Z-path IF-filters for mobile telephones

IF-filters for channel selection in mobile phone systems must offer narrow bandwidths together with very steep skirts and excellent stopband rejection. Surface acoustic wave (SAW) filters fulfill these requirements. But if conventional transversal design techniques are used chip sizes become too large. The propagation path of the SAW was folded in order to make better use of the total chip length. The use of two slightly inclined reflectors results in a Z-shaped filter which conserves the temperature stability of quartz substrate. At a center frequency of 45 MHz, filters were developed on small chips with good passband characteristics and an insertion loss better than 10 dB. A filter with signal suppression of 20 dB and an ultimate relative stopband rejection of 60 dB is shown