, significantly reducing the BOM count and cost. Eliminating the need to specifically match each individual filter reduces design cycle time and saves valuable PCB space, resulting in a strategic advantage in terms of pricing, performance, delivery and quality.

The electrical features include a 70 MHz bandwidth, very low in-band insertion loss with typical values of 1 dB (see Figure 1), steep filter response with excellent near-band rejection that exceeds all other known existing technologies, 50 Ω nominal terminal impedance for very low input and output return loss. The filters offer superior power handling capability, enhanced ESD robustness, stable performance, lower temperature drift and an operating temperature range from –40° to +85°C.

The BAW filters are fabricated using semiconductor wafer fabrication techniques similar to those already used by Skyworks to manufacture heterojunction bipolar transistor (HBT) power amplifiers (PA) and PHEMT switches. In doing so, the manufacturer leverages all of the known benefits of wafer fabrication to achieve the high precision, high repeatability, and large volume capability and manufacturing scalability to address the RF filter needs of the company’s wireless communications customers. Device reliability is excellent with no performance degradation over lifetime.

For next generation wireless standards, the BAW filters offer unprecedented frequency selectivity for a dramatic reduction in out-of-band spurious responses, enabling compliance with FCC regulations at a higher-in-band transmission power with improved handling for more robust architectures. Initially being deployed for wireless local area network (WLAN) 802.11 b, g and n access points, Skyworks is also developing BAW filters and duplexers for use in transmit front-end modules for personal communications system (PCS) and universal mobile telecommunications system (UMTS) cellular handsets. The SKY33100-360LF is available for sampling now and is priced at $2.95 for quantities of 10,000.

Skyworks Solutions Inc.,
Woburn, MA (781) 376-3000,

RS No. 302

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