A Ring Type Bandpass Filtering Impedance Transformer With Complex Load

In this letter, a ring type bandpass filtering impedance transformer (FIT) is proposed which consists of three cascaded transmission lines, a coupled transmission line, a real source and a frequency-independent complex load. In order to achieve ripple level of S
11
under -15 dB, the relationship between the real part and the imaginary part of complex load is determined. When the real part of the complex load is fixed at 62 Ω, the realizable range of the imaginary part can reach -j20 Ω to j20 Ω. For demonstration, a prototype of the proposed FIT with Z
S
= 50 Ω, Z
L
= 62 - j15 Ω, 2 GHz center frequency, 80% fractional bandwidth (FBW) is designed, manufactured and measured. The measurement results and the simulation results match very well.