Selective null placement in SAW coupled resonator filters

Selectively placed nulls in the frequency response of surface acoustic wave (SAW) coupled-resonator (CR) filters have been used to suppress local oscillator and image signals within single conversion receivers. Rejection of these signals to 50 dB by placement of a 450-kHz-wide null was demonstrated through the use of a CR filter utilizing the hiccup transducer. Effects of modifications to transduction lengths, placement of cavities within the structures, and variations of the reflection coefficient per wavelength at center frequency are examined to show how null width and placement are affected. Results of three devices at 328.400, 451.250, and 804.300 MHz are compared to the theoretical coupling-of-modes (COM) model. The ability to place selective nulls without detrimental effects to device response, insertion loss, or additional tuning complexity makes these devices unique for suppressing local oscillator and image signals.<
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