DMS filter having improved signal suppression in the stop band

#This patent application is a national phase filing under section 371 of PCT/EP2011/050601, filed Jan. 18, 2011, which claims the priority of German patent application 10 2010 005 306.6, filed Jan. 21, 2010, each of which is incorporated herein by reference in its entirety.

#SAW (Surface Acoustic Wave) filters of the DMS (Dual Mode SAW filter) type are preferably used as RF filters when the intention is either to change from asymmetrical to symmetrical signal routing or to achieve small filter dimensions. DMS filters comprise at least two resonators which are arranged in the same acoustic track, acoustically couple to one another and are preferably alternately connected to the input and output sides of the RF filter. The number of input and output transducers laterally arranged beside one another in the longitudinal direction determines the electrical performance of the filter substantially via the respective transducer impedances and the resonances occurring between the individual transducers.

#In order to improve the selectivity of DMS filters, it is known practice to connect the filters to individual resonators or to one or more basic elements of a ladder-type structure. In this case, a so-called basic element respectively comprises at least one acoustic resonator arranged in a series path and an acoustic resonator connected to ground in parallel therewith in a shunt branch.

#The invention is explained in more detail below using exemplary embodiments together with comparison tests and the associated figures. The figures are used only to illustrate the invention and are therefore only schematic and are not true to scale. For better illustration, size proportions may be distorted and individual parts may be illustrated on a larger or smaller scale. Identical or identically acting parts are denoted with the same reference symbols.

#FIG. 1 shows a DMS filter having three transducers and a variable capacitance,

#FIG. 2 shows a DMS filter having five transducers, a variable capacitance and a common series resonator,

#FIG. 1 shows a simple embodiment of the invention. The DMS filter has a DMS track DSP in which three transducers, two transducers E1, E2 of the first type and one transducer A1 of the second type, are arranged between two reflectors R1, R2. The two transducers of the first type E1, E2 are connected in parallel between a common node KG and a ground connection. The common node is directly connected to an asymmetrical signal connection TE1.

#A variable capacitance is connected here, as a concentrated element EK, between the common node KG and one of the transducers of the first type (here the transducer E2). The voltage U1 applied to the transducer E1 thus differs from the voltage U2 applied to E2.

#The connections TA1 and TA2 are connected to the transducer A1 of the second type arranged centrally in the DMS track DSP and together can form a symmetrical or an asymmetrical port. In the case mentioned last, one of the two signal connections is connected to ground.