A system for determining a composite signal level at which a signal path begins to generate non-linear distortion.[.. A.]. .Iadd.uses a .Iaddend.reference test signal.[., which is preferably a short-duration burst of repeatable broadband energy,.]. .Iadd.that .Iaddend.is passed through the signal path and received on a digital signal acquisition unit. An impaired received reference test signal .[.is comprised of.]. .Iadd.is formed from .Iaddend.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. Alternately, a reference test signal, such as an orthogonal frequency division multiplex (OFDM) reference signal with spectral holes, can be processed in the frequency domain to find the non-linear distortion energy that enters the spectral holes. Alternately, a transfer function of a signal path, showing an output voltage as a function of an input voltage, can be generated from a two-burst waveform comprised of a clipping high-level sinewave and non-clipping low-level sinewave. As the reference test signals are elevated in level, the magnitude of the non-linear distortion products can typically be observed to increase..].