1.34‐to‐16.37 GHz on‐chip balun based on asymmetric stacked spiral transformers

Abstract

A novel broadband balun of owning bandwidth of more than a decade octaves is proposed, which is comprised of two parallel stacked spiral transformers with slightly different dimensions. The equivalent coupled line model reveals that the amplitude imbalance of the balun with symmetric structure equals to the coupling coefficient of the coupled line in decibel. Therefore, with higher coupling degree comes smaller amplitude imbalance. Furthermore, combined with the high coupling degree of stacked spiral transformers, the balun of owning optimal amplitude balance can be realised by slightly adjusting the size of one side transformer. Finally, the balun is designed and fabricated in Sanan's integrated passive device (IPD) process and its core size occupies an area of 690 μ m × 240 μ m . The simulation and measurement agree well in general, exhibiting a bandwidth covering from 1.34 up to 16.37 GHz with good return loss, flat insertion loss and high stability of 180° phase difference.