Methods, apparatus and systems for determining a power spectral density (PSD) of a signal, in which modulation of the signal is employed to facilitate determination of one or more PSD values over a wide range of frequencies. In some implementations, the signal may represent a wide-sense stationary random process. In one example, the signal is measured/sampled during a plurality of measurement windows, during which an intensity of the signal is modulated at one or more modulation frequencies. A variance of a set of quantities determined from the signal samples is calculated and used to determine respective values of the power spectral density (PSD) for the signal at the one or more modulation frequencies. The one or more modulation frequencies may be chosen in excess of the Nyquist frequency of the sampling process, such that the signal may be characterized for one or more frequencies above the Nyquist limit.