Acoustic wave filter apparatus and duplexer

An acoustic wave filter apparatus having a balanced/unbalanced conversion function and defining a second band pass filter of an acoustic wave filter device that includes a first band pass filter that has a pass band at a relatively low frequency side and the second band pass filter that has a pass band at a relatively high frequency side, one end of the first band pass filter and one end of the second band pass filter being connected to a common terminal, the acoustic wave filter apparatus comprising:

• a piezoelectric substrate;
• and
• a first longitudinally coupled resonator acoustic wave filter portion and a second longitudinally coupled resonator acoustic wave filter portion provided on the piezoelectric substrate;
• wherein
• each of the first longitudinally coupled resonator acoustic wave filter portion and the second longitudinally coupled resonator acoustic wave filter portion includes a first IDT, a second IDT, and a third IDT arranged in sequence along a propagation direction of an acoustic wave;
• one end of the first IDT of the first longitudinally coupled resonator acoustic wave filter portion and one end of the first IDT of the second longitudinally coupled resonator acoustic wave filter portion are connected to each other, and one end of the third IDT of the first longitudinally coupled resonator acoustic wave filter portion and one end of the third IDT of the second longitudinally coupled resonator acoustic wave filter portion are connected to each other so that the first longitudinally coupled resonator acoustic wave filter portion and the second longitudinally coupled resonator acoustic wave filter portion are cascade-connected;
• one end of the second IDT of the first longitudinally coupled resonator acoustic wave filter portion is connected to the common terminal;
• one end of the second IDT of the second longitudinally coupled resonator acoustic wave filter portion is connected to a first balanced terminal and the other end of the second IDT of the second longitudinally coupled resonator acoustic wave filter portion is connected to a second balanced terminal;
• each of the first longitudinally coupled resonator acoustic wave filter portion and the second longitudinally coupled resonator acoustic wave filter portion includes a narrow pitch electrode finger portion that is provided at a gap-facing portion in a pair of the first, second, and third IDTs that are arranged adjacent to each other with a gap therebetween when viewed in the acoustic wave propagation direction;
• a pitch of electrode fingers of the narrow pitch electrode finger portion is less than a pitch of electrode fingers of an IDT portion excluding the narrow pitch electrode finger portion;
• and
• the number of the electrode fingers of the narrow pitch electrode finger portion of the second IDT of the first longitudinally coupled resonator acoustic wave filter portion is greater than the number of the electrode fingers of the narrow pitch electrode finger portion of the second IDT of the second longitudinally coupled resonator acoustic wave filter portion.