A novel design of reflection-type phase shifter for microwave life-detection system

Abstract

Summary form only given. In this work, a low-loss 360° reflection-type analog phase shifter is proposed for use in the microwave life-detection system. A photo of the fabricated prototype of our proposed phase shifter is shown in Fig. 1. A 90° branch-line coupler is adopted here as in (T. W. Yoo, et. al., Electron. Lett., vol.33, no.3, pp. 224-226, Jan. 1997). The input and isolation ports of the coupler are used as the input and output ports for the phase shifter, respectively. Meanwhile, the through and coupled ports are terminated by our proposed reflection loads. Each reflection load has two branches in shunt: one is formed by a section of transmission line (TL) of the electrical length θ and characteristic impedance Z <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> in series with a varactor shorting to the ground and the other is merely a varactor to ground. By sweeping the reverse bias voltage of the varactor, namely sweeping its capacitance, the associated impedance loci on the Smith chart could be obtained and are plotted in Fig. 2. As shown in Fig. 2(a), the two branches can provide impedance loci along different portions of the periphery of the Smith chart. Note that the electrical length of the TL section θ was chosen such that the associated impedance of the branch is equal to zero (short circuit) as the varactor is operated at its min. capacitance, that is, θ = tan <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> (ωZ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> i <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</sub> ) <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , where θ is the electrical length of the transmission line, ω is the angular frequency, and C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">min</sub> is the min. achievable capacitance of the varactor. As shown in Fig. 2(b), the proposed reflection load, namely the shunt circuit of the two branches, can provide a full 360° tuning phase range. The reasons are as follows. When the varactor is operated at its min. capacitance, the branch with the TL section functions as a short circuit, and thus its reflection phase is 180°. Meanwhile, when the varactor is operated at its max. capacitance, the branch without the TL section would also function as a short circuit with its reflection phase being 180°. For both branches, the impedance point on the Smith chart moves clockwise as the capacitance of the varactor is increased, and vice versa. The tunable phase range of the proposed reflection load, and hence the phase shifter, is thus 360°. The design concept has been verified via a 1150-MHz test piece fabricated on a lossy FR4 slab. Currently, we are trying to replace the branch-line coupler by Lange coupler and use low-loss substrate to further enhance its bandwidth and insertion loss.