Resonator filter structure having equal resonance frequencies

Abstract

The invention relates to a resonator filter structure (10) for radio frequency (RF) filters, especially a bulk acoustic wave (BAW) filter structure. According to the invention, a resonator filter structure (10) is constructed with a BAW lattice filter section (20), in which all of the BAW resonator elements (20-1, 20-2, 20-3, 20-4) within the BAW lattice filter section (20) have substantially equal resonance frequencies. According to the invention, there are parallel capacitances (30-1, 30-2) connected in parallel to the BAW resonators (20-2, 20-3) of one branch type of the BAW lattice filter section (20). Thus, anti-resonance frequency of the respective BAW resonator (20-2, 20-3) is tuned. That results in a very narrow passband which corresponds approximately to the difference in anti-resonance frequencies between diagonal and horizontal branches of the lattice filter section (20). The parallel capacitances (30-1, 30-2) are used to tune the bandwidth: the smaller the capacitance, the smaller the bandwidth. Moreover, due to the lattice structure at one port of the resonator filter signal guidance will be balanced while at the other port signal guidance can be unbalanced or balanced according to the application needs.

Dependencies are inferred from the claim text.