Elastic Wave Bandpass Filter Circuit for Steep Cutoff and Broad Stop Band

The patent combines multiple capacitor elements (first capacitor between input terminal and elastic wave resonator, second capacitor between elastic wave resonator and output terminal, third capacitor in parallel with the inductor) into a unified resonant circuit configuration. This merging of capacitor elements with the inductor and elastic wave resonator creates a coordinated system that achieves steep cutoff characteristics while maintaining reasonable complexity through functional integration rather than simple addition of components.

The patent applies different component configurations at different locations in the circuit: capacitors are strategically placed at specific nodes (input terminal side, output terminal side, and in parallel with inductor) to create localized impedance transformations. This local optimization of component placement and configuration enables steep cutoff characteristics at critical frequencies without requiring complex elements throughout the entire circuit.

The inductor in the circuit serves multiple functions simultaneously: it creates resonance with the capacitor elements to define the stop band frequency, provides impedance transformation to broaden the stop band, and works with the elastic wave resonator to establish the overall filter response. This multi-functionality of the inductor element broadens the stop band without requiring additional dedicated components for each function.

The patent achieves broad stop band by optimizing the parameter values of existing elements rather than adding more elements. The inductor value is specifically selected to create resonance at the desired stop band frequency, and the capacitor values are adjusted to control the impedance characteristics and resonance conditions. This parameter optimization allows a single inductor and a few capacitors to achieve broad stop band coverage.