Technique for accurate characterization of baluns using one-port S-parameters measurements

Accurate characterization of a three-port balanced-unbalanced (balun) device requires a four-port vector network analyzer (VNA), which might be not easily available. Thus, it is a common practice to terminate one of the three ports and to use a standard two-port VNA. However, applicability of the above approach is limited since the ports at the differential side of the balun are tightly coupled. This paper presents a technique that enables very accurate balun characterization by means of one-port measurements. The measurements are performed at the single-ended port of the balun, whilst the coupled ports are terminated using several loads. This enables very easy and highly accurate measurement and subsequent determination of all nine S-parameters of a balun. The applicability and accuracy of this approach is discussed. For verification, the derived analytical equations have been applied to test structures very accurately simulated in HFSS to characterize an on-board 24 GHz balun, based on rat-race hybrid coupler. Then, the de-embedded balun S-parameters are compared with directly simulated results.