Differential-/Common-Mode Conversion Loss and LCL/TCL Measurement Methods

In wired signal transmission, conversion between the differential and common modes deteriorates the quality of the transmitted signal and also causes electromagnetic compatibility problems in the environment. Therefore, the transmission-line components are requested to be tested on their conversion loss characteristics, such as the longitudinal conversion loss (LCL), transverse conversion loss (TCL), and mixed-mode S-parameters. Theoretical investigations are carried out to analyze the mode conversion of a differential one-port device in order to express the conversion loss parameters in terms of the modal impedances of the device. Consequently, the mutual relationships among the parameters are derived using the modal impedances. In addition, a new LCL measurement method is proposed that can accurately determine LCL values of a device under test by using a vector network analyzer instead of a traditional balun-transformer measuring instrument. Furthermore, it is found that the ratio of the TCL to LCL is not constant but varies with device characteristics. Therefore, a new definition of the TCL is proposed in place of the conventional one. The validity of these theoretical results is confirmed through SPICE simulation and measurements.