A new design of a total difference-frequency distortion (TDFD) meter having the highest sensitivity to date is described. It requires no critical filter stages and produces its own test signals. Quadrature synchronous detectors, simple passive and active filters, and phase-locked-loop synthesizers are used to achieve a measurement dynamic range of 135 dB or 0.15 ppm. The microprocessor displays TDFD and shows odd- and even-order distortion fractions. The possibility exists of locking to an external reference, allowing the measurement of CD players or a change of test signal frequency by shifting the master reference. Block diagrams and example measurements are presented.
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