Bandpass Filter for 5G Sub-6 GHz Bands

Abstract

The performance requirements for filters in the microwave frequency band are particularly stringent, particularly in terms of high bandwidth and out-of-band rejection. However, meeting these requirements within the constraints of a compact size presents a significant challenge. A coupled step-impedance resonator bandpass filter is proposed. The filter combines U-shaped branches and Lshaped branches to create multiple resonance points while expanding the bandwidth, and the in-band ripple is also improved by this folded structure that greatly reduces the filter size. The microstrip filter measures only 9.6 mm × 8.8 mm × 1.1 mm, has a center frequency of 4.65 GHz, and achieves a relative bandwidth of 60.2%. The filter can be used in 5G n77 (3300 ∼ 4200 MHz), n78 (3300 ∼ 3800 MHz), n79 (4800 ∼ 4960 MHz), and WLAN (5150 ∼ 5850 MHz) bands. In addition, the filter has a left-side rectangular coefficient of 1.12, insertion loss < 0.4 dB, and return loss better than 17 dB.