High frequency-selectivity impedance transformer

This paper presents the design of an impedance transformer (IT) with high selectivity and wide out-of-band suppression characteristics. The proposed IT provides two transmission poles in the passband and a sharp frequency selective characteristic. For validation, a 50-to-20 Ω IT has been implemented at a center frequency (f₀) of 2.6 GHz. The measurement results show a return loss higher than 20 dB over the passband of 2.2-3 GHz and the insertion loss less than 0.4 dB over the same passband. The out-of-band suppression higher than 17 dB from DC to 1.85 GHz and higher that 11 dB from 3.5 GHz to 7.2 GHz are obtained.