Frequency-domain multiplexing development for high-count-rate microcalorimeters

Future X-ray telescopes like XEUS will require a microcalorimeter array capable of handling a high photon-count-rate (250 cps per pixel, τ eff = 100 μ s ). This requirement, combined with an energy resolution of 1–2 eV and an energy range up to 3 keV, leads to very demanding requirements for the readout bandwidth and dynamic range. We discuss the development of a frequency-domain multiplexing (FDM) compatible TES bias circuit and bias and readout electronics, and focus on the effects of nonlinearity of the system. Furthermore, we compare between the experimentally observed IV characteristics of a TES under AC and DC bias.