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The common mode current generated by an AFE (analog-front-end) of a modem is a main cause of emission from the external wires connected with the AFE. In this paper, the measurement and the calculation are carried out about the common mode current and the magnetic field generated from the AFE by the equivalent circuit that consists of a signal generator and an output transformer. As a result, it is confirmed by the calculation and the measurement that the common mode current increases according to the coupling capacitances between the primary and the secondary windings of the output transformer and the parasitic capacitances caused by traces, and that the magnetic field strength emitted from the signal wires is proportional to the common mode current. And then, an output transformer with a balun is proposed. It is clarified that the transformer with a balun is effective to reduce the common mode current and the magnetic field.