Filter and resonator using langasite

Wide band filter and low loss resonator have been developed using the new piezoelectric material, Langasite. The filter has about three times wider passband compared with the filter utilizing quartz crystal, and the resonator has approximately two to five times smaller equivalent series resistance than that of quartz crystal. We have developed 71 MHz BAW filter for GSM base station. Since the filter is made in fundamentals mode, we have to obtain the blank of about 19 μm thickness because the frequency constant is 1380 kHz mm. We tried to obtain the required blank thickness by chemically etching the center area of the blank (thickness: 70 μm) from both sides. Special technology of langasite etching which has big differences compared with usual quartz technology was developed. We have successfully etched the blank to the required thickness using an efficient etchant. The etching area of the blank has a chemically polished surface and it is sufficient for high frequency filters. Two MCF units are used to make a 4-pole langasite filter. The size of the 4-pole langasite filter is 10.3×3.5 mm and it is five times smaller than conventional filler. In this study, the possibility of high frequency fundamental and overtone mode BAW resonator has been examined. We made the resonators ranging from 10 MHz to 200 MHz in both modes and checked the performance. Inverted-mesa blank that is formed with chemical etching is used for the resonator above 20 MHz. The blank for the resonator below 20 MHz is lapped with normal 2500 abrasive. The resonator at 11 MHz in fundamental mode shows 2.450 ohms of Equivalent Series Resistance (ESR) and 3×10⁴ of Q factor. The resonator has checked with its performance up to 9th overtone and shows satisfactory results