This paper describes a significantly improved method for calculating the harmonic distortion content of active-RC phase shift oscillators. The proposed method introduces the truncated harmonic transfer function (THTF) which is related to the two-sinusoidal-input describing function (TSIDF) of non-linear control theory. This allows accurate calculation of the distortion terms in oscillator systems containing higher signal levels and therefore higher levels of nonlinearity than is possible with conventional methods. The method is quite general in its application and this paper considers both cubic and quadratic nonlinearities. The method is demonstrated using examples employing the Wien RC-bandpass oscillator with a quadratic nonlinearity.
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