グループ型一方向性すだれ状電極を用いた低損失弾性表面波フィルタ

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Acoustic surface wave filters employing the ordinary interdigital transducer show an inherent minimum insertion loss of 6 dB because of bidirectionality and strong passband ripple for low insertion loss devices due to secondary effects. In order to avoid these flaws, the unidirectional transducer represents the most advanced method of tackling the above mentioned defects, and, among others, the following suggestions have already been made : 1) use of a λ_0/4-phase shifter 2) use of a 120° or 60° phase shifter (Fig. 1). Method 2) has produced a very low insertion loss of 0. 65 dB and a passband ripple of less than 0. 1 dB with maching network. However, this method needs highly sophisticated manufacturing techniques to fabricate the air-gap crossovers. In this paper we propose the new method shown in Fig. 2, which is capable of overcoming the above mentioned defects. Two interdigital transducers S (for Sender) and R (for Reflecter) with N electrodes respectively are arranged λ_0/2 apart (f=f_0, f_0 center frequency), and are connected to an electrical source with a phase difference φ, as shown in Fig. 3. If the phase difference is frequency dependent (φ=π/2・f/f_0), the transducers show unidirectionality and thus render an ideal insertion loss of 0 dB possible. At frequencies slightly removed from the f_0, however, the propagation direction will eventually reverse and a passband ripple appears. By arranging collinearly many transducers (called groups) having only a small number of pairs and being made unidirectional by applying an electrical 90° phase-shift (Fig. 2), we could obtain a filter with very low insertion loss and a small passgand-ripple. Experiments have been undertaken with the arrangement shown in Fig. 11. Rotated-Y128° cut X-propagated LiNbO_3 has been used as a propagation medium for the acoustic surface waves. The arrangement is a combination of an N-4-11 sending and a M-4-11 receiving transducer, both displaying a radiation impedance of 52 Ωat a center frequency f_0=99. 2 MHz. A 50 Ω coaxial cable is used as a 90°-phase shifter (length 43 cm). The result of the experiment is shown in Fig. 12 and shows a minimum insertion loss of 1. 0 dB without tuning and a passband ripple of less than 0. 2 dB. In another experiment, an N-2-22 and N-4-11 type transducer has been combined and the weighting pattern of Fig. 13 was applied. The result is shown in Fig. 14, with a minimum insertion loss of 2. 7 dB. The sidelobes at the frequencies f_s/f_0 and f_s/f_0=1. 2 are suppressed by more than 35 dB, as shown in Fig. 14. In all the experiments a 50 Ω coaxial cable was used as a phase shifter. Next, the application of an LC-network (Fig. 15) was applied, and we obtained the same result as that using coaxial cables. Moreover, a new group type of unidirectional transducers without sidelobe frequency were proposed, as shown in Fig. 8 and confirmed experimentally (Fig. 20).
社団法人日本音響学会の論文
1977-10-01

グループ型一方向性すだれ状電極を用いた低損失弾性表面波フィルタ

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