Considerations on accurate characterization of differential devices using baluns

Direct characterization of differential components requires multiport equipment, which is not always available. Thus, it is a common practice to attach balanced-unbalanced (balun) circuits to convert between single-ended and differential signals. However, removing impact of baluns from measured two-port S-parameters is a challenge, since baluns are three-port devices and classical de-embedding techniques are not applicable. Additionally, obtaining the S-parameters of baluns by two-port network analyzer is not trivial, since the ports at the differential side are tightly coupled. In this paper we address both challenges by proposing two techniques: how to characterize a balun using two-port measurements, and how to accurately de-embed the impact of balun characteristics from device-under-test (DUT) measurements. The applicability and accuracy of the proposed techniques is analyzed. For verification, the first technique has been applied to characterize an integrated ultrawideband (UWB) Marchand balun, and the second to de-embed the baluns impact from measured results of the balun-DUT-balun structure. The characterized DUT is a differential LNA operating in the range from 30 GHz to 110 GHz realized in 0.35 μm SiGe:C bipolar technology. The obtained de-embedded S-parameters are compared with the directly measured results.