Surface acoustic wave elements with varying electrode finger pitch and connection arrangements
A SAW element configured to suppress spurious emissions resulting from non-periodicity of an IDT electrode finger arrangement. In one example, the SAW element is a resonator comprising an IDT electrode including a first plurality of IDT electrode fingers connected to a first busbar and a second plurality of IDT electrode fingers connected to a second busbar, and reflectors having a plurality of reflector fingers. The pluralities of first and second IDT electrode fingers are alternatively connected to each busbar by either normal connections or reversed connections and include regions arranged by at least two different types of pitches. The normal connections are such that either the odd-numbered or even-numbered IDT electrode fingers connect to the first busbar, and the reversed connections are such that the opposite group of IDT electrode fingers connect to the first busbar.
1. A surface acoustic wave element comprising:
a piezoelectric substrate;
an interdigital transducer (IDT) electrode formed on the piezoelectric substrate, the IDT electrode including a plurality of electrode fingers, a first busbar, and a second busbar, the IDT electrode further including a first region in which the plurality of electrode fingers of the IDT electrode are connected to the first and second busbars by normal connections, a second region in which the plurality of electrode fingers are connected to the first and second busbars by reversed connections, and a third region in which the electrode fingers are connected to the first and second busbars by normal connections, the normal connections having odd-numbered electrode fingers sequentially counted from one end of the IDT electrode in a propagation direction of a surface acoustic wave excited by the IDT electrode connected to the first busbar and even-numbered electrode fingers connected to the second busbar, and the reversed connections having the odd-numbered electrode fingers connected to the second busbar and the even-numbered electrode fingers connected to the first busbar, the plurality of electrode fingers being arranged with a first pitch in the first region and a second pitch in the second region, the second pitch being greater than the first pitch, the first and second regions being symmetrically arranged with respect to a single reference position in the IDT electrode along the propagation direction, and the reference position being a mid-point of the IDT electrode along the propagation direction, the first region extending symmetrically from the reference position to a pair of first points located a first distance away from the reference position along the propagation direction, the third region extending from each first point to a corresponding one of a pair of second points located a second distance away from the reference position along the propagation direction, and the second region extending from each second point to a respective end of the IDT electrode, the electrode fingers in the third region being arranged with a varying pitch that varies linearly with distance from the reference position from the first pitch at each first point to a third pitch at each second point, the third pitch being less than the first pitch; and
a pair of reflectors disposed opposing one another on opposite sides of the IDT electrode in the propagation direction on the piezoelectric substrate.
2. The surface acoustic wave element of claim 1 wherein each reflector of the pair of reflectors includes a plurality of reflector fingers arranged with a fourth pitch greater than the second pitch.
3. The surface acoustic wave element of claim 2 wherein the first region has a first width in the propagation direction and the second region has a second width in the propagation direction, the first width being greater than the second width.
4. A ladder-type filter comprising:
an input and an output;
a piezoelectric substrate;
a plurality of series-arm resonators formed on the piezoelectric substrate and connected in series along a signal path between the input and the output, at least one of the series-arm resonators being implemented using a surface acoustic wave element including an interdigital transducer (IDT) electrode formed on the piezoelectric substrate and configured to excite an acoustic wave and a pair of reflectors disposed opposing one another on the piezoelectric substrate on opposite sides of the IDT electrode in a propagation direction of the acoustic wave, the IDT electrode having a plurality of electrode fingers, a first busbar, and a second busbar, the IDT electrode configured with a first region in which the plurality of electrode fingers of the IDT electrode are connected to the first and second busbars by normal connections, a second region in which the plurality of electrode fingers are connected to the first and second busbars by reversed connections, and a third region in which the electrode fingers are connected to the first and second busbars by the normal connections, the normal connections having odd-numbered electrode fingers sequentially counted in the propagation direction from one end of the IDT electrode connected to the first busbar and even-numbered electrode fingers connected to the second busbar, and the reversed connections having the odd-numbered electrode fingers connected to the second busbar and the even-numbered electrode fingers connected to the first busbar, the plurality of electrode fingers being arranged with a first pitch in the first region and a second pitch in the second region, the second pitch being greater than the first pitch, the first and second regions of the IDT electrode of the surface acoustic wave element being symmetrically arranged with respect to a single reference position in the IDT electrode along the propagation direction, the reference position being a mid-point of the IDT electrode along the propagation direction, the first region extending symmetrically from the reference position to a pair of first points located a first distance away from the reference position along the propagation direction, the third region extending from each first point to a corresponding one of a pair of second points located a second distance away from the reference position along the propagation direction, and the second region extending from each second point to a respective end of the IDT electrode, the electrode fingers in the third region being arranged with a varying pitch that varies linearly with distance from the reference position from the first pitch at each first point to a third pitch at each second point, the third pitch being less than the first pitch; and
a plurality of parallel-arm resonators formed on the piezoelectric substrate and connected between the signal path and a reference potential, at least one of the parallel-arm resonators being implemented using the surface acoustic wave element.
5. The ladder-type filter of claim 4 wherein each reflector of the pair of reflectors includes a plurality of reflector fingers arranged with a fourth pitch greater than the second pitch.