UNIDIRECTIONAL SURFACE ACOUSTIC WAVE FILTER

Abstract: A unidirectional SAW transducer (10) has a piezoelectric carrier (11) having first and second elements (15) arranged on the carrier surface. Each element (15) has a first exciting electrode (12), a second, antiphase exciting electrode (13), and a reflecting electrode (14). The elements (15) have a length corresponding to the SAW length lambda at an average operating frequency. The width b1 of the first electrode (12) equals lambda /6; the width b2 of the second electrode (13) equals lambda /8; the width b3 of the reflecting electrode (14) equals lambda /4; the distance between adjacent edges of first and second electrodes (12, 13) equals lambda /6; and the distance between adjacent edges of the second electrode (13) and the reflecting electrode (14) equals lambda /8.

Description: UNIDIRECTIONAL SURFACE ACOUSTIC WAVE FILTER Field of the Invention The present invention generally relates to surface acoustic wave (SAW) filters, and, more particularly, to a unidirectional transducer for an SAW filter. Background of the Invention Surface acoustic wave (SAW) filters have a number of applications in electronics. At corresponding frequencies, the acoustic wavelengths λ are shorter than the lengths of electromagnetic waves. SAW technology is important for applications where miniaturization is important or desirable. For example, SAW filters are used in radio telephones where the small size and weight of the SAW filter is highly advantageous over conventional technologies, such as ceramic, dielectric and other filters. In cellular phone systems, an insertion loss of the SAW filter should not exceed a range of about 3 to 10 dB at intermediate frequencies between 70 to 250 MHz, and the insertion loss should not exceed a range of about 2 to 4 dB at radio frequencies between 800 to 1800 MHz. FIG. 1 is a simplified schematic diagram of a transducer of the prior art. An SAW interdigital transducer (IDT) has piezoelectric acoustic guide 1 with exciting electrodes 2 and 3 located at its surface. When voltage is applied, electrodes 2 and 3 provide exciting in opposite phase. A useful reference is: [1] Herbert Matthews (editor): "Surface Wave Filters", Wiley-Interscience Publication, 1981. The dimensions are related to the SAW-length λ at an average operating frequency of the transducer. Electrode widths of electrodes 2 and 3 are b, = b 2 = λ / 4 and distances d between the edges of adjacent electrodes 2 and 3 are also λ / 4. Arrows A/2 and B/2 at the electrode edges illustrate so-called delta-sources (δ-source) which approximate the excitation at a location where the electrical field has a maximum. Details are explained in reference [1]. The transformation between acoustic and electric energy and vice versa can be estimated by vector calculations known in the art. An efficiency parameter of the transformation is obtained for IDT of FIG. 1 as E, = 2" 2 * A ~ 1.4 * A (subscript "1/2" for square root), wherein the absolute value "A" is a measure of the excitation strength at the electrode edges. E, is relatively high. However, the IDT-transducer suffers from relatively high insertion losses caused by the bidirectionality of the SAW radiation. Further, the IDT-transducer suffers from high distortions in the amplitude- to-frequency response (APR) because incoming waves reaching the transducer from the right of input line 1-1 (dashed) or from the left of input line 2-2 (dashed) are reflected from the electrode edges and summed up in-phase. This is not wanted for many applications. FIG. 2 is a simplified schematic diagram of a further transducer of the prior art. The transducer is a single phase unidirectional transducer (SPUDT) with distributed acoustic reflection (DART-type). A useful reference is: [2] T. Kodama et al. : "Design of low-loss SAW filters employing distributed acoustic reflection transducers", Proceedings of Ultrasonics Symposium 1986, pages 59-64. In FIGS. 1-2, like reference numbers stand for like elements. The DART-transducer of FIG. 2 further comprises reflecting electrode 4. Widths are b, = b 2 = λ / 8 for electrodes 2 and 3, and b 3 = (3 * λ) / 8 for reflecting electrode 4. Compared to the IDT-transducer of FIG. 1, the DART-transducer of FIG. 2 provides waves with a better directivity which means stronger wave propagation in X direction than in -X direction. However, with an efficiency E 2 ~ 0.6 *A, the DART-transducer exhibit an higher, unwanted insertion loss. The present invention seeks to provide transducers for SAW filters having lower insertion losses and which further mitigate or avoid disadvantages and limitations of the prior art. Brief Description of the Drawings FIG. 1 is a simplified schematic diagram of a transducer of the prior art; FIG. 2 is a simplified schematic diagram of a further transducer of the prior art; FIG. 3 is a simplified schematic diagram of a transducer according to the present invention; and FIG. 4 is a simplified source vector diagram in a complex plane. Detailed Description of a Preferred Embodiment A transducer of the present invention can be used in filters operating at intermediate and carrier frequencies, used, for example, in radios, pagers, mobile communication systems etc. The present invention accommodates the above mentioned requirements concerning insertion loss for many applications. FIG. 3 is a simplified schematic diagram of transducer 10 according to the present invention. Transducer 10 is a single phase unidirectional transducer (SPUDT) for an SAW filter with triple-electrode sections (TES). Transducer 10 comprises piezoelectric acoustic guide 1 1 having elementary sections 15 (dashed frame) located on its working surface. Section 15 has exciting electrodes 12 and 13 and reflecting electrode 14. For convenience, a coordinate system illustrates a forward direction ("X") and a reverse direction ("-X"). The origin 0-0 (dashed) is given by a dashed line in the center of electrode 12. A section length L is defined as the distance between conventional boundaries 1-1 and 2-2 and substantially equals SAW length λ at an average operating frequency of transducer 10 (L ~ λ). Electrode 12 has width b,; electrode 13 has width b 2 ; and reflecting electrode 14 has width b 3 . Width b, substantially equals λ / 6: width b., substantially equals λ / 8; and b 3 substantially equals λ / 4. The distance d, between adjacent edges of electrodes 12 and 13 is λ / 6; and the distance d 2 between adjacent edges of electrodes 13 and 14 is λ / 8. When an electric signal from an external generator (not shown) is applied to electrodes 12, 13 and 14, four δ-sources 21 to 24 generate surface waves (SAWs). With the underscoring for "vector", and "A/2" or "B/2" standing for the excitation strength (conveniently, A ~ B), sources 21 and 22 (A / 2) and sources 23 and 24 (B / 2) are excited opposite in phase (antiphase). FIG. 4 is a simplified source vector diagram in a complex plane with imaginary part (Im) and real part (Re). For a convenient analysis of transducer 10, sources 21 to 24 are combined to a single equivalent source E 3 (dashed arrow) having a conventional excitation center at X = 0. As illustrated in FIG. 4, the sources A / 2 and B / 2 have equal amplitudes but different phases. Referred to the coordinate origin 0-0 (cf. FIG. 3, dashed line), source 21 (in FIG. 3 at the right side of electrode 2) is advanced by λ / 12 (φ 2I = ω * t = + 30°) and source 22 (left side of electrode 2) lags behind by λ / 12 (φ 22 = - ω * t = - 30°). Source 23 (right side of electrode 13) lags behind by 90° and the source 24 (left side of electrode 14) lags behind by 270°, thus sources 23 and 24 equalizing in origin 0-0. Applying vector analysis known in the art, vector E 3 has a magnitude E 3 of (subscript "1/2" for square root): E 3 = 3 1/2 / (2 * A) - 0.87 * A Compared to the DART-transducer of the prior art (cf. FIG. 2), transducer 10 of the present invention has a 46% higher efficiency. A filter with transducers 10 according to the present invention (cf. FIG. 3) was compared to a filter with a DART-transducer (cf. FIG. 2) of the prior art. Both filters were dimensioned as intermediate frequency filters having a center frequency f 0 = 1 12.32 MHz with a 3 dB passband of Δf = 1.2 MHz. Such filters are used in cordless phones according to the European DECT standard. Both filters were manufactured on quartz substrates cut in ST-direction. For both filters, load impedances were 50 Ohm. The insertion loss of the filter according to the present invention was 1.2 to 1.5 dB lower than the insertion loss of the DART-filter of the prior art. While the invention has been described in terms of particular structures and devices, those of skill in the art will understand based on the description herein that it is not limited merely to such examples and that the full scope of the invention is properly determined by the claims that follow.

Claims: Claims 1. A unidirectional surface acoustic wave (SAW) transducer (10) with a piezoelectric carrier (1 1) having first and second elements (15) arranged on the surface, each element having a first ( 12) exciting electrode, a second, antiphase exciting electrode (13) and a reflecting electrode (14), said elements (15) having a length corresponding to the SAW length λ at an average operating frequency, said transducer characterized in that in each element the width b, of said first exciting electrode (12) substantially equals λ / 6 and the width b 7 of said second exciting electrode (13) substantially equals λ / 8, the width b 3 of said reflecting electrode (14) substantially equals λ / 4, wherein a first distance between adjacent edges of said first and second exciting electrodes (12, 13) substantially equals λ / 6, and a second distance between adjacent edges of said second exciting electrode (13) and said reflecting electrode (14) substantially equals λ / 8. 2. SAW-transducer having a first exciting electrode (12), a second exciting electrode (13) in opposite phase, and a reflecting electrode (14), said SAW-transducer characterized in that • said first electrode has a first width of λ / 6, wherein λ is the SAW length at an average operating frequency of the SAW-transducer, • said second electrode has a second width of λ / 8, • said reflecting electrode has a third width of λ / 4, • a first distance between adjacent edges of said first and second electrodes is λ / 6, and • a second distance between adjacent edges of said second electrode and said reflecting electrode is λ / 8.

Inventors: CHVETS VALERI BORISOVITCH, MAKAROV VLADIMIR MICHAILOVITCH, ORLOV VICTOR SEMENOVITCH, CHVETS, VALERI BORISOVITCH, MAKAROV, VLADIMIR MICHAILOVITCH, ORLOV, VICTOR SEMENOVITCH
Applicants: CHVETS VALERI BORISOVITCH, MAKAROV VLADIMIR MICHAILOVITCH, MOTOROLA INC, ORLOV VICTOR SEMENOVITCH, CHVETS, VALERI BORISOVITCH, MAKAROV, VLADIMIR MICHAILOVITCH, MOTOROLA INC., ORLOV, VICTOR SEMENOVITCH
Application year: 1998
Main classification: H03H9
IPC codes: H03H9