Thickness shear vibration type, crystal electromechanical filter

Abstract

Abstract 1,023,671. Piezo-electric crystals. TOYO TSUSHINKI KABUSHIKI KAISHA. Dec. 6, 1962 [Dec. 27,1961], No. 46134/62. Heading H1E A lattice filter is realized by means of a crystal plate having divided electrodes and vibrating in two fundamental thickness-shear modes. The plate is an AT or BT quartz cut, the major faces of such a plate being in the XZ plane. It is stated in the Specification that one fundamental thickness-shear mode exhibits a displacement distribution with one sine wave in the X direction, one cosine wave in the Y a direction and zero displacement in the Z a direction. Such a mode may be designated (1.1.0). Two further so-called inharmonic modes can be expressed as (2.1.0) and (1.1.1), these having similar sine and cosine distributions. In the filter of the invention, the fundamental mode and one of the inharmonic modes are excited in the plate. The plates of Figs, 2A, 2B utilize the (1.1.0) and (2.1.0) modes and have electrodes divided in the Z a direction so that transmission is in the X direction. By minimizing the Z a dimension of the plate as in Fig. 2B, the (1.1.1) mode can be suppressed; in the plate of Fig. 2A this mode is present but does not affect the electrical input or output. In the circular plates of Figs. 2D, 2E suppression of unwanted modes is achieved by means of slits 20. By dividing the electrodes in the X direction (so that transmission is in the Z a direction) a plate utilizing the (1.1.0) and (1.1.1) modes may be realized. In both types of plates the two modes of vibration have an antiphase relationship and this property is utilized in the design of a bandpass filter. The Specification describes in detail the equivalent lattice network for a plate vibrating in each of the modes discussed and it is shown how the equivalent reactances are related to electrode distribution and crystal size. The crystal is dimensioned so that the resonance frequencies of the fundamental and the inharmonic modes are adjacent, the difference being one-half of the band-width. A three-terminal filter may be realized by connecting one pair of divided electrodes to a common terminal as shown in Fig. 4A. Filters may be connected in cascade and capacitances connected across input and output terminals to provide attenuation poles.

Inventors

• 👤 Nakazawa Yuzo

Bibliographic Details