Miniaturization of Multi-Functional Filters Using Spiral Resonators Coupled to Their Scaled Negative Images

A novel technique for miniaturizing planar signal-interference coupled-resonator-based filters is proposed in this paper. The presented technique employs circular and logarithmic spiral resonators coupled with their scaled negative images. We demonstrate significant size reduction of approximately 2x compared to the state-of-the-art, with minimal penalty on loss and notch depth. Besides these benefits, the proposed technique also enables additional design degrees of freedom for the filter size, attenuation level and sharpness of its in-band notches. In addition, we demonstrate that by scaling the resonators’ coupled negative images, we can further reduce the filter size. This paper explores these basic design principles and demonstrates proof-of-concept UHF bandpass filter prototypes with tunable in-band notches.