Design Considerations of Ultra-Broadband Finite-Ground CPW Filters Using Multipole Generation Technique

An ultra-broadband finite-ground coplanar waveguide (FGCPW) filter is designed and demonstrated by applying a multipole generation technique. An extremely tight coupling in this case is realized by an FGCPW with defected ground structure (DGS). Its equivalent circuit model, which is composed of a /-inverter and uniform transmission line sections, is extracted by a commercial Method-of-Moments (MoM) simulator based on the short-open calibration (SOC) technique. The transmission poles of the filter are located at frequencies where the normalized coupling coefficient equals to one and also the electrical length of resonator equals to half a wavelength and one wavelength. Measured results agree well with simulations that are obtained by using both the circuit model and commercial electromagnetic software. Low return loss is achieved for a fractional bandwidth over 85%. In addition, the selectivity of this type of filters is increased by cascading two identical FGCPW filters interconnected with a transmission line section. As a result, the measured out-of-band attenuation is increased from 13.5 dB/GHz to 32.9 dB/GHz within a fractional bandwidth of over 72.2%.