Surface-acoustic-waver filter having a plurality of electrodes

A surface-acoustic-wave device, comprising:

• a generally rectangular substrate of a piezoelectric material and having an upper major surface;
• a plurality of input electrodes provided on the upper major surface of the substrate for inducing a surface acoustic wave on the upper major surface of the substrate in response to an input electric signal supplied thereto, each of said plurality of input electrodes comprising a plurality of first finger electrodes extending on the major surface of the substrate with a parallel relationship from each other in a first direction, each of the plurality of first finger electrodes having a first end connected with each other to form an input terminal for receiving the input electric signal, each of the plurality of first finger electrodes further having a second, opposing, free end, each of said plurality of input electrodes further comprising a plurality of second finger electrodes extending on the major surface of the substrate with a parallel relationship from each other in an opposing, second direction to the first finger electrodes, each of the plurality of second finger electrodes having a first end that is offset from the first end of the first finger electrodes in the first direction such that each of the first end of the second finger electrodes is connected with each other to the ground and each of the plurality of second finger electrodes further having a second, opposing, free end such that the second free end is close to the first end of the first finger electrodes, each of said first and second finger electrodes being disposed alternately in a third direction perpendicular to the first and second directions on the upper major surface of the substrate to form a pair such that each input electrode is characterized by a first pair number representing the number of pairs of the first and second fingers;
• a plurality of output electrodes provided on the upper major surface of the substrate for converting the surface acoustic wave to an output electric signal, each of said plurality of output electrodes comprising a plurality of third finger electrodes extending on the major surface of the substrate with a parallel relationship from each other in the second direction, each of the plurality of third finger electrodes having a first end connected with each other to form an output terminal, each of the plurality of third finger electrodes further having a second, opposing, free end, each of said plurality of output electrodes further comprising a plurality of fourth finger electrodes extending on the major surface of the substrate with a parallel relationship from each other in the first direction, each of the plurality of fourth finger electrodes having a first end that is offset from the first end of the third finger electrodes in the second direction such that each of the first end of the fourth finger electrodes is connected with each other to the ground and each of the plurality of fourth finger electrodes further having a second, opposing, free end such that the second free end is close to the first end of the third finger electrodes, each of said third and fourth finger electrodes being disposed alternately in the third direction on the upper major surface of the substrate to form a pair, such that each output electrode is characterized by a second pair number representing the number of pairs of the third and fourth fingers;
• each input electrode and each output electrode being disposed alternately in the third direction to form a row of electrodes extending in the third direction;
• and
• a reflector provided on the major surface of the substrate at both ends of the row of electrodes for reflecting the surface acoustic wave;
• wherein said first pair number is set different from said second pair number with a predetermined ratio therebetween, said second pair number being changed gradually in each second electrode, and said first pair number is changed in each first electrode in correspondence to the change of said pair number.