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Balanced Low-Loss SAW filters with Impedance Conversion


S. A. Doberstein

The reduction of the insertion losses in the SAW filters and expansion of their functionalities like balanced operation, impedance conversion, selfmatching in combination with the specified frequency response meets the advanced directions of the SAW technology progress. This paper presents the balanced low-loss selfmatched SAW filters with impedance conversion of 1:4 in a fractional bandwidth (∆ƒ/ƒ0) of 1.5–7 % on the base of the narrowband ring structures with reflective multistrip couplers (RMSCs), wideband resonators with the reflective arrays (RA) and unidirectional transducers (UT) with U-shape MSCs. These filters allow eliminating the cumbersome baluns and matching elements from an electronic scheme. For the ∆ƒ/ƒ0 = 1.5% the ring SAW filters with RMSCs on 128°YX LiNbO3, for the ∆ƒ/ƒ0 = 2–6% the three-transducer resonator SAW filters with RA on 42°YX LiТаO3, 64°YX, 41°YX LiNbO3 and for the ∆ƒ/ƒ0 = 3–7% the three-transducer SAW filters on UT with U-shape MSCs on 128°YX, 64°YX, 41°YX LiNbO3 are used. The balanced operation of the ring filters is realized by symmetrical connection of the input and output IDTs to the loads. The balanced operation of the three-transducer filters is realized by symmetrical connection of the input central and output side IDTs with RA or U-shape MSCs to the loads. In order for the low input impedance of the filter to be converted to the high output impedance in the ring filters the sectional output IDT with a series electrical connection of the two sections is used. An impedance conversion in the three-transducer filters is provided by a series connection of the output side IDTs with RA or UT with U-shape MSCs. To increase the selectivity of the filters a phase weighting in the input IDT or cascading two filters is used. The optimization of the SAW filter topology is provided with a computer simulation using an equivalent circuit model. The 45–500 MHz balanced filters have shown an insertion loss of about 3 dB, stopband attenuation of 60–100 dB and impedance conversion from 50–75 Ohm to 150–300 Ohm. In the passband the 1–2 dB un-balance in amplitude and 6–20° unbalance in phase are provided. The presented balanced low-loss SAW filters with impedance conversion offer a single-chip solution for the front-end of the handheld VHF transceivers for matching the low impedance antenna with modern high impedance double balanced mixer and also for suppression of the local oscillator and image frequencies.

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