Abstract
Errors in test results are minimized in a test system (FIG. 1) employing digital data acquisition units (121) when measuring intermodulation distortion of a voice frequency communication facility (105) by employing a unique 3-tone test signal. To this end, the three tones have predetermined amplitude, frequency and phase relationships to obtain a test signal having a desired optimum probability density function which is substantially Gaussian. Additionally, circuit noise components are eliminated from the test results by obtaining a measure of noise with a single tone test signal and subtracting it out from the three tone test results. In a specific embodiment, a measure of the power spectrum resulting from the three tone signal and the power spectrum of the single tone signal are obtained, combined and utilized to obtain measurements of second and third order intermodulation distortion products compensated for noise.
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