A New Synthesis Method to Design Broadband Continuously Tunable Phase Shifters Based on Coupled Line With Low Phase Error

This article introduces a new synthesis method for broadband continuously tunable phase shifters (PSs). In contrast to traditional reflective-type PSs (RTPSs) based on a 3-dB quadrature coupler, whose phase response is determined solely by the reflection load, we find that the even- and odd-mode impedances <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Z_{0e}$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Z_{0o}$ </tex-math></inline-formula> and the electrical length <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Theta $ </tex-math></inline-formula> of the coupled line can adjust the phase properties of the tunable PS, including the phase shift and phase slope. Taking both the coupled line and reflection load into account, the proposed method can reduce the phase slope variation with respect to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$C_{v}$ </tex-math></inline-formula> by more than 1.6~4.6 times compared to conventional RTPSs. Without introducing extra circuits or designing complex reflection loads, the bandwidth and phase error performance of tunable PSs can be significantly improved. The transmission-line (TL) model of the coupled line with the prescribed fractional bandwidth (FBW) and return loss (RL) is developed to synthesize the proposed broadband continuously tunable PS with all circuit parameters, thus allowing for a quick design process. Finally, two PSs are designed, fabricated, and tested to verify the proposed design method and predicted frequency responses.