A novel reflector-filter using a SAW waveguide directional coupler

Abstract

The SAW waveguide technique was used to form a novel reflector-filter for the realization of compact IF filters in CDMA handsets. The reflector-filter design is an attractive technique to obtain characteristics with steep skirts in a short device length, because the SAW propagation path is folded and the frequency response is synthesized by utilizing both IDT and reflector responses. In this paper, we propose a new reflector-filter structure using a SAW waveguide directional coupler. For the implementation of the proposed reflector-filter, the key technology is the design of SAW waveguides. We have formed SAW waveguides using Al gratings loaded on the surface of the substrate. The pitch of the Al gratings has been chosen unequal to half the acoustical wavelength to avoid the occurrence of the gratings stopband at the filters passband position. Using the proposed reflector-filter structure, PCS-CDMA IF filters were fabricated on quartz substrates. The filter exhibited an insertion loss of 8.5 dB, a 5 dB bandwidth of 1.45 MHz, and a rejection of more than 33 dB at the center frequency +/-1.25 MHz with the package size of 4.8x9.1 mm (2). i.e., half the size of a conventional transversal filter was achieved using the proposed reflector-filter technique.

Development and Application of SAW Filter

“…In 2000, Jun Tsutsumi formed a new type of reflection filter using SAW waveguide technology in [ 72 ], realizing a compact IF filter. In the second year, in 2001, Jun Tsutsumi studied the acoustic energy loss of the grating waveguide SAW filter.…”

Fabrication and Characteristics Analysis of SAW Filter Using Al 0.36 Ga 0.64 N Thin Film on Sapphire Substrate

“…Piezoelectric single crystals such as LiNbO 3 , LiTaO 3 , and ZnO, and AlN or ZnO thin films [1,2] have been widely investigated to determine their SAW characteristics such as velocity, loss, electromechanical coupling coefficient (k 2 ), and temperature coefficient of frequency (TCF) [1][2][3]. In this paper, experimental results were obtained for a SAW filter fabricated on an AlGaN/sapphire substrate, which was prepared by metal organic chemical vapour deposition (MOCVD).…”

Experimental Studies on Acoustic Energy Loss in Surface Acoustic Wave Filters Using Grating Waveguides

“…The SAW filter design using grating waveguides, such as the transversely coupled resonator filter 1,2) and the reflector filter using a SAW waveguide directional coupler proposed by the authors, 3) is a promising technique to achieve such excellent characteristics. However, this design technique has one drawback.…”

“…This energy loss may be due mainly to the weak confinement of the SAW energy in the IDT, because it decreases drastically upon increasing the film thickness, in other words, upon increasing the SAW velocity difference between the finger region and the busbar region. 3) Next, the cause of the large insertion loss with a short waveguide length was investigated. Figure 4 shows one possible mechanism of energy loss with a short waveguide length.…”

“…As a filter structure, the reflector filter using the SAW waveguide directional coupler shown in Fig. 6, which the authors previously proposed, 3) was used. Here, the period λ of the IDT was 15 µm, and the waveguide width was set at 7.5λ.…”