Abstract: A compact input-reflectionless balanced bandpass filter (BPF) with flexible bandwidth (BW) using a three-line coupled structure (TLCS) is presented in this paper. For the differential mode (DM), the TLCS is applied to achieve the bandpass response; meanwhile, the input coupled-feed line of the TLCS is reused in the input absorption network. This design shows a good fusion of the absorptive and BPF sections, effectively reducing the circuit size, and the BWs of the two sections that can be controlled separately result in a flexibly controllable DM response BW of the proposed input-reflectionless balanced BPF. Detailed analyses of the ratio of the two-part BWs have been given for the first time, which is vital for the passband flatness and reflectionless feature. In the codesign of this work, the input-reflectionless DM bandpass response can be optimized easily, while wideband common mode (CM) noise absorption is achieved by the input absorption network. To verify the design method, a prototype with a compact layout (0.52λ×0.36λ) is designed and measured in the 0‍‒‍7.0 GHz range. The DM center frequency (f₀) is 2.45 GHz with a measured 3 dB fractional bandwidth of 31.4%. The simulation and measurement results with good agreement are presented, showing good performance, e.g., low insertion loss (0.43 dB), wide upper stopband for the DM bandpass response (over 20 dB rejection level up to 2.72f₀), and wideband DM reflectionless and CM noise absorption (fractional absorption bandwidth of 285.7%).

Key words: Input-reflectionless filter; Balanced bandpass filter (BPF); Differential mode (DM); Common mode (CM); Three-line coupled structure (TLCS)

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