Low-loss DECT-IF SAW filters without matching networks

Abstract

This paper presents low-loss ring SAW filter on 128/spl deg/ YX LiNbO/sub 3/ with a 3-dB fractional bandwidth of about 1%. Reflective multistrip couplers (RMSCs) with 3 electrodes per /spl lambda/ (/spl lambda/ is SAW wavelength at the center frequency of the filter) and long IDTs with phase weighting were used in the filter. The IDTs have a self-matching effect when the static capacitance of the IDT is compensated by the acoustic radiation susceptance. The ring filter at a center frequency of 110.6 MHz showed an insertion loss of about 3 dB, 3-dB bandwidth of about 1.1 MHz, stopband attenuation of 25, 35 and 50 dB at /spl plusmn/1.1, /spl plusmn/1.7 and /spl plusmn/2 MHz offsets from the center frequency respectively. The filter did not require matching networks, was mounted in SMD package (14.2/spl times/8.4/spl times/2.7 mm) and was used for IF-filtering in mobile communication (DECT system).