Jpn. J. Appl. Phys. Vol. 40 (2001) pp. 3722–3725 Part 1, No. 5B, May 2001 c 2001 The Japan Society of Applied Physics
Longitudinally Coupled Resonator Filter Using Edge Reflection of Bleustein-Gulyaev-Shimizu and Shear Horizontal Waves with Various Bandwidths Realized by Selecting Substrates Michio K ADOTA∗ , Junya AGO, Hideya H ORIUCHI and Mamoru I KEURA Murata Mfg. Co., Ltd., Nagaokakyo-shi, Kyoto 617-8555, Japan (Received November 23, 2000; accepted for publication January 25, 2001)
A conventional surface acoustic wave (SAW) resonator filter requires reflectors with many grating fingers at both sides of the interdigital transducers (IDTs). On the other hand, a small-sized low-loss resonator filter without grating fingers can be realized by utilizing the reflection of a Bleustein-Gulyaev-Shimizu (BGS) wave or a shear horizontal (SH) wave at the edge of a substrate. As a result, the authors have developed longitudinally coupled resonator filters using the reflection of BGS and SH waves at the edges of a substrate. Longitudinally coupled resonator filters on piezoelectric ceramic substrates for the first intermediate frequency (IF) stage have been realized to be low-loss, small-sized and extremely wide bandwidth (fractional bandwidth of 15%) filters as compared to the ordinary resonator filters, and they can have various kinds of bandwidths (2 to 15%) by employing ceramic substrates with various electromechanical coupling factors. On the other hand, the edge-reflectiontype resonator filters on LiTaO3 single crystal for various first IF stages have also been realized to be low-loss and small-sized filters. In spite of being a wire-bonding type of IF filter, the package size of this filter is the same as that of the radio frequency (RF) SAW filter. KEYWORDS: Bleustein-Gulyaev-Shimizu wave, shear horizontal wave, edge reflection, longitudinally coupled resonator filter, small-sized package, low loss, various bandwidths, PZT, LiTaO3
1. Introduction Currently, surface acoustic wave (SAW) devices are very important and indispensable for mobile communication systems. In this market, SAW devices are strongly required to be low-cost, small-sized and low-loss devices. In particular, intermediate frequency (IF) stage filters, which are larger than radio frequency (RF) SAW filters, are required to be even smaller. A Bleustein-Gulyaev-Shimizu (BGS) wave and a shear horizontal (SH) wave have the characteristics of complete reflection at the edge of a substrate with a large dielectric constant.1) The authors have developed and commercialized BGS resonators on a piezoelectric ceramic substrate (PZT: Pb(Ti, Zr)O3 ) using the reflection at the edge of the substrate. They have been widely used as additional traps in the video IF (VIF) circuit of TV/VCRs and as resonators for voltagecontrolled oscillators (VCO) in digital video recorders.2, 3) Conventional longitudinally coupled SAW resonator filters require reflectors consisting of numerous grating fingers at both sides of the interdigital transducers (IDTs), as shown in Fig. 1(a). However, by applying the characteristics of complete reflection at the edge of a substrate to resonator filters, small-sized and low-loss resonator filters which have no reflectors consisting of grating fingers can be realized as shown in Fig. 1(b). However, there has been no report of any proposal or experimental results for a longitudinally coupled resonator filter utilizing the reflection at the edge of a substrate, except for the authors via presentations at symposia.4–6) With regard to this filter, this paper is the first such report. A SAW resonator filter has the merit of being extremely low-loss compared with a SAW transversal filter. But because the bandwidth of a SAW resonator filter is restricted by the electromechanical coupling factor of the substrate, the applicable bandwidth in a resonator filter is also limited. In this paper, the authors report low-loss and small-size IF filters ∗ E-mail
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(b)
Fig. 1. (a) Conventional longitudinally coupled SAW resonator filter with grating reflectors, and (b) BGS·SH wave resonator filter utilizing reflection at the edge of the substrate.
with extremely wide and various bandwidths (relative 3 dB bandwidth of 2 to 15%) using edge reflections of BGS waves on PZT substrates with various coupling factors. However, a BGS wave filter using a PZT substrate currently has a large insertion loss and a low impedance at frequencies higher than about 110 MHz because this substrate has a large propagation loss in that frequency range and a large dielectric constant. Hence, the PZT substrate is not currently suitable for filters at frequencies higher than 110 MHz. The authors have realized IF filters suitable for high frequencies with the low loss and extremely small size as the RF SAW filter, using the reflection at the edge of SH waves on a LiTaO3 substrate. 2. BGS Wave Resonator Filter on PZT By selecting PZT substrates with various kinds of coupling factors, longitudinally coupled resonator filters with various bandwidths are realized. In this section, three kinds of filters with relative bandwidths of 2 ∼ 15% are reported. The first IF filter of 40 MHz for an electronic toll collection (ETC) system requires a 3 dB bandwidth wider than 4 MHz. The bandwidth of a longitudinally coupled resonator filter is wider than that of a transversely coupled one. However, it is difficult to realize a filter with such a wide bandwidth utilizing conventionally available substrates even if the longitudinally coupled type is used. Thus, the IF filter for the ETC system is currently designed using the SAW transversal filter in spite of a high insertion loss. A piezoelectric substrate with a large electromechanical coupling factor is required in order to real-
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Jpn. J. Appl. Phys. Vol. 40 (2001) Pt. 1, No. 5B
ize a resonator filter for the ETC IF stage. A PZT substrate with a large electromechanical coupling factor (k 0.6) has been developed for the IF filter of the ETC system. The ETC IF filter is composed of two stages of a longitudinally coupled resonator filter, and each stage has normal IDTs of 6 pairs with a wavelength of 56.8 µm. Figure 2 shows the frequency characteristics of this filter. The insertion loss is very low (4.4 dB), the 3 dB bandwidth is very wide at 5.9 MHz (relative bandwidth of 15%) and the stop band rejection is larger than 45 dB. The chip size is 0.35 × 2.0 mm2 and the package size is 3.8 × 3.8 × 1.5 (height) mm3 . This size is very small compared with that of our previous filter whose package size is 5.1 × 9.6 × 2.0 mm3 . The development of a PZT substrate with a large electromechanical coupling factor enabled the realization of this filter. Figure 3 shows the frequency characteristics of an IF filter whose nominal center frequency is 63 MHz, insertion loss is 3.4 dB, and 3 dB bandwidth is 4.85 MHz (relative bandwidth of 7.7%); the chip size is 1.0 × 2.1 mm2 , and the package size is 3.8×3.8×1.5 mm3 . The electromechanical coupling factor k of the substrate used in the filter is 0.47. The IF filter can be applied to a global positioning system (GPS). In contrast, Fig. 4 shows the frequency characteristics of a longitudinally coupled resonator filter with a 3 dB bandwidth of 2%, insertion loss of 2.7 dB, and chip size of 0.8×2.5 mm2 for the clock filter of a digital audio broadcast (DAB) system, using a substrate with an electromechanical coupling factor k of 0.2.7)
Fig. 2. Frequency characteristics of 40 MHz longitudinally coupled BGS wave resonator filter for first IF of ETC with a wide bandwidth of 15%.
Fig. 3. Frequency characteristics of 63 MHz longitudinally coupled BGS wave resonator filter with a wide bandwidth of 8%.
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Since a major factor in this filter is the cost, an inexpensive resin package is used. The authors were able to realize longitudinally coupled resonator filters with various bandwidths (relative bandwidth of 2 ∼ 15%) by using various PZT substrates with various kinds of coupling factors (k = 0.2, 0.47 or 0.6). 3. SH Wave Resonator Filter on LiTaO3 Using the SH wave on 36◦ -rotated Y-X propagation (36◦ YX) LiTaO3 , resonator filters with low loss and high impedance are realized at frequencies higher than about 110 MHz. The first IF filter for the Bluetooth system requires a relative 3 dB bandwidth of more than 1%. There are currently no substrates with coupling factors of single finger electrodes suitable for this required bandwidth using either longitudinally coupled or transversely coupled types consisting of SH waves. The required bandwidth is narrower than that using the former type and wider than that using the latter type with single-finger electrodes on 36◦ Y-X LiTaO3 . Previously, when there was no substrate suitable for the required bandwidth in a 71 MHz filter for the first IF of a global system for mobile communications (GSM), the authors were able to realize a marked narrowing in bandwidth on a transversely coupled resonator filter using BGS waves on a PZT substrate by employing split finger electrodes.4, 7) To realize such a narrow bandwidth, a similar approach was applied to the longitudinally coupled filters for the IF filter of a Bluetooth system. The filter is composed of two stages of a longitudinally coupled resonator filter, and each stage has IDTs consisting of 65 pairs of split fingers and a wavelength of 37.6 µm. Figure 5 shows the frequency characteristics. The insertion loss of 5.7 dB, the 3 dB bandwidth of 1.13 MHz, the chip size of 2.5 × 1.6 mm2 and the package size of 3.5 × 6.0 × 1.65 mm3 are obtained. This filter is a very small-sized and low-loss filter compared with our previous filter whose package size is 5.0×11.4×2.0 mm3 . Figure 6 shows the frequency characteristics of a 135 MHz filter for a GPS IF filter. This filter is composed of two stages of a longitudinally coupled resonator filter using single-finger electrodes. The insertion loss of 3.3 dB, the 3 dB bandwidth of 3.7 MHz, and the stop band rejection of more than 30 dB are obtained. The chip size and the package size are very small (1.1×2.4 mm2 and 3.8×3.8×1.15 mm3 , respectively).
Fig. 4. Frequency characteristics of 49.2 MHz longitudinally coupled BGS wave resonator filter for clock filter of DAB with a bandwidth of 2%.
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Fig. 5. Frequency characteristics of 111 MHz SH wave longitudinally coupled resonator filter for first IF of Bluetooth system.
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Fig. 7. Frequency characteristics of 190 MHz SH wave longitudinally coupled resonator filter with a wide bandwidth and good GDT for Rx first IF of W-CDMA.
Fig. 6. Frequency characteristics of 135 MHz SH wave longitudinally coupled resonator filter for first IF of GPS.
We have two kinds of 190 MHz receiver side (Rx) first IF filters for a wide-band code division multiple access (WCDMA) system. Each filter is composed of two stages of a longitudinally coupled resonator filter using edge reflections of SH waves on 36◦ Y-X LiTaO3 . Figures 7 and 8 show their frequency characteristics. Figure 7 shows the characteristics of the filter with a low insertion loss (3.8 dB) and wide bandwidth (relative 3 dB bandwidth: 6.3 MHz) in which priority is given to the flatness of the group delay time (GDT) within the bandwidth (deviation: 50 ns or less). Generally, resonator filters are considered to have poor GDT flatness compared to transversal SAW filters. However, the resonator filter does not cause ripples due to triple transit echo (TTE), which is an unwanted response generated by reflection between two IDTs in the transversal SAW filters. The GDT characteristic within the passband region is relatively good. The filter shown in Fig. 8 is designed to have large attenuations at 190 ± 5 MHz, ±10 MHz, and ±20 MHz. When the insertion loss is 4.1 dB and the 3 dB bandwidth is 4.7 MHz, the attenuations at 190 ± 5 MHz are larger than 53 dB, and the attenuations at 190 ± 10 MHz and 190 ± 20 MHz are larger than 60 dB. The GDT deviation characteristics are comparable to those shown in Fig. 7. These longitudinally coupled filters have a wavelength of 21.7 µm and two stages. Each chip size is less than 0.9 × 1.7 mm2 and has a package size of 3.0 × 3.0 × 1.15 mm3 in spite of using a wire-bonding type of IF filter. This size is smaller than that previously reported by us5) and as small as that of the wire-bonding type of RF
Fig. 8. Frequency characteristics of 190 MHz SH wave longitudinally coupled resonator filter with large attenuations at 190 ± 5, ±10 and ±20 MHz for Rx first IF of W-CDMA.
SAW filter. These are probably the smallest in size and exhibit the lowest loss among Rx first IF SAW filters reported for the W-CDMA system to date. The conventional longitudinal coupled resonator filter with electrode reflectors has a transverse spurious response like a terrace in the right foot region of the passband, but the longitudinally coupled resonator filter utilizing the edge reflection does not. Thus, the latter filter has a good shape factor and large stopband rejection. Figure 9 shows the chips and packages reported here. 4. Conclusions Because the package size of the first IF filter which has a low nominal center frequency is larger than that of the RF SAW filter, it is essential that it is small. Utilizing the reflection of BGS or SH waves on a PZT or LiTaO3 substrates at the edge of the substrate, we were able to realize small and low-loss longitudinally coupled resonator filters. They have the following features. (1) IF filters with an extremely wide bandwidth (relative bandwidth of 15%), small size and low loss have been realized using PZT substrates with a large coupling factor (k = 0.6). (2) Low-loss and small-sized IF filters with various relative bandwidths of 2%, 8%, and 15% have been realized us-
Jpn. J. Appl. Phys. Vol. 40 (2001) Pt. 1, No. 5B
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BGS -SH wave filter ETC
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the wire-bonding type, the package size of this filter is as small (3 × 3 × 1.15 mm3 ) as that of the RF SAW filter. The loss and size are the smallest among the filters for Rx first IF of W-CDMA reported to date. Acknowledgement The authors thank Mr. S. Arai, Director of Murata Mfg. Co., Ltd., for his support.
Conventional SAW filter Package
Fig. 9. Chips and packaged devices of our newly developed BGS·SH resonator filters and our conventional filters.
ing substrates with various coupling factors k of 0.2, 0.48, or 0.6. (3) Low-loss and small-sized Rx IF filters for W-CDMA using 36◦ Y-X LiTaO3 have been realized. In spite of being
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