Impedance Transforming Inline Bandpass Filter with High Selectivity using Internal and External Frequency-Variant Couplings

In this paper, a technique is introduced to design impedance transforming inline microstrip bandpass filters with high selectivity. The proposed technique is based on employing frequency variance in both internal and external couplings. The frequency-variant couplings allow creation of transmission zeros close to the passband increasing the filter’s selectivity. The external frequency-variant coupling topology allows the core filter structure to be matched to real or complex termination impedance values. Therefore, the proposed circuit can be used as an impedance transformer with highly selective bandpass filtering response. To validate the proposed technique, a third order filter with 50 Ohms input termination and 100-j100 Ohms output termination is designed and fabricated for $2.4-2.5 \mathrm{GHz}$ band. The measured return loss is better than 11 dB over 100 MHz of bandwidth with insertion loss better than 2.9 dB. The achieved selectivity factor, defined as the ratio of the 3-dBc to $20-\mathrm{dBc}$ bandwidths, is $\mathbf{0. 6 7 9}$ providing $20-\mathrm{dB}$ roll-off rates higher than $360 \mathrm{~dB} / \mathrm{GHz}$ at the band edges. The size of the filter prototype is 0.36 by 0.25 times the wavelength at the center frequency.

I. Introduction

High-selectivity bandpass filters with superior stopband characteristics are needed in the RF front-ends of wireless communication/radar systems and test instrumentation. In RF systems, filters are often cascaded with other components such as power amplifiers, mixers, antennas, etc., which may present complex or real termination impedances other than the standard at the filter ports. This creates a need for co-design of two or more functionally different circuits, such as filters with impedance transforming capabilities. Moreover, inline filter structures with resonators arranged in a single row and feed ports on the opposite sides of the structure are desired for ease of integration in practical systems.