Piezoelectric ceramic filters using the edge‐mode vibration

Abstract

AbstractFor a conventional MF‐band piezoelectric filter, the contour vibration has mainly been used in which the whole of the vibrator body vibrates, and the vibrator is supported at its nodal point by a spring or a soldered wire. This degrades the characteristics of the filters, widens their manufacturing deviations, and limits their reliability and miniaturization. This paper proposes a new type of piezoelectric ceramic filter for the MF‐band using the edge‐mode vibration. This type of filter has a simple shape and a small size, and can be supported by vibration‐absorptive means such as a trapped‐energy type vibrator. The three‐terminal single‐mode filter is presented in which the electrode is divided with a gap into two parts to be used for the input and output. The relationship between the width of the gap and the fractional bandwidth of the filter is calculated theoretically, and the upper limit of its bandwidth is given. By using Pb(Zr, Ti)O3 pieaoelectric ceramics, two types of 455‐kHz filters having the 3‐dB fractional bandwidth of 1.3% and 1.6%, respectively, have been produced experimentally. By connecting 3 or 4 units of either type of the filters in cascade, a filter having the out‐of‐band attenuation greater than 50 dB over the wide frequency range up to ten times the center frequency has been produced successfully. To widen the bandwidth of the edge‐mode piezoelectric filter, the double‐mode filter consisting of two acoustically coupled resonators is also proposed. Experimental results confirm the practical usefulness of this type of filter.