Abstract
A SAW transducer and a SAW resonator are proposed composed of consecutively arranged unit cells of length L. Slight geometry or material variations such as variations of the metallization ration or the unit cell length L affecting the pitch) between these unit cells result in improved spurious mode suppression while the main mode performance is unaffected.
Claims
1.-13. (canceled)
14. A SAW transducer comprising unit cells that are arranged consecutively in a longitudinal direction, the unit cells having a length L.sub.i and a metallization ratio η.sub.I, wherein each unit cell comprises two or more electrode fingers connected to different electrical potentials, wherein the unit cells show a variation of L.sub.i and η.sub.i over a length of the transducer.
15. The SAW transducer of claim 14, wherein each unit cell is adapted to produce a wavelet W.sub.i, wherein the variation of L.sub.i and η.sub.i over the length of the transducer is such that for each unit cell, an eigenfrequency of a main operating SAW mode is unchanged and wavelets superimpose constructively at the frequency of a main operating SAW mode.
16. The SAW transducer of claim 15, wherein by the variation the eigenfrequency of unwanted resonances differs slightly between neighboring unit cells and therefore the wavelets superimpose at least partially destructively at the frequency of a spurious mode and/or the envelope of the spurious mode along the transducer is shaped so that the excitation maximum is reduced and/or its frequency is shifted.
17. The SAW transducer of claim 14, wherein the unit cells show a quasi-linear variation of the parameters L.sub.i and η.sub.i over the length of the SAW transducer.
18. The SAW transducer of claim 14, wherein L.sub.i continuously increases and η.sub.i continuously decreases over the length of the transducer.
19. The SAW transducer of one claim 14, wherein the variation of the parameters L.sub.i and η.sub.i is according to two sine functions having a phase difference of π.
20. The SAW transducer of claim 14, wherein one of the parameters L.sub.i and η.sub.i has a minimum and the other parameter has a maximum in the middle of the transducer.
21. The SAW transducer of claim 14, wherein the two or more electrode fingers have a transversal geometry profile such that a cross section thereof is at maximum near a tip of each electrode finger.
22. The SAW transducer of claim 14 comprising one or more reflectors.
23. The SAW transducer of claim 14, wherein the SAW transducer is one of at least two transducers arranged between two reflectors to form a DMS filter.
24. A SAW transducer comprising of unit cells that are arranged consecutively in a longitudinal direction, the unit cells having a length L.sub.i, wherein each unit cell comprises two or more electrode fingers connected to different electrical potentials, wherein the unit cells show a variation of L.sub.i and another frequency-relevant parameter over a length of the transducer.
25. The SAW transducer of claim 24, wherein the unit cells show a quasi-linear variation of the parameters L.sub.i and the other frequency-relevant parameter over the length of the transducer.
26. The SAW transducer of claim 24, wherein L.sub.i continuously increases and the other frequency-relevant parameter continuously decreases over the length of the transducer.
27. The SAW transducer of claim 24, wherein the frequency relevant parameter corresponds to at least one of a metal height of the two or more electrode fingers, or a dielectric layer height, or a broadness of the two or more electrode fingers, or a combination thereof.
28. A SAW resonator comprising of one or more transducers and one or more reflectors where the one or more reflectors is composed of reflector unit cells that are arranged consecutively in a longitudinal direction each reflector unit cell comprises two or more reflector fingers at least two frequency-relevant geometry and/or material parameters of the reflector unit cells differ from the corresponding parameters in the neighboring transducer.
29. A SAW resonator according to claim 28, where a resonance of one or more unwanted modes is outside a reflector stopband while a main SAW resonance is within the reflector stopband.
Description
[0031] FIG. 1 shows the variation of the two parameters length L.sub.i and metallization ratio η.sub.i over the unit cell index of a transducer
[0032] FIG. 2 shows the course of eigenfrequencies of the main mode and a spurious mode over the unit cell index of a transducer according to FIG. 1
[0033] FIG. 3 shows the real part of the admittance of a resonator with a transducer according to FIG. 1
[0034] FIG. 4 shows the magnitude of the admittance of the same resonator like FIG. 3
[0035] FIG. 5 shows a passband of a SAW filter comprising two resonators as shown in FIGS. 3 and 4
[0036] FIG. 6A shows unit cells UC.sub.i of a transducer
[0037] FIG. 6B shows unit cells RUC.sub.i of a reflector
[0038] FIG. 7 shows a resonator comprising the transducer and reflectors
[0039] FIG. 8 shows a SAW filter comprised of series and parallel resonators
[0040] FIG. 9 shows a SAW filter embodied as a DMS filter using transducers according to an embodiment.
[0041] FIG. 1 shows in a first embodiment a possible realization of the proposed parameter variation of two parameters of a transducer over the transducer in a longitudinal direction according to the unit cell index. In the example a transducer consisting of 100 unit cells is shown. The left y-axis is assigned to the length L.sub.i of the unit cells UC.sub.i that is proportional to the pitch and the wavelength that can be excited by this transducer. The right y-axis indicates the metallization ratio η.sub.i of a unit cell UC.sub.i. The metallization ratio can be derived by dividing the width of the electrode fingers through the pitch.
[0042] In this embodiment length L.sub.i and metallization ratio η vary linearly from the first to the last unit cell of the transducer. The relative variation is chosen so that the eigenfrequency of the main mode is the same in all unit cells, but the eigenfrequency of one (or several) spurious modes varies over the length of the transducer.
[0043] FIG. 2 shows the course of these two eigenfrequencies over the length of a transducer having a parameter variation as shown in FIG. 1 which is optimized to achieve the above mentioned result. The course of the main mode does not vary over the transducer but the spurious mode does. Hence, each unit cell UC.sub.i produces a spurious mode the eigenfrequency of which is different to that of a neighboring unit cell UC.sub.(i+1) or UC.sub.(i−1). As a result the spurious mode portions of the wavelets produced by all unit cells of the transducer together do not superimpose fully constructively and hence, the disturbing spurious resonance peak is reduced or vanishes completely. As far as not extinguished by superposition the spurious mode contributions spread over a range of frequencies and do not appear at a single spurious mode frequency.
[0044] The solid line in the diagram of FIG. 3 shows the real part of the admittance of a SAW resonator comprising a parameter variation as shown in FIG. 1 compared with a respective admittance of a standard SAW resonator depicted by a dashed line. Above the pronounced maximum at the frequency of the main mode the standard SAW resonator shows a small peak caused by excitation of a differently polarized acoustic mode. This small peak nearly vanishes or is at least markedly reduced in height in the admittance curve of a transducer according to the proposed parameter variation depicted as a solid line.
[0045] This positive effect can also be detected in the diagram of FIG. 4 comparing the magnitudes of admittance of a standard resonator and the resonator of the embodiment. While the resonance of the main mode keeps constant the peak of the spurious mode is reduced significantly.
[0046] A SAW filter with ladder type structure comprising one series resonator and one parallel resonator is formed using two resonators each having a transducer according to the embodiment. The calculated transmission in the passband region of such a filter is depicted in FIG. 5. For reference the dashed line depicts a respective passband of a filter comprising two standard SAW resonators. The admittance characteristics of series and parallel resonator correspond to those of a standard SAW resonator with non-vanishing spurious modes (dashed line) and a SAW resonator according to the embodiment (solid line) as shown in FIGS. 3 and 4. It can be seen that the solid line according to the embodiment shows a significantly reduced passband dip compared to the standard SAW resonator.
[0047] FIG. 6A shows the structure of a transducer composed of unit cells UC.sub.i. Usually each unit cell UC.sub.i consists of two electrode finger EF connected to different busbars BB and hence, to different electric potentials. Each length L.sub.i complies with the wavelength λ of an acoustic wave that can be excited when applying an RF signal to the two busbars BB. Dependent on the metallization ratio applied in a respective unit cell different geometrical lengths are required to excite the same frequency. Due to the small differences the variation of the two parameters are not visible by merely looking on the structure as depicted.
[0048] FIG. 6B shows unit cells RUC.sub.i of a reflector REF. Each reflector unit cell comprises two reflecting stripes that are shorted by two “busbars”. The same frequency determining parameters may be varied over the length of such a reflector similar like the transducer unit cell variation to yield a reflector performance wherein the wavelets of the main mode superimpose constructively and are hence totally reflected while the reflection of spurious modes is reduced due to non-constructive superposition of the respective wavelets. Alternatively, at least two frequency-relevant geometry and/or material parameters of the reflector unit cells differ from the corresponding parameters in the neighboring transducer so that the frequency of the reflector stopband relative to the transducer resonance differs for different acoustic modes.
[0049] FIG. 7 show a SAW resonator R that may comprise a transducer IDT with the proposed parameter variation. The SAW transducer IDT is arranged between two reflective gratings REF that may be shorted to form reflectors. On the right side of the figure the circuit symbol of a resonator R as used in a block diagram is shown.
[0050] FIG. 8 shows a possible structure of a SAW filter with ladder type structure. Such a filter consists of a number of series resonators R.sub.S connected in series in a series signal line and a number of parallel resonators R.sub.P each arranged in a shunt arm branching from the series signal line. The number of resonators in the filter is set dependent on the desired degree of selectivity. According filter design rules are well known in the art.
[0051] At least one resonator of any type chosen from series and parallel resonators has a transducer and/or reflector with the proposed parameter variations. Also at this type of a SAW filter the advantageous reduction of spurious modes can be achieved resulting in improved insertion attenuation, smoother skirts, reduced group delay ripple, improved out-of-band reflection, improved compression and improved power durability behavior.
[0052] FIG. 9 shows an exemplary simplified SAW filter of the DMS type that may be construed from transducers and/or reflectors having the proposed parameter variation. A DMS filter comprises a number of transducers IDT to IDT.sub.3 that are arranged in an acoustic track between two acoustic reflective gratings REF. The IDTs are alternatingly connected to input and output terminal respectively of the filter. A symmetrical structure is preferred. At least one of the resonators of one type chosen from input transducer and output transducer shows the proposed parameter variation. Also at this type of SAW filters the advantageous reduction of spurious modes can be achieved resulting in improved insertion attenuation, smoother skirts, reduced group delay ripple, improved out-of-band reflection, improved compression and improved power durability behavior.
[0053] Despite being explained on the depicted embodiments only the invention is not restricted by the shown embodiments and figures. The scope is defined by the claims only and may comprise variations deviating from the figures.
TABLE-US-00001 List of used terms and reference symbols BB busbar EF electrode finger IDT SAW transducer L.sub.i length of unit cell UC.sub.i REF reflective grating/acoustic reflector R.sub.P series SAW resonator R.sub.S series SAW resonator RUC.sub.i reflector unit cell UC.sub.i transducer unit cell η.sub.i metallization ratio of unit cell UC.sub.i W.sub.i wavelet eigenfrequency longitudinal direction main mode pitch of transducer SAW filter spurious mode
