A highly miniaturized recursive Z-path SAW filter

In the last couple of years Z-path SAW filters have been widely used as intermediate frequency filters in mobile phones especially for GSM at center frequencies below 100 MHz. This design technique offers a great potential for size reduction compared to inline structures due to the folded signal path. The last IF-filter generation for CDMA-1900 realized as inline structures was typically mounted in a 13.3×6.5 mm2 package. This paper presents an extension of the classical Z-path concept for a CDMA-1900 filter at 210.38 MHz with a 5 dB-bandwidth of 1.6 MHz on Quartz in a 5×5 mm2 ceramic package. In order to achieve the dramatic size reduction of 70% compared to inline structures, the Z-path concept has been combined with the recursive design technique resulting in a recursive Z-path filter. Both transducers have a weighted excitation and reflection. The reflection functions exhibit changes in sign, thus forming longitudinally coupled resonant cavities leading to steep skirts. The degradation of stopband performance due to electromagnetic coupling of the closely adjacent transducers can't be neglected. Therefore, a novel method which allows for independent compensation of both inductive and capacitive cross coupling was introduced. A filter having a typical insertion attenuation of 8.2 dB and a selectivity at f0-±1.25 MHz of typically 39 dB is presented. The deviation from linear phase amounts to 2.3° rms typically in a frequency range of f0±0.63 MHz. It will be shown that the theoretical and experimental results are in very good agreement.