Abstract: A system for determining a composite signal level at which a signal path begins to generate non-linear distortion. A uses a reference test signal, which is preferably a short-duration burst of repeatable broadband energy, that is passed through the signal path and received on a digital signal acquisition unit. An impaired received reference test signal is comprised of is formed from the transmitted reference test signal, linear distortion components, and non-linear distortion components. The impaired received reference test signal is digitally processed to reveal the non-linear distortion components. The impaired received reference test signal may be processed with a stored reference test signal to find a time-domain impulse response from which the uncorrelated distortion energy can be measured.