Composite crystal filter circuit

Claim 1

A crystal filter circuit comprising: a first transformer having a primary winding and a secondary winding, said primary winding being coupled between first and second terminals; a first crystal resonator coupled between said first and second terminals; a first capacitor coupled in parallel with said secondary winding; a second crystal resonator and a frequency sensitive impedance element coupled in series with one another and in parallel with first capacitor; a second transformer having a primary winding and a secondary winding, said primary winding of said second transformer being coupled between said second terminal and the junction between said second crystal resonator and said frequency sensitive impedance element; a second capacitor coupled in parallel with said primary winding of said second transformer; a third capacitor and a third crystal resonator coupled in parallel with said secondary winding of said second transformer; a first inductor and a fourth capacitor coupled in parallel between an electrode of said third capacitor and a third terminal; a second inductor, a fifth capacitor and a fourth crystal resonator coupled in parallel between said second and third terminals; said circuit providing minimum attenuation for signals within a preselected frequency passband; and each of said first, third and fourth crystal resonators having a series resonant frequency essentially equal to a frequency near an extremity of said passband.