A test instrument for testing of a line under test having a first conductor and a second conductor, having: a coupling transformer having a line under test side and a test instrument side; a positive side transmitter electrically connected to said test instrument side of said coupling transformer and including a positive side driver, a negative side transmitter electrically connected to said test instrument side of said coupling transformer and including a negative side driver, a positive side receiver electrically connected to said test instrument side of said coupling transformer for receiving a signal from said first conductor of the line under test, and a negative side receiver electrically connected to said test instrument side of said coupling transformer for receiving a signal from said second conductor of the line under test, characterized in that it comprises: a positive side DC isolation circuit providing an electrical connection between said line under test side of said coupling transformer and the first conductor of the line under test, said positive side DC isolation circuit being responsive to signals from the first conductor of the line under test and configured to pass the received signals from the first conductor of the line under test to the positive side transmitter, wherein the received signals from the first conductor of the line under test are broad band signals; a negative side DC isolation circuit providing an electrical connection between the line under test side of said coupling transformer and the second conductor of the line under test, said negative side DC isolation circuit being longitudinal balanced with said positive side DC isolation circuit and being responsive to signals from the second conductor of the line under test and configured to pass the received signals from the second conductor of the line under test to the negative side transmitter, wherein the received signals from the second conductor of the line under test are broad band signals. a coupling transformer having a line under test side and a test instrument side; a positive side transmitter electrically connected to said test instrument side of said coupling transformer and including a positive side driver, a negative side transmitter electrically connected to said test instrument side of said coupling transformer and including a negative side driver, a positive side receiver electrically connected to said test instrument side of said coupling transformer for receiving a signal from said first conductor of the line under test, and a negative side receiver electrically connected to said test instrument side of said coupling transformer for receiving a signal from said second conductor of the line under test, a positive side DC isolation circuit providing an electrical connection between said line under test side of said coupling transformer and the first conductor of the line under test, said positive side DC isolation circuit being responsive to signals from the first conductor of the line under test and configured to pass the received signals from the first conductor of the line under test to the positive side transmitter, wherein the received signals from the first conductor of the line under test are broad band signals; a negative side DC isolation circuit providing an electrical connection between the line under test side of said coupling transformer and the second conductor of the line under test, said negative side DC isolation circuit being longitudinal balanced with said positive side DC isolation circuit and being responsive to signals from the second conductor of the line under test and configured to pass the received signals from the second conductor of the line under test to the negative side transmitter, wherein the received signals from the second conductor of the line under test are broad band signals.