Abstract: A device for testing phase inversion of a circuit includes a signal generator having an assymetrically shaped positive phased test signal output electrically connected through a buffer circuit to the input of the circuit under test. A differentiator circuit electrically connected to the output of the test circuit, produces in response to the test circuit output, rectangular pulses whose amplitudes are proportional to the slopes of the test waveform. The differentiated output signals are isolated into positive and negative signals by a plurality of peak detectors whose outputs are fed to a summing amplifier. The summing amplifier amplifies the difference between the positive and negative isolated peak voltages and in turn drives a polarity indicating device which was previously calibrated to indicate a positive or negative polarity as either a phase or non-phase inversion. An alternate mode of measuring pulses directly and bypassing the differentiator is also provided.