Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Patent
1981-05-28
1983-07-12
Shoop, William M.
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
333150, 333193, H01L 4110
Patent
active
043933211
DESCRIPTION:
BRIEF SUMMARY
FIELD OF INVENTION
The invention relates to surface acoustic wave (SAW) transducers, and more particularly to capacitive-weighting transducers.
DESCRIPTION OF THE PRIOR ART
Known in the art is an interdigital capacitive-weighting SAW transducer comprising a piezoelectric substrate and an array of electrodes laid down on said substrate, said array of electrodes including a first group of electrodes arranged in parallel with one another and connected, by means of a capacitive coupling, to a first current-conducting pad, and a second group of electrodes connected to a second current-conducting pad by means of a capacitive coupling and arranged in parallel with said electrodes of said first group, the electrodes of the first group being positioned in the corresponding spaces between the adjacent electrodes of the second group so that overlap regions of the electrodes of the both groups are formed, said overlap regions having a constant length.
The capacitive coupling between the electrodes and their respective current-conducting pads is provided by depressions made in the latter and the electrode ends that enter said depressions. The capacitance between each electrode and the respective current-conducting pad depends on the depression perimeter and the width of the gap which is available between the end of the electrode in the depression and that pad. The fashion in which the capacitances vary with the transducer length depends on a given impulse response of the transducer and is selected by varying the depression depth and the width of the gap between the ends of the corresponding electrodes and their current-conducting pad (cf. U.S. Pat. No. 3,904,996, Sept. 9, 1975).
In the known transducer, there results a considerable energy loss in the working frequency band. This is due to the fact that in the frequency passband surface acoustic waves are excited not only within the overlap regions of the electrodes of the both groups, but also in the gaps between the current-conducting areas and the ends of the electrodes that enter these areas. In addition, diffraction effects occur in the transducer since surface acoustic waves with circular front are excited at the areas having shallow depressions. It is known that diffraction results in a distortion of the impulse response and, consequently, to a distortion of the frequency response in the passband of the transducer, which in turn causes poor suppression of the signals beyond this passband.
There is a capacitive-coupling SAW transducer offering smaller energy loss in the working frequency range (cf. U.S. Pat. No. 4,162,415, July 24, 1975).
This transducer comprises a substrate of a piezoelectric material in which surface acoustic waves may be propagated and at least two rows of electrodes laid down on said substrate, a first main row including a first group of electrodes arranged in parallel with one another and connected electrically to a first current-conducting pad, and a second group of electrodes arranged in parallel with said electrodes of said first group. The electrodes of said first group are positioned in the corresponding spaces between the adjacent electrodes of the second group so that overlap regions of the electrodes of the both groups are formed, said overlap regions having a constant length. The electrodes of said second group are connected to a second current-conducting pad. A second (additional) row of said two rows is positioned between said main row and said second current-conducting area and includes a first group of electrodes arranged in parallel with one another and electrically connected to said second current-conducting pad, and also includes a second group of electrodes arranged in parallel with said electrodes of said first group in said additional row. In said two groups of said additional row certain number of their electrodes are positioned in the spaces therebetween in a manner such that overlap regions of the electrods are formed which have a variable length depending on a given impulse response of the transducer, said electrod
REFERENCES:
patent: 4066985 (1978-01-01), Kuny
patent: 4162415 (1979-07-01), Andreev et al.
patent: 4166257 (1979-08-01), Subramanian
patent: 4185218 (1980-01-01), Bagdasarian et al.
Gulaev, "Saw Transducer With Capacitive Electrode Weighting", Sov. Tech. Phys. Lett. 5(6), Jun. 1979, pp. 287-288.
Bagdasarian Alexandr S.
Fedorets Vladimir N.
Gulyaev Jury V.
Kmita Anatoly M.
Shoop William M.
Wong Peter S.
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