Performance of an X-ray single pixel TES microcalorimeter under DC and AC biasing

We are developing Frequency Domain Multiplexing (FDM) for the read-out of TES imaging microcalorimeter arrays for future X-ray missions like IXO. In the FDM configuration the TES is AC voltage biased at a well defined frequencies (between 0.3 to 10MHz) and acts as an AM modulating element. In this paper we will present a full comparison of the performance of a TES microcalorimeter under DC bias and AC bias at a frequency of 370kHz. In both cases we measured the current-to-voltage characteristics, the complex impedance, the noise, the X-ray responsivity, and energy resolution. The behaviour is very similar in both cases, but deviations in performances are observed for detector working points low in the superconducting transition (R/RN < 0.5). The measured energy resolution at 5.89keV is 2.7eV for DC bias and 3.7eV for AC bias, while the baseline resolution is 2.8eV and 3.3eV, respectively.