Abstract: A testing system including a swept frequency signal generator to apply a signal to a device under test, a broadband detector for receiving and detecting that signal returned from the device under test, a cathode ray tube connected to display the detector output level as a function of frequency applied to the device under test after high gain low frequency amplification of the detector output signal. The detector input is provided with an input impedance across its two conductors that is matched to the impedance of the circuit under test, with a capacitor connected subsequent to the load impedance in each of the two detector leads. The detector circuit between the capacitors and an input to the high gain amplifier is unconnected to ground except at one of the amplifier input terminals, thereby to improve the accuracy and sensitivity of measurements.