Low-loss DECT-IF SAW filters without matching networks

This paper presents low-loss ring SAW filter on 128° YX LiNbO ₃ with a 3-dB fractional bandwidth of about 1%. Reflective multistrip couplers (RMSCs) with 3 electrodes per λ (λ 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 ±1.1, ±1.7 and ±2 MHz offsets from the center frequency respectively. The filter did not require matching networks, was mounted in SMD package (14.2×8.4×2.7 mm) and was used for IF-filtering in mobile communication (DECT system)