  At SRON we are studying the performance of a Goddard Space Flight Center
single pixel TES microcalorimeter operated in an AC bias configuration. For
x-ray photons at 6 keV the pixel shows an x-ray energy resolution $dE_{FWHM} =
3.7\mathrm{eV}$, which is about a factor 2 worse than the energy resolution
observed in an identical DC-biased pixel. In order to better understand the
reasons for this discrepancy we characterised the detector as a function of
temperature, bias working point and applied perpendicular magnetic field. A
strong periodic dependency of the detector noise on the TES AC bias voltage is
measured. We discuss the results in the framework of the recently observed
weak-link behaviour of a TES microcalorimeter
