A testing device including: a first terminal configured to be connected to an equipment under test; a common-mode detector configured to detect a common-mode part of a signal emitted from the equipment under test; and a first impedance unit connected in series between the first terminal and the common-mode detector. The first impedance unit is configured to adjust a longitudinal conversion loss of the testing device to a predetermined value. A corresponding method determines a common mode signal of an electrical telecommunication.