Compact Elliptic-Function Coplanar Waveguide Low-Pass Filters Using Backside Metallic Patterns

In this letter, compact elliptic-function low-pass filters (LPF) implemented in coplanar waveguide technology by etching stepped impedance resonators (SIR) in the back substrate side, are presented. The required transmission zeros of the elliptic-function response are provided by the shunt connected SIRs. As an illustrative example, an order-3 prototype LPF with 1 GHz cutoff frequency is designed and fabricated. The measured stop-band rejection is better than 30 dB up to 6.4 GHz, and a narrow transition band is obtained. To demonstrate the possibilities of the approach to synthesize arbitrary elliptic-function responses and order, a fifth-order elliptic LPF with 1 GHz cut-off frequency is also designed and fabricated. Filter dimensions are as small as 6.6 mm × 9.2 mm, i.e, 0.034λ × 0.047A, for the third-order filter, and 12.1 mm × 13 mm (0.062λ × 0.067λ) for the fifth-order filter, λ being the guided wavelength at cutoff.