Wide-Band Balanced Low-Loss SAW Ring Filters with Impedance Conversion

This paper presents the new wide-band balanced SAW ring filters with the reduced insertion loss, low input and output impedances and impedance conversion on 64 degrees YX and 49 degrees YX LiNbO3. To decrease the input and output impedances and resistive losses of the IDT electrodes the parallel connection of the two SAW ring filters is used simultaneously reducing the apertures of each filter. The balanced operation of the filters is provided by symmetrical connection of the parallel input and output IDTs to the loads. For the impedance conversion a series connection of the output IDTs in the two filters or changing the polarity of output IDT in one of the filters are used. To increase the selectivity of the filters the cascaded connection and phase weighting are employed. The constructional and topological optimization of the SAW filters is provided with a computer simulation using an equivalent circuit model. The 45 MHz balanced SAW ring filters have shown an insertion loss of 1.2-1.5 dB, stopband attenuation of 35-40 dB, impedance conversion from 75 Omega to 300 Omega in a wide fractional bandwidth of 3.87-5.48%. Two cascaded filters with the phase weighting have provided an insertion loss of 3.5 dB and stopband attenuation over 90 dB. The similar filters for 146-174 MHz handheld transceivers were presented. 158.6 MHz filter on 49 degrees YX LiNbO3 has shown insertion loss of 1.4 dB, 3-dB bandwidth of 7.48 MHz, stopband attenuation of about 40 dB. 149 MHz filter on 64 degrees YX LiNbO3 has provided an insertion loss of 1.4 dB, 3-dB bandwidth of 5.9 MHz, stopband attenuation of 35 dB. The filters are housed in the 9.1x7.1x1.8 mm SMD packages.