Balanced bandpass filter with common-mode reflectionless and linear-phase characteristics based on negative group delay circuit

Abstract

In this paper, a balanced bandpass filter (BPF) with common-mode reflectionless and linear-phase characteristics is proposed, which consists of a balanced 3-order bandpass filtering network and two common-mode absorptive networks to obtain the common-mode reflectionless and suppression performance. The negative group delay circuits are loaded on the proposed balanced BPF to realize the full-passband linear-phase filtering performance. The flatter the group delay of the BPF (the fluctuation of the group delay within the passband tends to 0), the better the phase-frequency linearity, which will realize the undistortion transmission of the communication systems. For demonstration, a balanced linear-phase BPF prototype operating at 1.5 GHz is designed, simulated, and manufactured. The results demonstrate that the group delay fluctuation is reduced from 1.20 ns to 0.11 ns by 90.8 %, the differential-mode insertion loss is 1.04 dB, the minimum common-mode suppression is 59.3 dB within the passband and the common-mode reflectionless band is 1.22–1.91 GHz. The developed topology requires high precision in the processing, and the proposed balanced BPF can be applied to digital microwave communication systems to improve the transmission quality and reduce the bit error rate.