Longitudinally coupled double mode resonator filters using shallow bulk acoustic waves

A filter comprising a shallow bulk acoustic wave (SBAW) longitudinally coupled double mode resonator (LCR) and a reactance coupled between an input and an output of the resonator, the reactance and the resonator being arranged to couple a signal from the input to the output with generally opposite phases at frequencies in a shoulder region in an upper stop band on a high frequency side of a pass band of the resonator where the resonator has a relatively poor stop band attenuation, the reactance having a magnitude such that in said region attenuation of the filter comprising the resonator and the reactance is greater than attenuation of the resonator.

A method of improving a relatively poor stop band attenuation in a shoulder region in an upper stop band on a high frequency side of a pass band of a longitudinally coupled double mode surface wave resonator comprising at least two interdigital transducers on a surface of a piezoelectric substrate with an orientation for transducing and propagating shallow bulk acoustic waves, comprising providing a reactance between an input and an output of the resonator, the reactance coupling a signal from the input to the output with a phase change that is generally opposite to a phase change of a signal at frequencies in said shoulder region coupled from the input to the output via the resonator, and with an attenuation such that in said shoulder region attenuation of the combination of the resonator and the reactance is greater than attenuation of the resonator.