High selectivity IF SAW filters for CDMA mobile phones

The Z-path structure is a good choice for intermediate frequency SAW filters in highly miniaturized mobile phones. In order to achieve the dramatic size reduction of up to 70% compared to inline structures, the Z-path concept has been combined with the recursive filter design technique resulting in a recursive Z-path filter. One of the advantages of this structure is the multiple mechanisms which can be exploited to shape the passband and to achieve high selectivity. However, at frequencies above 100 MHz the selectivity of a Z-path filter on a chip of minimum size is affected by inductive as well as capacitive crosstalk between the input and the output transducer. To meet the rejection characteristics required in modern telecommunication systems, a novel method to improve the stopband performance of Z-path filters was developed. It allows for an independent cancellation of both the inductive and the capacitive crosstalk by balancing the net current flow to a transducer and by equalizing the effective coupling capacitance from the input to the output transducer. The finger-electrodes themselves remain unchanged and thus the electro-acoustical behaviour is not affected. CDMA-filters having a selectivity of more than 65 dB are presented. The new technique is applied to filters for the CDMA-800 system at 128.1 MHz on quartz in a 13×6.5 mm² ceramic package. A CDMA-1900 filter at 210.38 MHz in a 5×5 mm² ceramic package is also presented