{"id":3822,"date":"2014-05-27T19:32:11","date_gmt":"2014-05-27T15:32:11","guid":{"rendered":"https:\/\/butis-m.ru\/?p=3822"},"modified":"2014-08-17T11:19:47","modified_gmt":"2014-08-17T07:19:47","slug":"design-of-saw-filters-on-langasite","status":"publish","type":"post","link":"https:\/\/butis-m.ru\/en\/articles-en\/design-of-saw-filters-on-langasite\/","title":{"rendered":"Design of SAW Filters on Langasite"},"content":{"rendered":"

Due to good temperature stability and moderately high electromechanical coupling coefficient langasite (Li3<\/sub>Ga5<\/sub>SiO4<\/sub> or LGS) is known as a promising material for SAW devices. The characteristics of the Rayleigh wave propagation on LGS have been studied fairly well. However, no design of SAW devices on LGS suitable for modern communication systems has been reported as yet. In this paper, transversal filters on LGS based on bidirectional interdigital transducers, and low-loss filters on LGS based on natural single phase unidirectional transducers (NSPUDT), are described. The filters on LGS have exhibited better electrical parameters than their analogues on quartz and lithium tantalate.<\/p>\n

INTRODUCTION<\/h2>\n

The substrate material for SAW devices must satisfy a number of requirements, such as high electromechanical coupling coefficient , good temperature stability (the best temperature coefficient of delay is TDC=0), small power flow angle (the best is=0o<\/sup>), low level of parasitic modes which determines the selectivity of a device in a wide frequency band (not less than =-(50-55) dB). To avoid diffraction distortions, it is also desirable that the propagation direction of SAW should coincide with direction, where the anisotropy parameter =\u00a0-1. None of widely used crystallographic orientations in known materials\u00a0 (quartz,\u00a0 lithium\u00a0 tantalate, lithium niobate) meets\u00a0 all these requirements. Therefore, the search for, and study of, new materials and their cuts is an important task.<\/p>\n

Recently the family of temperature stable LGS cuts\u00a0 with the Euler angles 15\u043e<\/sup><<10o<\/sup>; 120o<\/sup><<165o<\/sup>; 20o<\/sup><<45o<\/sup>\u00a0 has been found [1]. In addition to TCD\u00bb0 ppm, some of these cuts exhibit high piezoelectric coupling (=0.28-0.38), low PFA (\u00bd\u00bd<2o<\/sup>) and anisotropy parameter close to the optimal value \u00bb-1. Later the natural unidirectionality of SAW generation has been\u00a0 discovered for some cuts in this family [2]. One more advantage of the LGS cuts indicated is the low level of parasitic modes. It is =-(50-55) dB in a wide frequency band, being 30 dB and 20 dB less than for YX and ZY cuts of LGS, respectively [3], and 5 dB less than for (90o<\/sup>,90o<\/sup>,112o<\/sup>) lithium tantalate cut. Since the effective velocity of SAW on LGS is low, =2400-2600 m\/s, the size of devices on LGS can be reduced at least by 20-30 % as compared with that of their analogues on quartz and lithium tantalate. So, because of the\u00a0 above listed properties langasite is a promising substrate material for SAW filters with good electrical parameters.<\/p>\n

In this paper, we report the results of the study of transversal filters, based on bidirectional interdigital transducer, and low-loss filters, based on NSPUDT with stepped electrodes. The filters of the first and second types are for use in relay lines of communication systems and radiotelephones of W-CDMA standard, respectively. As a substrate material, the [0o<\/sup>,140o<\/sup>,25o<\/sup>] cut Li3<\/sub>Ga5<\/sub>SiO4 <\/sub>\u00a0is utilized in both the types of filters. This cut is close to the optimal one. Besides, it has been chosen for study because the phase shift between the mechanical and electrical components of SAW in NSPUDT is known, =-51\u00b0 [2].<\/p>\n

STUDY OF SUBSTRATE MATERIAL CHARACTERISTICS<\/h2>\n

The parameters of SAW propagating on LGS were computed in [1,2,5]. However, these computations have yielded different results, since the different sets of elastic and temperature constants were used. So, we had to experimentally determine some of the important characteristics\u00a0 of [0o<\/sup>,140o<\/sup>,25o<\/sup>] cut LGS in order to provide an adequate accuracy in designing SAW filters on this cut.<\/p>\n

By the method resembling the one described in [4], we have measured the SAW velocity on the free () and the metallized () surfaces, the electromechanical coupling coefficient , the temperature coefficients of delay (TCD) of the first () and the second () orders, and the dielectric permittivity . Table 1 summarizes the results of measurements for the case of the aluminum film of thickness \u00bb1 %. They agree well with the data obtained in [1]. For comparison the analogous characteristics for (0\u00b0,140\u00b0,22.5\u00b0) cut LGS [5] are also given in Table 1.<\/p>\n

\u00a0Table 1. Characteristics of piezoelectric substrate<\/h3>\n\n\n\n\n\n\n\n\n\n\n
Characteristic<\/strong><\/td>\nSiO2<\/sub><\/strong><\/td>\nLGS<\/strong><\/td>\n<\/tr>\n
(0o<\/sup>,132.75o<\/sup>,0o<\/sup>)<\/strong><\/td>\n(0o<\/sup>,140o<\/sup>,22.5o<\/sup>)<\/strong><\/td>\n(0o<\/sup>,140o<\/sup>,25o<\/sup>)<\/strong><\/td>\n<\/tr>\n
Vaf, m\/s<\/td>\n3157<\/td>\n2742.1<\/td>\n2736<\/td>\n<\/tr>\n
K^2<\/td>\n0.14<\/td>\n0.32<\/td>\n0.38<\/td>\n<\/tr>\n
a2*108<\/sup>\/o<\/sup>C2<\/sup><\/td>\n- 3.2<\/td>\n- 7.8<\/td>\n- 6.8<\/td>\n<\/tr>\n
Tt, o<\/sup>C<\/td>\n25<\/td>\n25<\/td>\n23<\/td>\n<\/tr>\n
e<\/td>\n4.92<\/td>\n-<\/td>\n27<\/td>\n<\/tr>\n
, degr.<\/td>\n0<\/td>\n52<\/td>\n51<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Fig.1 shows the temperature dependence of the central frequency shift of the filters fabricated on the substrates made\u00a0 of\u00a0 different materials at \u00bb1 %. For the lithium tantalate, the scale \/o<\/sup>C is used. The temperature stability of the filter on the (0o<\/sup>,140o<\/sup>,25o<\/sup>) cut LGS is seen to be substantially better than that of the filter on the (90o<\/sup>,90o<\/sup>,112o<\/sup>) cut lithium tantalate and practically the same as that of the (0o<\/sup>,132.75o<\/sup>,0o<\/sup>) cut quartz. The temperature dependence of the central frequency shift for the filter on the (0o<\/sup>,140o<\/sup>,25o<\/sup>) LGS cut is described accurately enough in a wide temperature range by the parabola , where \u00a0is the temperature deviation from the turnover temperature . For the 1% thick\u00a0 aluminum electrodes and metallization ratio =0.5, we have obtained the first order TCD \u00a0\u00bb\u00a00 and the second order TCD \u00a0\u00bb\u00a0\u20116.8\u042710-8<\/sup> (1\/o<\/sup>C)2<\/sup> and =23o<\/sup>.<\/p>\n

\"Fig.1.<\/a>

Fig.1. Temperature dependence of the center frequency of SAW filters: 1 - LiTaO3, cut (90o,90o,112o), (*f\/f0)*10-5\/oC; 2 - SiO2, cut (0o,132.75o,0o); 3 - LGS, cut (0o,140o,22.5o);\u00a04 \u2013 LGS, cut (0o,140o,25o)<\/p><\/div>\n

For design of transversal filters with small amplitude and phase passband ripples as well as of SPUDT based low-loss\u00a0 filters, the reflection coefficient from an electrode\u00a0 is\u00a0 an important parameter. Referred to the centre of a strip of thickness , this coefficient becomes [11]<\/p>\n

where \u00a0is the metallization ratio, \u00a0is the number of\u00a0 electrodes per wavelength \u00a0in IDT, , , \u00a0are the coefficients determining the contribution of piezoelectric effect, mechanical loading, and energy storage effect, respectively. In contrast to quartz, the storage effect is the predominant mechanism of the reflection in LGS. At =0.00143; =0.047, and =0.28 the above formula describes the experimental values of the reflection coefficient from the aluminum electrode on the [0o<\/sup>,140o<\/sup>,25o<\/sup>] LGS cut with a good accuracy.<\/p>\n

TRANSVERSAL FILTERS ON LGS<\/h2>\n

In addition to the temperature stability, the low shape factor SH=1.1-1.2, selectivity larger than UR=50 dB, and ripples of group delay time at most GDT<50 ns are also important requirements of transversal filters for relay line.<\/p>\n

To achieve such parameters, we have designed the 117 MHz filter with the bandwidth BW3=7.36 MHz (5.2%), and SH(40\/3)=1.12 on (0o<\/sup>,140o<\/sup>,25o<\/sup>) cut LGS. The filter uses the input apodized and output withdrawal weighted transducers. The aperture of both the transducers is =1.8 mm\u00a0 or 50. The IDT\u2019s use split-finger electrodes to ensure the bidirectionality on NSPUDT substrate. The electrode thickness is =2500 Ao<\/sup>. The chip dimensions are 15.0 x 3.0 x 0.5 mm. The frequency dependences of\u0405S21\u0405and GDT for the filter under the condition of matching with the impedances of the source and load =50 Ohm are shown in fig. 2a,b.\u00a0 In this case matched insertion loss is IL=20.5 dB at ripples GDT=50 ns. Ultimate rejection in stop-band equals UR=50-55 dB.<\/p>\n

For comparison, using the same mask, we have fabricated a filter on (90o<\/sup>,90o<\/sup>,112o<\/sup>) LiTaO3<\/sub> cut. The filter has not been specially optimized but the electrode thickness has been reduced to =2000 Ao<\/sup>\u00a0 in order to decrease the reflection coefficient. Fig.2c shows the frequency dependences of \u0405S21\u0405 and GDT for this filter. The central frequency has increased up to 140 MHz, or by 17 %,\u00a0 which\u00a0 corresponds to the SAW velocity on LiTaO3<\/sub>=3290 m\/s. In the filter on LiTaO3<\/sub> matched insertion loss\u00a0 (IL=25.2 dB)\u00a0 is larger by 4.5-5 dB at the same ripples GDT=50 ns and ultimate rejection (UR=45 dB) is worse by 5-8 dB as compared with the filter on LGS.<\/p>\n

\"Fig.2.<\/a>

Fig.2. Comparison of measured frequency responses of transversal filters: (a) \u2013 LGS, cut (0o,140o,25o) in wide band; (b) \u2013 LGS, cut (0o,140o,25o) in passband; (c) - LiTaO3, cut (90o,90o,112o)<\/p><\/div>\n

The first effect can be attributed to better matching conditions for the filter on LGS, because the dielectric permittivity of the substrate (\u00bb27) is less approximately by the factor of 2. The second effect can be explained by weaker influence of diffraction (\u00bb-1) in the vicinity of the passband and lower level of parasitic modes (\u00bb55-60 dB) in the far stopband [3].<\/p>\n

To summarize, the parameters of transversal filters on LGS with the bandwidth BW3=3-6 % are better than that of the filters on LiTaO3<\/sub>. In addition, the chip dimensions can be decreased by 15-20 %, since the SAW velocity in LGS is lower than in LiTaO3 <\/sub>.<\/p>\n

LOW-LOSS FILTERS ON LANGASITE<\/h2>\n

\u00a0Natural single phase unidirectional transducers (NSPUDTs) are the most elegant solution among numerous alternatives of unidirectional transducers for low-loss filters. The unidirectionality of NSPUDT is achieved by \u00b1\/8 shift between the transduction and reflection centers due to electrode reflection of transducer and asymmetry of the crystal [6]. In designing NSPUDT low-loss\u00a0 filters, the basic challenge is how to make reversal\u00a0 transducers having the opposite directivity relative the direct NSPUDT with two \/4\u00a0 electrodes\u00a0 per wavelength [6]. Among reversal NSPUDT structures the most preferable are the configurations fabricated using a single stage technological process with a single operation of metallization. Such configurations of Reversal\u00a0 Directivity Transducer or RDT have recently been described in\u00a0 [7]: Electrode Width Difference (EWD) type RDT, Transduction Center Shift (TCS) type RDT, Group-type RDT.<\/p>\n

In the filters on [0o<\/sup>,140o<\/sup>,25o<\/sup>] cut LGS, we used the SAW propagating along the X+25 o<\/sup> direction. In this case the phase shift between the electrical potential and the mechanical displacement of the reflected SAW is =-51o<\/sup> [2].\u00a0 This phase angle was used as a parameter in the simulation technique based on equivalent circuit model. As reversal transducers, we used modified\u00a0 TCS-types RDT and Group-type RDT.<\/p>\n

In order to increase the efficiency of SAW generation in TCS-type RDT we have introduced a shorted reflected electrode. This has eliminated the negative effect of the charges\u00a0 induced on the isolated reflected electrode [7]. The width of the shorted electrode and its location with respect to excited electrodes in the new TST-type RDT have\u00a0 been optimized to obtain the maximum reflection and better directivity at =-51o<\/sup>. The minimum size of the electrodes was .<\/p>\n

To improve the shape factor and selectivity, the direct and reversal NSPUDT have been divided along the aperture into several parallel acoustic channels. Similar channels of the direct and reversal NSPUDT formed the subfilter with the central frequency \u00a0corresponding to the period of electrodes in -th channel. Such transducers have stepped electrodes with weighted electrode period and aperture of channels (WEPAC). The WEPAC transducers\u00a0 operate\u00a0 similarly\u00a0 to\u00a0 the\u00a0 tapered SPUDT [9].<\/p>\n

They can be called \"quasi-slanted\" transducers [8]. The WEPAC transducers, however, do not use slanted electrodes.\u00a0 The\u00a0 electrode period and aperture in neighbour channels have been weighted to improve the shape-factor and to increase ultimate rejection. To adjust the phase ripples of the filter in total, the subfilters have been shifted relative to each other in the direction of SAW propagation.<\/p>\n

Fig.3 shows the frequency dependences of \u00a0and phase for the W-CDMA 240 MHz filter on [0o<\/sup>,140o<\/sup>,25o<\/sup>] LGS\u00a0 cut.\u00a0 The filter uses the direct WEPAC NSPUDT with the \/4 electrodes and the reversal WEPAC NSPUDT (first modified TCS-type). Each of the transducers has been divided into 10 channels. The length of\u00a0 each\u00a0 channel was 125\u00a0in the direct NSPUDT and 197 \u00a0in the reversal\u00a0 one. The chip dimensions were 5.5 x 2.0 x 0.5 mm.\u00a0 The 9.0 x 7.0 x 1.5 mm SMD package was utilized. The filter was matched with the impedances =50 Ohm. Insertion loss did not exceed IL=10.8 dB. Ultimate rejection was more than 55 dB in the frequency range \u00b1100 MHz.<\/p>\n

\"Fig.3.<\/a>

Fig.3. Measured frequency responses of 240 MHz W-CDMA filter with reversal WEPAC NSPUDT, TCS-type 1<\/p><\/div>\n

For comparison we have designed the analogous W-CDMA 230 MHz filter, which uses two WEPAC bidirectional transducers on the (0o<\/sup>,132.75o<\/sup>,0o<\/sup>) quartz cut. The\u00a0 matched\u00a0 with\u00a0 50\u00a0 Ohm\u00a0 insertion\u00a0 loss\u00a0 and\u00a0 ultimate rejection\u00a0 of the filter are IL=16.6 dB and UR=50 dB in the\u00a0 frequency range =\u00b1100 MHz, respectively. The chip dimensions are 7.0 x 3.0 x 0.5 mm. The 9.0 x 7.0 x 1.5 mm SMD package is utilized.<\/p>\n

Thus, as compared to filters on quartz, the low-loss filters on LGS exhibit lower insertion loss, improved selectivity, and smaller dimensions with nearly the same temperature stability.<\/p>\n

Fig.4 shows experimental and calculated dependences of\u0405S21\u0405 for the filter that uses alternative modification of the reversal WEPAC NSPUDT on LGS. The main parameters\u00a0 of the filter are: =115 MHz, BW3=3.2 MHz\u00a0 or 2.8%; IL=12.3 dB on 50 Ohm system, UR=50 dB, phase ripples RMS=2.4o<\/sup>. The calculated values agree well with the experimental results.<\/p>\n

\"Fig.4.<\/a>

Fig.4. Measured (1) and calculated (2) frequency responses of 115 MHz filter with reversal WEPAC NSPUDT, TST type 2<\/p><\/div>\n

As is known, SAW propagation loss for radio frequencies of 1.2-2.4 GHz is small in LGS. However, the low SAW velocity, being an advantage at intermediate frequencies, complicates the fabrication of filters for the radio frequencies, since the electrode size must then be small.<\/p>\n

To overcome this obstacle, the high frequency Group-type\u00a0 unidirectional transducer [10] has been changed to the reversal NSPUDT. The minimum size of electrodes and period of sections in this transducer were \/6 and ,\u00a0 respectively. In fig. 5 the frequency dependences of\u0405S21\u0405 for the W-CDMA filter with the central frequency\u00a0 =250 MHz and bandwidth BW3=5.2 MHz or 2.1 % are shown. Like in the previous cases, the filter uses the direct WEPAC NSPUDT with electrodes.\u00a0 The\u00a0 reversal WEPAC NSPUDT is, however, of the Group-type with \/6 electrodes. Within the frequency range \u00b12.4 MHz the ripples are GDT=90 ns\u00a0 and phase ripples\u00a0 are\u00a0 RMS=3o<\/sup>. Matched insertion loss has increased up to IL=12.1 dB as compared to the 230 MHz filter, because the excitation efficiency of the Group-type NSPUDT is lower [10]. Nevertheless, this filter on [0o<\/sup>,140o<\/sup>,25o<\/sup>] LGS cut also exhibits a high selectivity UR=50 dB in the band\u00a0 \u00b115 MHz and UR=70 dB in a wide band of 50-500 MHz.<\/p>\n

\"Fig.5.<\/a>

Fig.5. Measured frequency responses of 250 MHz W-CDMA filter with reversal NSPUDT Group-type: (a) \u2013 in wide band;
(b) \u2013 in passband.<\/p><\/div>\n

CONCLUSION<\/h2>\n

This\u00a0 paper\u00a0 reports the practical results of the designing SAW filters on langasite. The transversal filters on LGS with passbands of 2-6% based on bidirectional transducers are shown to exhibit better electrical parameters and temperature stability than the filters on LiTaO3<\/sub>. The low-loss filters on LGS that use the new reversal NSPUDT's with the shifted center of transduction and the reversal Group-type NSPUDT possess lower insertion loss and better selectivity than the filters on quartz. Thus, we have demonstrated that LGS cut with temperature stable characteristics is a very promising substrate material for high performance SAW devices for communication systems.<\/p>\n

REFERENCES<\/h2>\n
    \n
  1. \u00a0N.F.Naumenko, V.S.Orlov, \"High Frequency SAW Device (on Langasite)\". Russian patent No\u00a02099857. Int.\u00a0 Cl6 HO3H 9\/00, dated 10 th January 1996, United States Patent No 05917265, dated\u00a0\u00a029th July,\u00a01999.<\/li>\n
  2. M.Takeuchi, M.Tanaka, Y.Imanishi, \"SAW Reflection Characteristics and NSPUDT Orientations on Langasite\", IEEE 1998 Ultrasonics Symposium Proc., pp.297\u2011300.<\/li>\n
  3. V.S.Orlov, P.G.Ivanov, V.M.Makarov, V.B.Chvets, \"New Types Low-Loss SAW Filters for Radiotelephone and Satellite Communication Systems\", Communication, Television, Broadcast Equipment and Systems, No 1, 1998, pp.38-43 (in Russian).<\/li>\n
  4. T.Thorvaldsson and B.P.Abbott, \"Low-Loss SAW Filters Utilizing the Natural Single Phase Unidirectional Transducer (NSPUDT)\", IEEE 1990 Ultrasonics Symposium Proc., pp.43 \u2013 48.<\/li>\n
  5. K. Inoue and K. Sato, \"Temperature Stability of SAW on Langasite Single Crystals\", IEEE 1998 Ultrasonics Symposium Proc., pp.301-306.<\/li>\n
  6. P.V.Wright,\u00a0 \"The Natural Single-Phase Unidirectional Transducer: A New Low-Loss SAW Transducer\", IEEE 1985 Ultrasonics Symposium Proc., pp.58-63.<\/li>\n
  7. \u00a0M.Takeuchi, H.Odagawa at al, \"SAW Transducer Configurations for Reversing the Directivity of NSPUDT Substrates\", IEEE 1995 Ultrasonics Symposium Proc., pp.17\u201122.<\/li>\n
  8. V.B.Chvets, P.G.Ivanov, V.M.Makarov, V.S.Orlov, \"Low-Loss Slanted SAW Filters With Low Shape-Factor\", IEEE 1999 Ultrasonics Symposium Proc., (to be published).<\/li>\n
  9. L.Solie, \"Tapered Transdusers Design and Application\", IEEE 1998 Ultrasonics Symposium Proc., pp.27-37.<\/li>\n
  10. V.B.Chvets, P.G.Ivanov, V.M.Makarov, V.S.Orlov, \"Low-Loss Filters Using New SPUDT Structures\", IEEE 1997 Ultrasonics Symposium Proc., pp.69-72.<\/li>\n
  11. P.V.Wright, \"Modelling and Experimental Measurements of the Reflection Properties of SAW Metallic Grating\", IEEE 1984 Ultrasonics Symposium Proc., pp.54-63.<\/li>\n<\/ol>\n","protected":false},"excerpt":{"rendered":"

    Due to good temperature stability and moderately high electromechanical coupling coefficient langasite (Li3Ga5SiO4 or LGS) is known as a promising material for SAW devices. The characteristics of the Rayleigh wave propagation on LGS have been studied fairly well. However, no design of SAW devices on LGS suitable for modern communication systems has been reported as […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"inline_featured_image":false,"footnotes":""},"categories":[8],"tags":[],"class_list":["post-3822","post","type-post","status-publish","format-standard","hentry","category-articles-en"],"acf":[],"yoast_head":"\nDesign of SAW Filters on Langasite - Butis<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/butis-m.ru\/en\/articles-en\/design-of-saw-filters-on-langasite\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Design of SAW Filters on Langasite - Butis\" \/>\n<meta property=\"og:description\" content=\"Due to good temperature stability and moderately high electromechanical coupling coefficient langasite (Li3Ga5SiO4 or LGS) is known as a promising material for SAW devices. The characteristics of the Rayleigh wave propagation on LGS have been studied fairly well. 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