Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
1999-09-28
2001-07-31
Enad, Elvin (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S31300R
Reexamination Certificate
active
06268680
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a surface acoustic wave transducer with a piezoelectric substrate and at least two electrodes on the surface of the substrate wherein the electrodes comprise fingers which overlap with the fingers of another electrode in a projection parallel to the surface of the substrate and a filter consisting of SAW transducers.
Surface acoustic wave (SAW) transducers can be used to design electronic filters which are optimized for a broad, flat passband with steep transition edges to the stopbands. A transducer comprises at least two electrodes which are located on the surface of a piezoelectric substrate, e. g. a crystal of quartz or lithium tantalate in a specific cut and orientation. Typically, the electrodes are manufactured by surface metallization of the substrate and consist each of a contact pad with fingers which project from one edge of the contact pad. The contact pads of two electrodes are arranged parallel to each other with the fingers extending inwardly between them and being interdigitated. Therefore, the fingers of both electrodes overlap in projection parallel to the contact pads.
When an electric signal is applied to the electrodes, the voltage causes elastic deformations of the substrate in the gaps between the fingers. The deformations propagate as surface acoustic waves in direction parallel to the contact pads and can be received by another transducer on the same substrate. The frequency of the maximum response of the SAWs to an electric signal is related to the fundamental period &lgr; of the fingers, i. e. an optimum coupling is achieved if the wavelength &lgr; of the surface acoustic waves is approximately equal to the width of two fingers on adjacent electrodes with their corresponding gaps. The location and overlap of the fingers of the transducer correspond approximately to the Fourier transform of its frequency response which consists of a main lobe and several side lobes for a transducer with a single passband as it is well known in the art.
It is of high importance that transducers have the lowest possible insertion loss and a flat passband without curvature at the edges. Besides of bidirectional loss and SAW propagation loss, the insertion loss of such a transducer with a suitably tuned feeding circuit is dominated by the Q-value if the ohmic losses are sufficiently low. The Q-value of the transducer is defined by
Q=|m(Y)|/Re(Y)
wherein Y is the transducer admittance and Re and Im denote the real and imaginary value respectively. A low Q-value can easily be obtained with &lgr;/4 wide fingers separated by gaps of equal width. This structure has a high coupling to the surface acoustic waves and a correspondingly low Q-value. Even if a filter with a broad passband is to be designed which requires a small number of fingers for a given length in the main lobe of the transducer, a low Q-value may be attained in this way.
Due to the surface inhomogeneities caused by the fingers, a part of the propagating surface acoustic waves is reflected at each finger. This effect distorts the surface acoustic waves, especially if large numbers of fingers are present on the substrate. The effect is most pronounced for &lgr;/4 wide fingers with gaps of equal width because in this case all SAW reflections add constructively. Consequently, fingers with a width of &lgr;/4 are avoided in SAW devices in the state of the art. As alternatives, split fingers consisting of two adjacent fingers with a width of &lgr;/8 on the same electrode or combinations of fingers with a width of 3&lgr;/8 and &lgr;/8 have been proposed. Both structures are low reflecting because the partial waves reflected at different fingers do not interfere in phase. However, the Q-value is higher than that of a structure with &lgr;/4 wide fingers.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to propose a transducer which has a low Q-value and causes only small reflections of the surface acoustic waves. It is a further object of the invention to develop a SAW filter which causes low reflections of the surface acoustic waves and has a high coupling to the surface acoustic waves. It is a third object of the invention to propose a broadband SAW filter with a large relative band width, steep edges of the passband and a low insertion loss.
According to the present invention, the fingers comprise coupling fingers which are located adjacent to a finger of another electrode and which have a high coupling efficiency to the surface acoustic waves, a structure with low surface acoustic wave reflection is located between at least two coupling fingers of the same electrode and the surface acoustic wave propagation velocity of the low reflecting structure and the coupling fingers is equal.
The invention is based on the concept that the number of fingers per length unit which have a high coupling efficiency to the surface acoustic waves has to be small in a broadband transducer. These coupling fingers which are located adjacent to a finger of another electrode are spaced at predetermined intervals which ensure the proper phase delay between the fingers for the desired shape of the frequency response. In the intervals between the coupling fingers, a structure with a low SAW reflections is located. The low reflecting structure ensures that the propagation velocities of the surface acoustic waves in the region of the coupling fingers and the intervals in between are equal or at least nearly equal. In this way, distortions of the acoustic waves and difficulties in the design of the transducer are avoided. The SAW velocity inside the transducer is influenced by the surface area covered by the electrodes, the amount of electrode material on the surface of the substrate, the number of finger edges per length unit and the depth of grooves in the substrate. The proposed structure may be confined to the main lobe and the strongest sidelobes of the transducer were the largest part of energy is transferred to the SAWs while the structure of the other sidelobes can be a customary transducer structure.
The proposed transducer has a low Q-value due to the fingers with high coupling efficiency to the SAWs and consequently a low insertion loss even if the relative bandwidth is large. Preferably, a material and orientation of the substrate is chosen which ensures that only Rayleigh waves are excited on the surface of the substrate while bulk waves are avoided. However, other substrate materials and orientations can be used for other, e.g. leaky, surface waves. Distortions of the surface acoustic waves are avoided because the low reflecting structure ensures a nearly constant propagation velocity of the SAWs throughout the transducer while a constructive interference of partial waves generated by this structure is excluded. Furthermore, the efficient coupling to the SAWs causes a low sensitivity of the transducer to temperature changes. Preferably, the width of a coupling finger is equal to the width of the gap to the adjacent finger of the other electrode. However, structures with ratios of finger width to gap width differing slightly from unity are possible, e. g. for adjustments of the excited phase or velocity of the SAWs.
In a preferred embodiment of the invention, the width of the coupling fingers and the spacing of adjacent coupling fingers on different electrodes is &lgr;/4 wherein &lgr; is the wavelength of the surface acoustic waves at the first harmonic frequency response of the transducer. It can be shown by theoretical calculations that this structure has the lowest Q-value if only transducers with two electrodes are treated. Slightly different widths of the fingers and gaps are possible.
Especially if the coupling fingers have a width of &lgr;/4, constructive interference of partial waves reflected at them may lead to undesired distortions of the SAWs. It is therefore proposed to embed the coupling fingers at least partially into the substrate. The depth of the embedding is to such an extent that the net reflection coefficient of
Addison Karen
Alcatel
Enad Elvin
Sughrue Mion Zinn Macpeak & Seas, PLLC
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