A Differential Low-Pass Filter With Wide Upper Stopband and Excellent Common-Mode Suppression

A compact high-performance differential low-pass filter (LPF) based on the double-sided parallel-strip line (DSPSL) structure is proposed simultaneously, featuring a wide upper stopband and excellent common-mode (CM) suppression. The intrinsic CM noise suppression can be realized with a compact circuit size due to the DSPSL phase inverter. For the differential-mode signal excitation, the low-pass response with small insertion loss (IL) and superior out-of-band rejection is exhibited. For practice, the fabricated and measured differential LPF prototype with the 1-dB cutoff frequency <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$f_{\text {c}}$ </tex-math></inline-formula> of 1.0 GHz demonstrates up to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$14f_{\text {c}}$ </tex-math></inline-formula> of a wide upper stopband rejection over 20 dB. In addition, the excellent CM suppression is better than 40 dB over the entire measured frequency range from 0 to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$14f_{\text {c}}$ </tex-math></inline-formula>.