Abstract: A test system having a bifilar wound isolation network disposed in between a device under test (DUT) and a device under test support system for providing a high isolation impedance to common mode signals present at the input terminals of the isolation network, while still providing a relatively lossless path for differential signals present across the input terminals of the isolation network. The isolation network comprises a plurality of four resonators each having a precise, predetermined number of turns and serially interconnected by a bifilar winding. The bifilar winding allows coupling to the inputs of a balanced DUT and enables a worst case operating environment to be provided for the balanced DUT to assist in testing and/or design of the DUT so as to minimize the effects on the DUT of electromagnetic interference and to assist in modifying the balanced DUT so as to immunize it from susceptible EMI receptors therewithin.