A neuromodulator accurately measures—in real time and over a range of frequencies—the instantaneous phase and amplitude of a natural signal. For example, the natural signal may be an electrical signal produced by neural tissue, or a motion such as a muscle tremor. The neuromodulator generates signals that are precisely timed relative to the phase of the natural signal. For example, the neuromodulator may generate an exogenous signal that is phase-locked with the natural signal. Or, for example, the neuromodulator may generate an exogenous signal that comprises short bursts which occur only during a narrow phase range of each period of an oscillating natural signal. The neuromodulator corrects distortions due to Gibbs phenomenon. In some cases, the neuromodulator does so by applying a causal filter to a discrete Fourier transform in the frequency domain, prior to taking an inverse discrete Fourier transform.