ABSTRACT
Crystal filters have the distinction of being the oldest of the electromechanical-type filters. The resonance frequency of a quartz crystal, as with any bulk mechanical resonator, is determined primarily by its external dimensions. The newest type of crystal filter is the acoustically coupled or “monolithic” filter. A monolithic crystal filter incorporates at least two thickness-shear resonators on a single quartz wafer which are acoustically coupled to each other to provide a multipole filter response. The quartz crystal resonator consists essentially of a thin quartz wafer with coincident metal electrodes placed on opposing surfaces. The design of filters using discrete crystals is accomplished by developing a filter network that can absorb the equivalent circuit of the crystal resonators. A critical component in discrete-crystal filters is the RF transformer which is required for the hybrid lattice circuit. This “hybrid transformer” must be linear and stable and tightly balanced.
