High selectivity and wideband bandpass filtering impedance transformer

An impedance transformer (IT) with wideband and high selectivity is presented in this paper. Three transmission poles were obtained within the passband and provides a wide passband response. Moreover, there are two transmission zeros (TZs) close to the passband that were achieved by a shunt parallel coupled line and provide high selectivity. For validation of proposed circuit, the ITs were designed at a center frequency (f₀) of 3.5 GHz using an impedance-transforming ratio (r) of 5 and 10. For r = 5, |S₂₁| and |S₁₁| are 0.36 dB and 28 dB at f₀, respectively. Within the passband, |S₂₁| and |S₁₁| are better than 0.65 dB and 18 dB, respectively. The TZs at 2.65 GHz (0.76f₀) and 1.22f₀, close to the passband, were measured with attenuation better than 37 dB. The out-of-band attenuations are higher than 20 dB from DC to 2.77 GHz of the lower stopband and from 4.16 GHz to 9.73 GHz of the higher stopband. For r = 10, |S₂₁| = −1.4 dB and |S₁₁| = −23 dB were measured at f₀. An 18 dB return loss was obtained from 3.415 GHz to 3.694 GHz (FBW = 7.96%).