Longitudinally-coupled-resonator-type elastic wave filter device

A longitudinally-coupled-resonator-type elastic wave filter device comprising:

• a piezoelectric substrate;
• and
• at least two IDTs disposed on the piezoelectric substrate along a direction of propagation of an elastic wave;
• wherein
• at least one of the at least two IDTs includes, in a portion in which the at least two IDTs are adjacent to each other, a first electrode finger that is an outermost electrode finger nearest to the other one of the at least two IDTs, a second electrode finger adjacent to the first electrode finger, and a dummy electrode disposed between the first electrode finger and the second electrode finger;
• the dummy electrode includes a first dummy electrode finger portion, a second dummy electrode finger portion, and a connecting portion connecting the first dummy electrode finger portion and the second dummy electrode finger portion, the first dummy electrode finger portion is disposed along an extension line of a longitudinal axis of the first electrode finger with a gap between the first dummy electrode finger portion and a forward end of the first electrode finger and extending substantially in parallel to the second electrode finger, the second dummy electrode finger portion is disposed along an extension line of a longitudinal axis of the second electrode finger with a gap between the second dummy electrode finger portion and a forward end of the second electrode finger and extending substantially in parallel to the first electrode finger, and the connecting portion is disposed between the forward end of the first electrode finger and the forward end of the second electrode finger;
• a length of the first electrode finger and a length of the second electrode finger are different from a length of the other electrode fingers to provide apodization weighting;
• the dummy electrode is disposed to provide series weighting;
• an elastic wave propagation portion in the longitudinally-coupled-resonator-type surface acoustic wave filter device includes a first acoustic track, a second acoustic track disposed on either side of the first acoustic track in a direction substantially perpendicular to the propagation direction of an elastic wave, and a third acoustic track disposed outside the second acoustic track in the direction substantially perpendicular to the propagation direction of the elastic wave, the first acoustic track passing through the connecting portion of the dummy electrode, the second acoustic track including a gap between the forward end of the first electrode finger and the connecting portion of the dummy electrode and a gap between the forward end of the second electrode finger and the connecting portion of the dummy electrode, the third acoustic track including the first electrode finger and the second electrode finger;
• and
• a series-weighted portion includes at least one of a metallization ratio increasing portion arranged to increase a metallization ratio of the second acoustic track and a metallization ratio reducing portion arranged to reduce a metallization ratio of the first acoustic track so as to reduce a difference between the metallization ratio of the first acoustic track and the metallization ratio of the second acoustic track.