Abstract This paper presents a novel planar balanced-to-balanced diplexer with high isolation and harmonic suppression. Both the lower and higher channels of the diplexer consist of the two-stepped impedance resonators and one uniform impedance resonator. To enhance the inter-channel isolation of the diplexer, the cross-coupling between the resonators is used to introduce extra out-of-band transmission zeros. By utilizing stepped impedance resonators with different impedance ratios, the harmonics of the three resonators, except the fundamental harmonics, are different from each other so as to suppress the odd- and even-mode harmonics effectively and ultimately to achieve the purpose of broadening the stopband and enhancing the common-mode (CM) suppression. To verify the feasibility of this design, a microstrip prototype with dual-channel center frequencies of 2.01 GHz and 3.54 GHz was manufactured. Experimental results demonstrate that the in-band differential-mode (DM) isolation is higher than 41.6 dB and 41.5 dB for the lower and higher channels, respectively. CM suppression is greater than 20 dB in the 0–13.7 GHz range for the lower channel and 20 dB in the 0–9.0 GHz range for the higher channel. The stopband suppression level achieves 20 dB up to 18.5 GHz.
Geng et al. (Wed,) studied this question.