A Parallel-Strip Balun for Wideband Frequency Doubler

A parallel-strip phase inverter with a pair of simple impedance matching networks is designed. The phase inverter introduces the almost frequency independent 180° phase shift and was employed in the wideband parallel-strip balun. The balun was designed and measured with the maximum magnitude imbalance of 0.5？dB and the maximum phase imbalance of 6.0°. The proposed balun is used as input network for the wideband balanced frequency doubler. The proposed frequency doubler achieves significant conversion gain from 0.1？GHz to 1.7？GHz. The frequency doubler achieves 7.4？dB conversion gain and 23？dB fundamental signal suppression at 1？GHz. 1. Introduction Differential radio frequency (RF) circuits are commonly found in the integrated circuits for wireless communication systems. The RF ports of these chips are not standalone, but they consist of both “positive” and “negative” terminals. However, most of the off-chip RF components such as antenna and switch are single-ended [1]. Balun, that, is a device for conversion between a single-ended signal and differential signals, is essential for the entire wireless module [2]. The differential signals are composed of two separated signals equal in magnitude but 180° out-of-phase. Differential amplifiers and differential oscillators outperform single-ended circuits in terms of the even-order harmonic signal and common-mode noise suppressions [3]. A high performance balun design is critical for integrating the single-end and differential circuit in an efficient way. Besides, impedance bandwidth, phase imbalance, and magnitude imbalance are the three important design issues for the balun design. Wilkinson power divider divides signal with high port-to-port isolation, and the impedance of all ports is matched [4]. However, the divided signals are in-phase. Additional and fixed 180° phase shifter is required for the balun based on the Wilkinson power divider. The frequency responses of both insertion loss and phase flatness of the phase shifter are keys of the balun performance. Many high performance balun designs based on the Wilkinson power divider were reported in [1, 4]. In [1], the 180° phase shifter was based on the combination of open circuit and short circuit stubs and microstrip lines. The measured impedance bandwidth is about 64%. Another work based on microstrip metamaterial lines that was designed using lumped elements achieved impedance bandwidth of about 77% [4]. A parallel-strip line is a balanced transmission line, which consists of two conductors separated by a dielectric substrate. The parallel-strip line