String weighted surface acoustic wave transducer

Details String weighted surface acoustic wave transducer String weighted surface acoustic wave transducer

2001-10-05

2003-05-06

Summons, Barbara (Department: 2817)

Wave transmission lines and networks

Coupling networks

Electromechanical filter

C310S31300R

Reexamination Certificate

active

06559739

ABSTRACT:

BACKGROUND OF INVENTION
1. Field of Invention
The invention relates generally to surface acoustic wave transducers, and more particularly to transducers using string weighting for impedance transformation with improved rejection properties.
2. Background Art
The use of surface-acoustic-wave (SAW) devices as filters or resonators is well known for having the advantages of high Q, low series resistance, small size, and good frequency-temperature stability when compared with other frequency control methods such as LC circuits, coaxial delay lines, or metal cavity resonators. As described in U.S. Pat. No. 4,600,905 to Fredricksen, typically, a SAW device contains a substrate of piezoelectric material such as quartz, lithium niobate, or zinc oxide. Input and output transducers are formed upon the substrate. The transducers convert input electrical signals to surface acoustic waves propagating upon the surface of the substrate and then reconvert the acoustic energy to an electric output signal. The input and output transducers are configured as interdigital electrode fingers that extend from pairs of transducer pads, with the electrodes alternately connected to the hot and the ground bus bars, which forms an unweighted transducer. Interdigital transducers may be formed by depositing and patterning a thin film of electrically conductive material upon the piezoelectric substrate.
Alternating electrical potential coupled to the input interdigital transducer induces mechanical stresses in the substrate. The resulting strains propagate away from the input transducer along the surface of the substrate in the form of surface acoustic waves. These propagating surface waves arrive at the output interdigital transducer, where they are converted to electrical signals.
The basic tapered transducer has been reported in the literature and in particular in an article by P. M. Naraine and C. K. Campbell (“Wideband Linear Phase SAW Filters Using Apodized Slanted Finger Transducers,” Proceedings of IEEE Ultrasonics Symposium, Oct. '83, pp 113-116). Naraine et al. discuss wide-band linear-phase SAW filters using apodized slanted or tapered finger transducers. In earlier publications, tapered finger transducer geometries have all the transducer fingers positioned along lines that emanate from a single focal point. A performance improvement was shown in U.S. Pat. Nos. 4,635,008 and 4,08,542 to Solie, the inventor of the present invention, by using hyperbolically tapered electrodes.
In Solie '008 a dispersive SAW filter comprises hyperbolically tapered input and output transducers that are aligned such that normals from the transducers to a dispersive reflective array are aligned at substantially the same angle. The dispersive reflective array includes a multiplicity of parallel conductive strips or grooves formed in the device substrate on which the transducer rests. Constant spacing between the transducer fingers causes a relatively narrow band of frequencies to be generated by the input transducer and received by the output transducer. In Solie '542, a reduction in the resistive loss associated with the long narrow electrodes in wide acoustic aperture devices is sought; a hyperbolically tapered transducer is provided with fingers having configuration paths that are subdivided into patterns that segment the acoustic beam width. Further disclosed is a means of transforming the impedance and thus reducing the insertion loss by a division of the SAW transducer structure into a plurality of subtransducers.
The use of tapered finger geometries on both input and output transducers permits the transduction of a wide range of surface acoustic wavelengths from input to output transducer, and thus provides an electrical filter with a wide frequency passband. Typically, high-frequency components are transduced in the regions of the transducer where the finger-to-finger distance is the least. Low-frequency components are transduced in the regions of the transducer where the finger-to-finger distance is the greatest. At any given frequency, a surface wave may be transmitted or received in a limited portion of the total acoustic aperture, and the width of this active portion is called the “effective aperture” of the SAW beam.
The Naraine article states that for filters employing tapered finger transducer geometries, where the electrodes or fingers are straight-line segments emanating from a single point, there is an inherent negative slope of the amplitude response with increasing frequency, as large as 5 dB for a 50% bandwidth case reported in the IEEE article. Naraine's article describes a method of flattening the amplitude-response curve of a tapered finger filter by utilizing finger apodization. Apodization is a technique in which the length of individual transducer fingers is selectively adjusted so that the overlap between fingers of opposite polarities changes along the path traveled by the surface acoustic wave.
A “tap” is defined as the gap between two adjacent electrodes. The strength of the tap, or the relative tap weight, is proportional to the voltage difference between the two electrodes. If each gap is surrounded by two electrodes of opposite polarity (hot and ground), then every tap has its maximum tap strength and has a relative tap weight of 1.0. This tap weighting is acceptable for coupling the strongest SAW in the shortest possible length of transducer; however, this configuration is not useful in making a filter with out-of-band rejection. In order to achieve useful rejection levels, the tap weights should assume predetermined values varying, for example, from 0.0 to 1.0. The technique used to achieve relative tap values is called tap weighting. Tapered transducers also need to be weighted for maximum rejection. One technique, as described above, apodization, is not applicable to tapered transducers. Three previously known techniques that are applicable to tapered transducers include withdrawal weighting, block weighting, and linewidth weighting. Linewidth weighting, wherein the electrode width is varied, cannot achieve an acceptable range in tap values owing to fabrication considerations, giving relative tap weights of approximately 0.8 to 1.0.
Another objective of the weighting technique is impedance transformation. Typically tapered transducers have very low impedance, resulting in a high insertion loss owing to the impedance mismatch between the source, which is typically 50 &OHgr;, and the transducer, which may be <1 &OHgr;. The impedance values of tapered transducers are low because they have wide apertures to reduce diffraction, and the impedance is inversely proportional to the aperture width. Second, tapered transducers are typically long, several hundred wavelengths or even over one thousand wavelengths. Since all taps are connected in parallel across the two bus bars, this results in a very low impedance. Withdrawal weighting, which includes the elimination of some electrodes in the previously described alternating hot-ground connection pattern, does not increase the impedance.
The last of the previously known transducer configurations, block weighting, does provide a means for increasing impedance. An exemplary unweighted transducer
80
of length N (see
FIG. 1
) comprises a number, here 7, of subtransducers
81
-
87
, all in the same acoustic path and phased so that all the subtransducers are acoustically in phase with each other. This depends upon the electrical connection as well as the spacing between subtransducers. The subtransducers are arranged in strings
88
-
90
. The subtransducers within a string are connected in series, and the strings are connected in parallel across the two major bus bars
91
,
92
. There is always an odd number of subtransducers in a string. The block-weighted transducer
80
of
FIG. 1
has 1 or 3 subtransducers per string.
Each subtransducer
81
-
87
in
FIG. 1
is represented by a capacitive impedance element
81
′-
87
′ in
FIG. 2
, which shows how the voltage division within a string determines effective tap

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