This paper presents a new time domain technique for computing the noise spectral density, The power spectral density is interpreted as the asymptotic value of the expected energy spectral density per unit time. We show that the methodology of stochastic differential equations can be used to derive a set of ordinary differential equations for the expected energy spectral density. This set of equations can then be integrated in time until the steady state value of the power spectral density is obtained. The method is quite general and can be used to find the noise spectrum in any circuit in which noise can be treated as a perturbation. In this paper, the algorithm has been used to simulate the spectrum of the noise voltage in switched capacitor circuits.