First results of a TES microcalorimeter AC-biased at 500 kHz

First performance results of a transition edge sensor (TES) microcalorimeter, AC-biased at 500 kHz, are presented in succession to earlier results at 46 kHz. Measurements on the bias circuit reveal a series resistance due to dielectric losses in the superconducting Nb/Ta-Oxide capacitor, which degrades the voltage bias. To improve frequency stability of the bias circuit against a large signal-amplitude range ways to reduce coupling between the SQUID feedback and input coil have been explored. I–V curves measured for different bath temperatures show hysteresis in the switch-on and switch-off behaviour of the TES for bias points low in the resistive transition.