Each of a plurality of circuit node voltages is sampled at high speed with quadrature mode samples being stored at every quarter wavelength of a reference waveform and spurt mode samples stored over a quarter wave of a reference waveform. The stored signals are processed to determine whether the sampled waveform is DC, AC, AC on DC, digital or noise and whether it is within a tolerance value. If the sampled signal is outside of tolerance the circuit node is slow sampled and compared to a reference tolerance. If an out-of-tolerance condition still persists, the testing apparatus makes an internal check of its circuitry. A failure at the node is indicated if the internal check determines that the testing apparatus is operating properly.