Frequency Selective Impedance Transformer With High-Impedance Transforming Ratio and Extremely High/Low Termination Impedances

This article presents a novel frequency selective impedance transformer (ITs) with high transforming ratio and extremely low/high termination impedances ( 1 Ω or 2 Ω and several kΩ). The design equations are derived for both low and high termination impedances and effectively demonstrated. For transforming extremely low or high termination impedances, the characteristic impedances of open or short-circuited transmission line (TL) load can be very high or low, which cannot be fabricated with normal substrates because of the very narrow or broad width of TL. To solve the practical realization problem, the parallel-coupled lines can be adopted instated of TL. For the validation, two types of ITs are designed and measured at a center frequency ( f
₀
) of 3.5 GHz with impedance transforming ratios of r = 25 and r = 71.43 ( 2 Ω: 50 Ω and 35 Ω: 2500 Ω). The measurement results are consistent with simulation and theoretical predicated results. For achieving frequency selectivity, transmission zeros are generated very close to passband with stop-band suppression of higher than 40 dB.