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World's First SAW Device for TV Receivers

SAW devices surpassed LC filters in performance but realizing their practical use was difficult.
Single-crystal growth technology and unique filter design opened the door to production.

World's First SAW Device for TV Receivers

A surface acoustic wave (SAW) device consists of a reflective pattern formed over a piezoelectric substrate and two thin, comb-shaped, metallic transducers called interdigital transducers (IDTs). The device is used to selectively filter out electrical signals at specific frequencies. A SAW is a wave of mechanical vibration that travels along the surface of a solid substance. By placing transducers on the surface of a piezoelectric substrate, electrical signals can be converted into acoustic waves. At high frequencies, a SAW device produces less transmission loss on the substrate, and the reflection transducer can more efficiently reflect waves from its extremely sensitive surface. A SAW device makes use of the property that the efficiency of digital-analogdigital energy conversion depends on the frequency imposed by the IDTs, and on the SAW resonance phenomena in the reflection transducer.

In 1977, Toshiba was the first company in the world to mass produce SAW filters for use in intermediate-frequency filters for TV receivers. In order to produce these filters, the company had to grow single-crystal lithium tantalite (LiTaO3) into piezoelectric SAW crystals, establish a new processing technology and determine the correct crystal cut angle, and develop a comb-shaped transducer design technology to determine the precise phase characteristics of the filter amplitude. In order to determine the phase characteristics, engineers looked for the causes of interference in surface acoustic waves and formulated methods to compensate for deterioration characteristics in the targeted frequency range. From this, an automatic design method was developed by which the design could be completed more efficiently in a short time.

Toshiba was also the first in the industry to mass produce radiators making use of SAW resonance phenomena and low-loss high-frequency filters, which contributed to the simplification of beeper circuits and made video tape recorder (VTR) modulators compatible with integrated circuits. Research into SAW filters continued as higher frequency bands were required and a lighter, more compact version was needed for mobile communication equipment. Today, the SAW filter is one of the key components in mobile phones.

By incorporating the reflection transducer in the internal digital transducer, surface acoustic waves could be transmitted in a single direction, and Toshiba's engineers were able to successfully build SAW devices with low transmission loss. The mode multiplexing filter structure was developed to achieve wider bandwidths, by placing a number of IDTs between the reflection transducers and integrating a higher order resonance mode inducer. As a result, low filter loss and wider bandwidth were achieved. Since the center frequency of a SAW filter is determined by the space between the IDT wires, microprocessing technology for IDTs became essential to obtain higher frequency bands. By making use of dry etching technology in semiconductor processing, mass production of 0.4 μm wire widths and 2.5 GHz filters was made possible, and these new filters covered the frequency bands assigned to third-generation mobile phones (W-CDMA) and wireless terminals (Bluetooth).

Toshiba developed dual filters consisting of two filter chips in one package and, in response to market demand, successfully implemented chip-scale packaging (CSP). With digital televisions dramatically changing the world of media, the line between communication and broadcasting is becoming unclear as TV tuner functions are being incorporated into mobile phones, mobile computers, and wireless terminals. Small and thin SAW devices, with their rapid frequency response characteristics, have contributed to the increased sophistication and size reduction of many such products.

Used as intermediate-frequency filters for TV receivers (18T32)

Used as intermediate-frequency filters for TV receivers (18T32)

Single-crystal lithium tantalite and SAW filter

Single-crystal lithium tantalite and SAW filter

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