Considerations on the measurement of active differential devices using baluns

The characterization of active differential devices requires a four-port vector network analyzer. Thus, it is a common practice to attach balanced-unbalanced (balun) circuits to convert between single-ended and differential signals and perform the measurement using a lower cost two-port vector network analyzer (VNA) or a spectrum analyzer. However, removing the impact of baluns is a challenge. This paper presents an analytical analysis of the back-to-back interconnection of the baluns and considers the common de-embedding technique insertion loss. This technique is commonly applied for two-port measurements of differential devices to remove the impact of the baluns. The applicability and accuracy of this approach is discussed. The analytical results have been verified on a differential low-noise-amplifier (LNA) at 24 GHz realized in Infineon's B7HF200 SiGe:C technology. It is measured directly using a four-port network analyzer and the results are compared with a measurement using baluns and a two-port network analyzer.