A Low-Loss and High Phase Matching Balun Chip Based on Integrated Passive Technology for S-band RF System

With the rapid development of 5G and mobile wireless communication systems, the market demand for high-performance RF front-end devices is increasing dramatically. As a balanced-unbalanced signal converter, the balun has attracted the attention of many researchers because of its widely-used advantages in various circuit systems. In this paper, we propose a miniaturized RF balun chip based on integrated passive technology, which can transmit signals in two different transmission environments with less loss. The designed balun chip consists of a T-type lowpass filter, a $\pi-$type highpass filter, transmission line, and coplanar waveguide feedline. The cut-off frequency of the T-type lowpass filter is larger than that of the $\pi-$type highpass filter, so the input signal can pass through the T-type lowpass filter and the $\pi-$type highpass filter to complete the transmission of the balanced signal to two unbalanced signals with a phase difference of 180° in microwave frequency band. The combination of T-type and $\pi$-type filters can reduce the phase mismatch and misalignment, reduce the layout area, which are conducive to miniaturized design and improve the transmission performance. Simulation results indicate the insertion loss is low to -35.56dB in 2.4 GHz, and the die is only 940 $\mu \mathrm{m}\times 907\mu \mathrm{m}$. The RF balun chip based on the integrated passive process proposed in this paper can be applied to the S-band RF system and effectively integrated into active or passive circuit systems.