Abstract: A fixed amplitude test waveform is concurrently applied to a reference circuit and a circuit under test that is supposed to be substantially identical to the reference circuit. The waveform comprises a composite signal whose frequency varies through a range of values during each period and is produced by a generator whose effective impedance also varies in accordance with the variation in frequency. The voltages developed at the test points of the reference and test circuits are compared and the difference between them, which is a measure of an undesired condition of the test circuit, is displayed.