  The X-ray integral field unit for the Athena mission consists of a
microcalorimeter transition edge sensor pixel array. Incoming photons generate
pulses which are analyzed in terms of energy, in order to assemble the X-ray
spectrum. Usually this is done by means of optimal filtering in either time or
frequency domain.
  In this paper we investigate an alternative method by means of principle
component analysis. This method attempts to find the main components of an
orthogonal set of functions to describe the data.
  We show, based on simulations, what the influence of various instrumental
effects is on this type of analysis. We compare analyses both in time and
frequency domain. Finally we apply these analyses on real data, obtained via
frequency domain multiplexing readout.
