Method for realizing surface acoustic wave filter arrangements and filter arrangement obtained in accordance to this method
View Patent ↗The invention relates to a method for realizing surface acoustic wave filter arrangements of the kind comprising a piezo-electric substrate and on said substrate metallic strip elements constituting impedance filter elements arranged in a network scheme. In such an arrangement by electrostatic interaction between the network elements is produced a parasitic resonance at a frequency different from the operational resonance frequency. The method is characterized in that means are provided in the filter arrangement which produce a shifting of the parasitic resonance frequency to a frequency substantially equal to said operational resonance frequency. The invention can be used for filter arrangements.
1. A surface acoustic wave filter comprising:
bus-bars extending in the direction of propagation of surface acoustic waves and inter-digital electrodes extending perpendicularly from said bus-bars, said electrodes being spaced apart by an inter-digital electrode pitch P, said bus bars further having parasitic resonance frequency shifting cut outs formed therein, said cut outs in the bus bars being spaced apart by said inter-digital electrode pitch P in a parallel relationship to the direction of the surface acoustic wave propagation;
wherein the cut outs create a dummi finger structure with electrode to electrode gaps less than an inter-digital electrode width M.
2. The surface acoustic wave filter of claim 1 wherein center lines of the cut outs in the upper and lower bus-bars align with center lines between two adjacent inter-digital electrodes.
3. The surface acoustic wave filter of claim 1 wherein the length of the cut outs in the direction perpendicular to the surface acoustic wave propagation direction is rn the range of 0.4 to 20 times the inter-digital electrode pitch P.
4. The surface acoustic wave filter of claim 1 wherein the length of the cut outs in the direction perpendicular to the surface acoustic wave propagation is variable.
5. The surface acoustic wave filter of claim 1 , wherein the edges forming said gap and the surface acoustic wave propagation direction form an angle in the range of 0 to 60°.
6. The surface acoustic wave filter of claim 1 , wherein the inter-digital electrodes have a mark to period ratio M/P substantially equal to a mark to period ratio m/p of the cut outs.