The problem of aliasing in relation to the use of Wigner distributions to process discrete time (DT) non-stationary
signals from fusion plasma diagnostics is comprehensively addressed. Three DTWigner distributions, two of which
are alias free (AF), are thoroughly studied and compared, their properties being provided as well as, making use
of so-called pseudo-Wigner (p-Wigner) distributions, formulae for their efficient computation. Of the three, one is
singled out as that AF form which simultaneously obeys an arguably more complete set of desirable properties for
time–frequency distributions and which has a lower computational complexity. Such an AF DTWigner distribution
has been known in quantum mechanics as the rotational Wigner function, where it has been introduced to treat
rotation angles and their canonically conjugate angular momenta, whose spectra have the same discrete nature as
DTsignals. As an illustration of the advantages and shortcomings of each of these three distributions, they are applied
to the time–frequency analysis of MHD activity data. Thus, the three DTWigner distributions, more precisely their
p-Wigner counterparts, together with the spectrogram for the sake of comparison, are used to obtain representations
in the time–frequency plane of non-stationary events from neoclassical tearing mode (NTM) and toroidal Alfvén
eigenmode (TAE) experiments in the JET tokamak. Besides aliasing, the appearance in the time–frequency plane
of spurious images due to cross-term interference between different signal components is also discussed. If such
undesirable artefacts are generated by negative frequencies interfering with positive frequencies, it is shown that the
analytic signal can be of great help in reducing cross-term effects.
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