Complex impedance of TESs under AC bias using FDM readout system

TL;DR

This paper presents a novel method for measuring the complex impedance of AC-biased TES detectors using an FDM readout system, enabling comprehensive characterization of detector performance in array configurations.

Contribution

Developed a new complex impedance measurement technique for AC-biased TESs with FDM readout, facilitating array-level characterization and improved thermal modeling.

Findings

Successful impedance measurements for various TES microcalorimeters.

Enhanced understanding of detector thermal models and performance differences.

Comparison of calculated and measured noise spectra validates the method.

Abstract

The next generation of Far-infrared and X-ray space observatories will require detector arrays with thousands of transition edge sensor (TES) pixel. It is extremely important to have a tool that is able to characterize all the pixels and that can give a clear picture of the performance of the devices. In particular, we refer to those aspects that can affect the global energy resolution of the array: logarithmic resistance sensitivity with respect to temperature and current ($\alpha$ and $\beta$ parameters, respectively), uniformity of the TESs and the correct understanding of the detector thermal model. Complex impedance measurement of a TES is the only technique that can give all this information at once, but it has been established only for a single pixel under DC bias. We have developed a complex impedance measurement method for TESs that are AC biased since we are using a MHz frequency domain multiplexing (FDM) system to readout an array. We perform a complete set of AC impedance measurements for different X-ray TES microcalorimeters based on superconducting TiAu bilayers with or without normal metal Au bar structures. We discuss the statistical analysis of the residual between impedance data and fitting model to determine the proper calorimeter thermal model for our detectors. Extracted parameters are used to improve our understanding of the differences and capabilities among the detectors and additionally the quality of the array. Moreover, we use the results to compare the calculated noise spectra with the measured data.