A Tunable Planar Balun with an Isolation Circuit Using Varactors Loaded Coupled Lines

Shaojun Fang,Hongmei Liu,Xiaojian Guo,Zhongbao Wang

doi:10.3390/electronics9030401

Abstract

In this paper, a frequency tunable planar balun composed of varactors loaded coupled lines (VL-CLs) is presented. By tuning the capacitances of the varactors, a wide frequency tuning range is obtained. Moreover, good impedance match, balanced output ports amplitude, and consistent output ports phase difference (PD) are maintained during the tuning. A detailed theoretical analysis using the signal flow diagram and the even-odd mode method is presented to clarify the characteristics of the proposed balun. To achieve ideal output matching and isolation for the proposed balun, a novel frequency tuned isolation circuit (IsC) is designed and connected to the balun. In theory, a frequency tuning range of 200% can be realized. In practice, due to the limited capacitances of the varactors, a prototype with a tunable frequency of 1.0 GHz ~ 2.0 GHz (66.7%) is designed, fabricated, and measured. The measured results show that more than21 dB of return loss (RL), 180° ± 1.8° of phase difference, 0.43 dB of amplitude imbalance (AP), and 22 dB of isolation are obtained at all tuning center frequencies, agreeing well with the simulated results.

Highlights

Balun [1,2], which transforms unbalanced signals into balanced signals, is an important component widely used in RF circuits, such as frequency doubler [3], mixers [4], power amplifiers [5] and antenna feed networks [6]
It is is composed composed of of two two varactors loaded coupled lines (VL-CLs) with one port of the first connected with the adjacent port of the second identical VL-CLs with one port of the first VL-CL connected with the adjacent port of the second
The frequency tuning range is chosen from 1.0 GHz to 2.0 GHz

Summary

Introduction

Balun [1,2], which transforms unbalanced signals into balanced signals, is an important component widely used in RF (radio frequency) circuits, such as frequency doubler [3], mixers [4], power amplifiers [5] and antenna feed networks [6]. The planar baluns [7,8,9] have received extensive attention and research due to their compact structures and good performance. Two ports of the Marchand balun should be terminated to ground by using via-holes. This complicates the fabrication process, and causes additional parasitic effects. A novel structure of planar balun [16] was proposed. Since it is based on trans-directional coupled lines, no via-holes are required and the inductive effect can be avoided

Methods

Results

Discussion

Conclusion