Polylithic SAW filters using coupled-resonator filter elements

Discrete SAW filters consisting of electrically coupled SAW resonators in ladder structure have strong finite attenuation poles, which are a severe limitation in the achievable bandwidth and stopband attenuation, and in the realizability of arbitrary filter characteristics. Lattice structures are difficult to realize at higher frequencies. Transversely and Longitudinally Coupled SAW filters (TCF, LCF) are the SAW analogue of monolithic crystal filter “duals” (MCF). Their simplified electrical diagram is equivalent to that of a two-pole lattice crystal filter section. Polylithic SAW Filters (PSF) can be generated by electrical coupling of several TCFs/LCFs in a tandem configuration with interstage coupling reactances. This structure allows the realization of filters of a large variety of responses with “all-pole”-characteristics, including linear-phase and selective-linear-phase filters. The paper describes the synthesis of PSFs based on the SAW element values. Alternatively an approximative method based on the transmission characteristic of the SAW elements is given. Experimental results achieved with TCF elements at 620 MHz are reported