Design of a Compact DC-Block and Bias-Tee for 28 GHz mm-Wave Frequency Applications

This paper introduces an innovative microstrip bias-tee (BT) circuit incorporating a compact U-shaped DC-block interdigital capacitor (IDC) combined with a newly developed sawtooth-shaped radial stub. The IDC design consists of two opposing U-shaped structures, with two adjacent $^{A}8$ parallel open stubs placed transversely within the U-shaped layout. This design results in a 50% reduction in size, achieving λ/8 in length and width at a 28 GHz operating frequency, occupying a compact area of $0.8 \times 0.8\text{mm}^{2}$. Additionally, the circuit offers a broad bandwidth of 10 GHz, ranging from 22 GHz to 32 GHz, corresponding to a percentage bandwidth of 37%. The proposed structure is modelled and optimized on a 0.406 mm RO4003C Rogers substrate using CST. Simulation results demonstrate a return loss (RL) greater than 20 dB, an insertion loss (IL) of less than 0.5 dB, and an isolation loss (IS) exceeding 25 dB over almost the entire operational frequency range.