Tunable First-Order Balanced Filter with Reflectionless Characteristics for Both Common- and Differential- Mode Signals

This work presents a novel tunable first-order absorptive balanced filter. It consists of a coupled-line bandpass filter with absorption networks connected at its input and output ports. The absorption network is designed using semi-lumped elements. With the proposed architecture, reflectionless performance can be achieved for both the common-mode (CM) noise and out-of-band differential-mode (DM) signal. Besides, the circuit is compact by nature due to the use of semi-lumped elements. To demonstrate the proposed idea, a microstrip prototype is fabricated and measured. The measured results show excellent reflectionless performance for both CM noise and DM signal. The measured DM transmission loss varies from 3.7dB to 2.2dB within a frequency tuning range of 1.6GHz-1.9GHz. Meanwhile, the DM reflection coefficient is lower than -9.5dB from 0.1GHz-5.6GHz. The CM reflection coefficient is lower than -10dB while the circuit still exhibits suppression higher than 22dB for the transmission of CM noise from 0.1GHz-2.3GHz.