Friday, June 20, 2014

Acoustic Wave Filters: SAW & BAW

A electronic filter is a device or combination of devices that are engineered to block signals in specific and (hopefully) controlled frequency ranges.The simplest filters are common board components. Inductors naturally block high-frequency signals and capacitors naturally block low-frequency signals. (For the record, resistors have no natural frequency-selective properties.) Other board components do such as diodes can also be used in filters but usually with other components. So lets get away from passive filters, L filters T and π filters and into the more interesting filters you might find under the chassis. I'm also going to skip M and K filters because I wrote them and George Campbell up previously. Today is about SAW filters.

Surface Acoustic Wave (SAW) filters are electromechanical filters used in radio. In these filters, electrical signals are converted to a physical wave piezoelectric crystal. This conversion causes a time delay as the wave propagates through the unit. At the output it is converted back into an electrical signal. SAW filters are limited to frequencies up to 3 GHz. They are commonly used as band-pass filters in superheterodyne receivers and 2G wireless devices.

They were first developed by Prof. Edward "Ted" George Sydney Paige a British physicist. His interest began in college at Reading University. His doctorate was an investigated into the optical properties of wavelengths in crystalline quartz caused by x-rays. His interest in piezoelectric crystals didn't end there. At the Radar Research lab at Malvern he lead a team of physicists that included Graham Marshall, Meirion Lewis and Dennis Maines. Together they developed the SAW filter in 1968. This was somewhat presaged by the development of pseudo-SAW filters. In 1965 R. White and F. Voltmer at the University of California developed a thin-film interdigital transducer (IDT) There are also Temperature Compensated SAW filters (TC-SAW.)  These have a coating of the IDT structures with layers that increase stiffness at higher temperatures. They are used in the middle ground as a cheaper component than a BAW filter (below) and more robust than a standard SAW filter.

Bulk Acoustic Wave (BAW)  filters are also electromechanical filters. BAW filters can be implemented as ladder or lattice filters. In fabrication they are isolated acoustically by silicon substrate. They are used typically for their low insertion loss and steep narrow band rejection. These can be configured two ways:

1.  BAW-SMR (solidly mounted resonator) where the resonator is mounted over a stack of alternating materials.
2. BAW-FBAR (Film Bulk Acoustic wave Resonator)  where the resonator is is suspended by an air-bridge.

One notable filter based on the BAW-SMR is the BAW-CRF (coupled resonator filter). This design stacks two BAW resonators separated by passive layers, to enable an exchange of acoustic energy between the lower and the upper resonator. These may soon be used in CDMA devices.  These usually operate at frequencies between 2 GHz to 16 GHz.