Balanced electrical circuits are designed with at least three ports, or more commonly four ports, as is standard for a simple differential transmission line. Many engineers prefer to perform differential measurements with 4-port instruments when characterizing electrical properties such as insertion loss and return loss, and S-parameters such as transmission and the reflection of single-ended, common-mode, and differential-mode signals. This application note gives a brief introduction on balanced and differential circuits and provides a demonstration of such measurements on a common-mode filter through the use of split-ring resonators.