A Broadband 90° Balun With Low-Phase-Imbalance Performance Based on Periodic Slow Wave Structure

We propose a method to construct the broadband 90° balun with low-phase-imbalance performance based on a U-shaped periodic slow wave structure (PSWS) transmission line (TL), whose dispersion curve has a linear region in a wide frequency band. By introducing meander lines and U-shaped TLs, the linearity, cutoff frequencies and slope of dispersion curves of PSWS can be controlled flexibly. Simulated and measured results show good agreements. Both reflection coefficient and isolation of the proposed balun are below -15 dB from 1.5 to 2.0 GHz. The phase deviation is within ±3°, covering a fractional bandwidth of 28.6%. Compared with that based on the traditional corrugated periodic slow wave TL unit, the operating bandwidth of the proposed balun is increased by more than six times. Moreover, the proposed PSWS keeps excellent ability of field confinement and therefore improves the isolation between the output ports. In a word, based on the proposed method, the microwave devices and circuits which realize the function of phase shifting can be designed.