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..].