[Objective] In high-speed data acquisition terminals, differential lines are commonly employed to carry high-speed digital signals, enhancing anti-interference performance. However, mutual interference frequently occurs when high-speed acquisition terminals and wireless devices coexist. For instance, 5G base stations and Gbps high-speed signal acquisition terminals in current 5G high-speed acquisition systems interfere with each other. This issue is addressed by installing electromagnetic interference (EMI) filters on a printed circuit board (PCB); however, EMI filters have disadvantages, including fixed frequency points and high cost, which hinder their practicability. In this work, a design method for a low-cost, easy-to-implement distributed printed differential low-pass filter is proposed based on a step impedance filter that can be designed using simple synthesis tools and applied to multilayer PCBs of various radiofrequency systems. [Methods] Based on a typical LC low-pass filter, step impedance resonators were introduced as a solution to high-frequency parasitic parameters and harmonic suppression. The high- and low-impedance transmission lines serve as an inductor and capacitor, respectively, realizing the transformation from a single-end LC filter to a single-end microstrip filter. Due to the common-mode suppression requirements of differential circuits, symmetrical design and equivalent processing were performed for conversion into a microstrip differential low-pass filter, where the high-impedance line is realized by loosely coupled differential lines and the low-impedance line is realized by tightly coupled differential lines. The larger the impedance difference between high and low impedance in the design, the more it contributes to reducing the filter size, and the device size can be further reduced by adhering to this principle. In addition, considering the influence of the transmission line on the ground impedance, an independent non-grounded pair of differential lines was selected for the design. The PCB board used FR4 with a dielectric constant of 4.2 and a multilayer structure with thicknesses of 0.127 mm and 0.508 mm for the two dielectric layers. The final dimensions were 22 mm × 16 mm. [Results] Simulation results showed that the distributed differential filter is a 0—3.5 GHz low-pass filter under ideal conditions. The passband and return loss values were less than 0.5 dB and greater than 20 dB, respectively. Two baluns were added to the circuit design used for differential circuit measurement. Test results revealed that the balun possesses considerable interference of approximately 600 MHz, but the curve without the balun’s influence is highly consistent with the simulated curve. The passband and return loss values were <5 dB and <17 dB, respectively. [Conclusions] The findings of this work indicate that the method of using differential step impedance to realize a distributed printed filter based on a multilayer PCB board is feasible and can be effectively applied in high-speed data acquisition terminals as a solution to the mutual interference between high-speed acquisition and wireless interconnection. © 2024 Press of Tsinghua University. All rights reserved.
