Design, Analysis, and Fabrication of a Sharp Band-Pass Filter with Improved Pass-Band and Stop-Band Characteristics for 5G Mid-Band Applications

In this paper, a sharp band-pass filter (BPF) with enhanced pass-band and stop-band specifications is presented. The filter can be utilized for 5G mid-band applications along with LTE42 (3.4–3.6 GHz) and LTE43 (3.6–3.8 GHz) to comply with the standards of different countries. The initial design of the filter consists of a high impedance transmission line connecting the filter’s two ports, which are terminated by 50 Ω resistors. Although this design results in reasonable pass-band characteristics, its stop-band and transition-band characteristics are relatively weak. Due to this shortcoming, the stop-band and transition-band characteristics of the filter are improved by locating sharp transmission zeros within the stop-band to determine the filter bandwidth and to significantly increase the stop-band attenuation. These modifications are achieved by inserting a pair of shunt short- and open-circuit stubs, a pair of short-circuited U-shaped parasitic elements, and a central short-circuited U-shaped element. The filter is mathematically analyzed and modeled, and the design steps are also demonstrated. The simulation and measured results are compatible, and the measurements show an insertion loss of 0.35 dB, stop-band attenuation larger than 20 dB, sharp transmission zeros, and a transition-band value within 10% of the center frequency.