Pseudofrequency

Pseudofrequency is a term used in signal analysis to describe an apparent frequency component that does not correspond to a genuine, steady-state oscillation of the system under study. It typically arises from sampling, data processing, or modeling choices that transform or distort the spectrum.

Common causes include aliasing from undersampling, where a frequency f above half the sampling rate maps to an apparent lower frequency, f_alias = |f − k fs| for some integer k; modulation or nonstationarity that yields a time-varying instantaneous frequency; spectral leakage from windowing in a Fourier transform; and nonlinear mixing that generates combination tones. These effects can produce spectral features that look like distinct frequencies but do not represent true, independent oscillations of the underlying process.

Interpretation and handling: to minimize pseudofrequencies, use appropriate sampling rates and anti-alias filters, and apply time-frequency analysis methods to distinguish true components from artifacts. It can also be informative to interpret a pseudofrequency as a rate of modulation or a result of the measurement process, rather than as a separate intrinsic mode of the system.

In practice, pseudofrequency is not a fixed technical term with universal definition; its meaning varies across disciplines. It is often described as an apparent or artifact-induced frequency rather than a true spectral line, and care is taken to separate genuine dynamics from processing-induced effects. See also aliasing, instantaneous frequency, and spectral leakage.