Patent ID: 7425879

Claim:
A surface acoustic wave filter apparatus comprising: a piezoelectric substrate; N first longitudinally coupled resonator SAW filters, where N is an integer equal to or greater than 2, each including at least two IDTs disposed on the piezoelectric substrate in a direction in which a surface acoustic wave propagates and a pair of reflectors disposed on two respective sides, in the direction in which the surface acoustic wave propagates, of an area in which the at least two IDTs are disposed; and M second longitudinally coupled resonator SAW filters, where M is an integer equal to or greater than 1, each including at least two IDTs disposed on the piezoelectric substrate in the direction in which the surface acoustic wave propagates and a pair of reflectors disposed on two respective sides, in the direction in which the surface acoustic wave propagates, of an area in which the at least two IDTs are disposed; wherein the first longitudinally coupled resonator SAW filters are connected in a cascade arrangement to the second longitudinally coupled resonator SAW filters; the surface acoustic wave filter apparatus further comprises an unbalanced signal terminal and first and second balanced signal terminals; one end of each of the N first longitudinally coupled resonator SAW filters is connected to the unbalanced signal terminal; one end of each of the M second longitudinally coupled resonator SAW filters is connected to the first balanced signal terminal; the other end of each of the M second longitudinally coupled resonator SAW filters is connected to the second balanced signal terminal; and N and M are selected such that N>M.