Elastic wave filter including a grating reflector between tapered input and output IDT electrodes
To provide an elastic wave filter capable of obtaining steep attenuation at a high band side or a low band side in a pass band and having high selectivity. An elastic wave filter is configured in a manner that when seen in a propagation direction of an elastic wave, as for respective electrode fingers composing each group of electrode fingers in an input side tapered IDT electrode and an output side tapered IDT electrode, width dimensions thereof are the same one another, and a distance dimension between the adjacent electrode fingers is formed to be the same as the width dimension of the electrode finger, when seen in the propagation direction of an elastic wave, as for a group of electrode fingers in a grating reflector, width dimensions of respective electrode fingers composing the group of the electrode fingers are the same, and a distance dimension between the adjacent electrode fingers is formed to be the same as the width dimension, and the width dimension of the electrode finger and the distance dimension between the electrode fingers are set to be equal to or less than a minimum value or equal to or more than a maximum value of the width dimension of the electrode finger and the distance dimension between the electrode fingers in the input side tapered IDT electrode and the output side tapered IDT electrode.
1. An elastic wave filter comprising:
an input side tapered IDT electrode including a pair of bus bars formed to be parallel to each other on a piezoelectric substrate and a group of electrode fingers formed in a comb shape by extending the electrode fingers alternately from each of the paired bus bars respectively, and formed in a manner that a width of the electrode finger and a distance between the electrode fingers are widened from the bus bar on one side toward the bus bar on the other side;
an output side tapered IDT electrode including a pair of bus bars provided on the piezoelectric substrate to extend in an extending direction of the bus bars in said input side tapered IDT electrode and to be apart from said input side tapered IDT electrode in a propagation direction of an elastic wave and a group of electrode fingers formed in a comb shape by extending the electrode fingers alternately from each of the paired bus bars respectively, and formed in a manner that a width of the electrode finger and a distance between the electrode fingers are widened from the bus bar on one side toward the bus bar on the other side;
a grating reflector including a pair of bus bars provided on the extensions of the bus bars of the input side tapered IDT electrode and the output side tapered IDT electrode, respectively, as well as an electrode finger extending from an end portion on an input side of one bus bar of said pair to an end portion on an input side of the other bus bar of said pair and an electrode finger extending from an end portion on an output side of the one bus bar to an end portion on an output side of the other bus bar, and further comprising a group of electrode fingers, which are arranged between said input side tapered IDT electrode and said output side tapered IDT electrode along the propagation direction of an elastic wave, and which extend from the one bus bar toward the other bus bar, and wherein
the grating reflector is provided between the input side tapered IDT electrode and the output side tapered IDT electrode and formed apart thereof such that elastic waves propagating at any position in a length direction of the input side electrode fingers propagate through the elastic wave filter along linear lines,
when seen in the propagation direction of an elastic wave, the respective electrode fingers composing the groups of electrode fingers in said input side tapered IDT electrode and said output side tapered IDT electrode are formed in a manner that width dimensions thereof are the same one another, and a distance dimension between the adjacent electrode fingers is the same as the width dimension of the electrode finger, and
when seen in the propagation direction of an elastic wave, the group of electrode fingers in said grating reflector is formed in a manner that width dimensions of the respective electrode fingers composing the group of electrode fingers are the same, and a distance dimension between the adjacent electrode fingers is the same as the width dimension, and the width dimension of the electrode finger and the distance dimension between the electrode fingers are set to be equal to or less than a minimum value or equal to or more than a maximum value of the width dimension of the electrode finger and the distance dimension between the electrode fingers in said input side tapered IDT electrode and said output side tapered IDT electrode.
2. The elastic wave filter according to claim 1 , wherein
the width dimension of the electrode finger and the distance dimension between the electrode fingers in said grating reflector are constituted to be the same as the minimum value of the width dimension of the electrode finger and the distance dimension between the electrode fingers in said IDT electrodes or to be slightly smaller than the minimum value thereof.
3. The elastic wave filter according to claim 1 , wherein
the width dimension of the electrode finger and the distance dimension between the electrode fingers in said grating reflector are constituted to be the same as the maximum value of the width dimension of the electrode finger and the distance dimension between the electrode fingers in said IDT electrodes or to be slightly larger than the maximum value thereof.
4. The elastic wave filter according to claim 1 , wherein
the width dimension of the electrode finger and the distance dimension between the electrode fingers in said grating reflector in a direction perpendicular to the propagation direction of an elastic wave are the same respectively.
5. The elastic wave filter according to claim 1 , wherein
the width dimension of the electrode finger and the distance dimension between the electrode fingers in said grating reflector are gradually widened when seen in a direction perpendicular to the propagation direction of an elastic wave.
6. The elastic wave filter according to claim 1 , wherein
the group of electrode fingers in said grating reflector is formed to stride from an extension region of the bus bar on one side to an extension region of the bus bar on the other side in said input side tapered IDT electrode or said output side tapered IDT electrode.