Band-pass crystal filters

Claim 1

A PIEZOELECTRIC CRYSTAL FILTER ARRANGEMENT FOR PASSING A NARROW FREQUENCY BAND OF ELECTRICAL SIGNALS FROM A SOURCE HAVING A RELATIVELY WIDE FREQUENCY BAND OF ELECTRICAL SIGNALS COMPRISING: AN INPUT CIRCUIT FOR RECEIVING SAID WIDE BAND OF SIGNALS; AN OUTPUT CIRCUIT FOR RECEIVING SAID NARROW BAND OF SIGNALS; AT LEAST ONE UNSYMMETRICAL HALF SECTION INTERPOSED BETWEEN SAID INPUT AND OUTPUT CIRCUIT TO PASS SAID NARROW FREQUENCY BAND AND TO SUBSTANTIALLY ATTENUATE THE REMAINDER OF SAID WIDE BAND, SAID HALF SECTION COMPRISING A FIRST LEG COUPLED TO A SECOND LEG; A FIRST PIEZOELECTRIC CRYSTAL ELEMENT INCLUDED IN SAID FIRST LEG, SAID FIRST CRYSTAL ELEMENT RESONATING AT A HIGH FREQUENCY NEAR THE HIGH FREQUENCY SIDE OF SAID NARROW FREQUENCY BAND; A SECOND PIEZOELECTRIC CRYSTAL ELEMENT INCLUDED IN SAID SECOND LEG, SAID SECOND CRYSTAL ELEMENT RESONATING AT A LOW FREQUENCY NEAR THE LOW FREQUENCY SIDE OF SAID NARROW FREQUENCY BAND, THE COACTING OF SAID FIRST AND SECOND CRYSTAL ELEMENTS ESSENTIALLY FORMING THE FREQUENCY BAND-PASS OF SAID HALF SECTION; A THIRD PIEZOELECTRIC CRYSTAL ELEMENT COUPLED TO SAID FIRST LEG; AND A FOURTH PIEZOELECTRIC CRYSTAL ELEMENT COUPLED TO SAID SECOND LEG, SAID THIRD AND FOURTH CRYSTAL ELEMENT RESONATING AT FREQUENCIES NEAR ONE SIDE OF SAID NARROW BAND TO PROVIDE SUBSTANTIALLY GREATER SELECTIVELY ON SAID ONE SIDE THAN ON THE OTHER SIDE OF SAID HALF SECTION BAND-PASS.