Balanced-type IF SAW filters with different input/output impedances

Transversely coupled resonator SAW filters, which are suitable for IF-stage filters of recent cellular phone applications, have been developed. Balanced-type SAW filters have better performances than unbalanced-type, because of suppressing EMI caused by weak grounding. Furthermore, best matching conditions with front and back stage circuits are strongly required for simplifying matching circuits around the IF-stage filter. The authors have developed a new interdigital transducer (IDT) structure which can control the filter input/output impedances. The filter input/output impedances mainly depend on the static capacitance of the IDT. This structure consists of three IDT blocks. These blocks are connected in serial so that surface waves of each block can propagate with same phase. Then, by changing the number of the IDT electrodes, the input/output impedances were successfully controlled. The filter samples show excellent performances.