The present invention is directed to an adaptor for measuring an echo return loss in a communication system. The adaptor includes two transformers, each having a primary winding and two secondary windings. The secondary windings are cross-connected to form first and second out of phase circuits. The first circuit includes impedance elements which match an impedance of an external device. When the external device is attached to the second circuit, the first and second circuits are schematically equivalent, albeit out of phase. When an oscillation signal is input to one transformer, the matched impedance and out-of-phase connection of the secondary windings cancel the signal such that only an echo signal is output by the other transformer.