Evaluation method of common-mode choke coil used for high speed telecommunications port

A method of measuring characteristics of the common-mode choke coil, which is used for a high-speed telecommunications system using UTP cable, is needed to evaluate the performance. We present a new method of measuring the S-parameters of the coil. The two-port F-matrix parameters of the balun were obtained from three-port S-parameters, and the S-parameters of the coil were measured at the unbalance port of the baluns, which was connected to both ends of the coil. The F-matrix parameters of the coil were obtained from the F-matrix parameters of the balun and the measured S-parameters. When two baluns were connected to each other at the balance ports, the S-parameters were measured find compared with the calculated value from the F-matrix parameters of the balun. The results indicated that the difference was within 0.1 dB from 30 MHz to 100 MHz, and within 1 dB from 100 MHz to 300 MHz. The differential mode insertion losses were also measured for five types of the coils. The results indicated that the proposed method obtained a realistic insertion loss value which was not less than 0 dB although the insertion loss measured by the usual method was not a realistic value because it was less than 0 dB up to 100 MHz.