A time-domain algorithm for computation of the noise power spectral density (PSD) is proposed in [17]. When applied to periodically varying circuits, this formulation requires efficient methods to obtain the quasi-periodic steady-state solution of a set of linear time-varying ordinary differential equations. We use both the mixed frequency-time technique(MFT) and the shooting Newton method for this computation. We show that, at each frequency for which the spectral density is sought, MFT requires only two integrations over a duration of the clock period. The results are compared with published experimental and computed data.