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.