Abstract-A surface acoustic wave (SAW) two resonator filter with coupling interdigital transducers (IDTs) located inside the reflectors, was designed, fabricated, ...
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IEEE TRANSACTIONS ON ULTRASONICS, FERROELECTRICS, AND FREQUENCY CONTROL, VOL.
44,NO. 1, JANUARY 1997
ondence Surface Acoustic Wave Two Resonator Filter Using Synchronous Coupling Interdigital Transducers Waldemar Soluch, Senior Member, IEEE Abstract-A surface acoustic wave (SAW) two resonator filter with coupling interdigital transducers (IDTs) located inside the reflectors, was designed, fabricated, and tested. In the filter, electrodes of the coupling IDTs are in spatial synchronism with the reflectors’ electrodes. The filter, fabricated on ST-cut quartz, had a center frequency of 302.6 MHz, insertion loss about 13 dB, and minimum rejection in the stopbands of about 40 dB.
I. INTRODUCTION OUPLING NETWORKS in SAW two resonator filters are
usually located between reflectors [l].To obtain reasonable coupling in the case of low electromechanical coupling substrates, the coupling networks must be long. Therefore, resonators are also long, the nearest longitudinal modes are close to the center frequency, and rejection near the passband is low. A possible solution to this problem is placement of the coupling network inside the reflectors. For example, it was recently shown that a coupling network can be formed by connecting open reflectors’ strips [2]. This structure is equivalent to a stopband multistrip coupler (SMSC). Single electrode interdigital transducers (IDTs), located inside the reflectors, can be used as a coupling network in the case of the short circuited reflectors’ strips. Therefore they can be called synchronous coupling interdigital transducers (SCIDTs). In this case, the bus bars of the input and output IDTs can also be located inside the reflectors. The SCIDT responses can be easily changed by apodization or withdrawal weighting. It is also expected that the cross-channel coupling coefficient of the SCIDTs’ filters should be larger than that of the SMSC since in the latter case some SAW energy is lost in the area between the two acoustic channels.
11. FILTER DESIGN The structure of the filter is shown in Fig. 1. Each reflector consists of three parts: bus bars for input or output IDTs, SCIDTs, and the remaining part of short circuited strips. The SCIDTs are connected in parallel by the rear parts of the reflectors. Input and output IDTs are apodized, while SCIDTs are unapodized. Scattering matrix theory was used for calculating the amplitude transfer function of the filter [l]. To determine the scattering coefficients of the reflectors, the reflection coefficient for Manuscript received March 5, 1996; accepted June 27, 1996. This work was supported by the State Committee for Scientific Research (KBN) under grant number 8 S501 051 04. W. Soluch is with the Institute of Electronic Materials Technology, Wolczynska 133, 01-919 Warsaw, Poland (e-mail: soluch-wQsp.itme.edu.pl).
Fig. 1. SAW two resonator filter with coupling IDTs located inside reflectors. (1)input IDT, (2) bus bar, (3) coupling IDT, (4) rear part of reflector, (d) lateral shift of resonators.
one aluminium strip on ST-cut quartz, r = -0.5h/X, was used, where h is the thickness of the aluminium layer, and X is the wavlength [3]-[4]. The scattering coefficients of the apodized IDTs were calculated from their input admittances [5].ST-cut quartz parameters were taken from Slobodnik [6].The crosschannel coupling coefficient was used as a parameter, and will be determined by comparing the calculated and measured amplitude responses of the filter. The following data of the filter were used: acoustic aperture: lmm; total number of strips in each reflector: 600; number of bus bar strips: 10; number of electrodes of coupling IDTs: 101; number of electrodes of the input and output IDTs: 61; period of electrodes: 5.2 ym; width of electodes: 2.6 pm; lateral shift of resonators: 0.3 mm (d in Fig. 1);and aluminium layer thickness: 0.12 ym. The input and output IDTs were placed at the positions of maximum coupling.
111. MEASUREMENTS AND CALCULATIONS The photomask was made with electron beam photolithography. The filters’ aluminium electrodes were deposited on the ST-cut quartz substrates by the lift-off method. The filters were mounted in metallic packages and the filters’ transfer properties were measured n a 50 R system (HP Network Analyzer 8752A). The typical amplitude response of the filter is shown in Fig. 2 . The characteristic asymmetry of the transfer response is the result of the synchronous placement of the coupling IDTs. In this case, the center frequency and insertion loss were equal to about 302.6 MHz and 13.3 dB, respectively. The 3 dB bandwidth was equal to about 40 kHz, which is equivalent to the loaded Q of about 7,600. The stopband rejection was higher than 40 dB at frequencies below the passband, and higher than 50 dB at frequencies above the passband. It was found that to obtain an agreement between the measured and calculated results, the SAW velocity in the periodically metallized areas u = 3147.46 m/s, and the cross-channel coupling coefficient e12 = 0.032, should be used. Additionally, an inductance L = 0.05 pH in series with 50 C2 load resistance was needed to take into account the parasitic inductances of connecting wires. The amplitude response, calculated from an analytical expression [l],is shown in Fig. 3.
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IV. CONCLUSIONS
A SAW two resonator filter with SCIDTs was designed, fabricated, and tested. As compared to the filter with coupling IDTs located between reflectors, these filters are shorter and better rejection is obtained in the stopbands. However, the asymmetry of the frequency response, which is characteristic for the synchronous placement of IDTs, makes the rejection smaller at frequencies below the passband. Compared to the filter with the SMSC couplers [a],application of the SCIDTs gave a higher cross-channel coupling coefficient. Location of the coupling IDTs in the reflector areas can be used as a method of obtaining additional coupling in filters with coupling IDTs located between reflectors. When coupling IDTs are located inside reflectors, a so-called QARP (quasi-constant acoustic reflection periodicity) method can be used for SAW to bulk scattering loss suppression [7]. In this case, no additional gaps between input and output IDTs and reflectors exist, but the period of the IDTs is smaller than that of the reflectors. This makes it possible to use a large reflection coefficient for one strip. Reflectors can then be shorter which leads to further reduction of filter length. It also means that the aluminium layer will be thicker, which is important for lowering ohmic losses at high frequencies.
Fig. 2. Measured amplitude response of the filter.
ACKNOWLEDGMENTS
I would like to thank M. Teodorczyk and E. Lipinska for filter fabrication and T. Wrobel for mask design and filter measurements.
REFERENCES [l] R. L. Rosenberg and L. A. Coldren, “Scattering analysis and design of SAW resonator filters,” IEEE Trans. Ultrason., Fer-
reelect., Freq. Contr., vol. SU-26, no. 3, pp. 205-230, May 1979. [2] W. Soluch, “Application of a stopband multistrip coupler in a SAW resonator filter,” in IEEE Ultrason. S y m p . Proc., 1994,
start 298.8 MHz
stop
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Fig. 3. Calculated amplitude response of the filter. (fo) center frequency, (IL) insertion loss.
Reasonable agreement exists between the measured and calculated amplitude responses of the filter. In particular, the asymmetry of the responses is similar and the stopband rejections are in good agreement. The shape of the stopband is determined mainly by the frequency responses of the coupling IDTs and reflectors. It is worthy of note that excellent feedthrough supression was obtained in the filter.
pp. 245-251. [3] C. Dundrowicz, “Reflection of surface waves from periodic discontinuities,” IEEE Ultrason. Symp. Proc., 1976, pp. 386-390. [4] W. J. Tanski ad H. van de Vart, “The design of SAW resonators on quartz with emphasis on two ports,” IEEE Ultrason. S y m p . Proc., 1976, pp. 260-265. [5] W. Soluch, “Admittance matrix of a surface acoustic wave interdigital transducer,” IEEE T r a n s . Ultrason., Ferroelect.,Freq. Contr., vol. 40, no. 5, pp. 828-831, Nov. 1993. [SI A. J. Slobodnik, “Surface acoustic waves and SAW materials,” Proc. IEEE, vol. 64, no. 5, pp. 581-595, May 1976. [7] Y. Ebata, “Suppression of bulk-scattering loss in SAW resonator with quasi-constant acoustic reflection periodicity,” IEEE Ultrason. S y m p . Proc., 1988, pp. 91-96.
