This invention relates to a piezoelectric monolithic crystal device, filters, resonators or the like, operating in the trapped energy mode and having a capacitor plated on the piezoelectric wafer within energy coupling distance of the main electrodes and it is an object of the invention to provide an improved device of this nature.
Piezoelectric monolithic crystal devices of the nature here involved are known to the art, as for example in the U.S. Pat. No. 3,898,489, Aug. 5, 1975 to John J. Grady and Theodore E. Lind and assigned to the same assignee as the present invention. As disclosed in the Grady et al patent electrodes are plated on the major surfaces of a piezoelectric crystal wafer, for example, AT cut quartz. The dimensions of the electrodes and those of the crystal wafer were selected, together with the frequency, so that the device whether acting as a filter, resonator, or the like, was operating in the trapped energy mode.
Conducting tabs connected to the electrodes on the appropriate surfaces of the crystal wafer were provided, and on the opposite sides of the crystal wafer from the conducting tabs there were provided pseudo tabs opposite the conducting tabs but terminating short of the actual electrodes for trapping spurious modes propagating within the crystal structure and conducting them to the edges of the crystal wafer for dissipation. Such spurious modes, accordingly, did not appear in the output response.
Connecting capacitors across the input and output electrodes of a monolithic crystal filter element for improving filter response also is well-known. In this respect, reference is made to the following U.S. Pat. Nos.: 3,947,784, Mar. 30, 1976 Arvanitis et al, 3,716,808, Feb. 13, 1973 Malinowski et al and 3,633,134, Jan. 4, 1972 Barrows et al all assigned to the assignee of the present invention. The capacitors illustrated in the patents referred to are not capacitors whose electrodes are plated onto the piezoelectric cyrstal surface but are separate, discrete, capacitors connected to the terminals as shown. However, it is also known to plate capacitor electrodes on the major surfaces of the piezoelectric element and connecting the capacitor electrodes to the input and output terminals. In such structures, though, the plated capacitor has been disposed far enough away from the main electrodes so that any frequency modes that were propagated in the piezoelectric material by virtue of the exitation of the capacitor did not couple to the main electrodes and thus did not produce any undesirable responses in the filter output. Thus such structures require relatively large piezoelectric crystal wafers in order to provide sufficient distance between the plated capacitor and the plated main electrodes to avoid any of the indicated coupling. Accordingly, such piezoelectric monolithic devices resulted in relatively large structures that were expensive in and of themselves and increased the cost by virtue of requiring larger packages. Moreover, it is essential that the device and the resulting package be made as small as possible.