Abstract: Test set apparatus for measuring the phasor impedance of a circuit under test, such as a telephone line. The test set apparatus also calculates for display the return loss of a circuit under test, against predetermined values of resistance or certain RC networks.To measure phasor impedance, the test set apparatus includes a signal generator providing a measurement frequency signal to a constant current source having an individual battery power source. The current source is connectable to measurement terminals, which provide for connection of a circuit under test, and produces measurement current flow through the circuit under test. During an impedance measurement, the battery-powered current source is connected to the measurement terminals and isolated from ground, thereby permitting operation of the test set apparatus to measure the impedance to a balanced circuit in the presence of externally-induced longitudinal voltages.