Comparing complex impedance and bias step measurements of Simons Observatory transition edge sensors

TL;DR

This paper compares two methods for characterizing the response times of transition-edge sensors in the Simons Observatory, highlighting the advantages and limitations of each approach for in-situ calibration during CMB observations.

Contribution

It introduces a comparative analysis of bias step and complex impedance measurements for TES characterization in the context of the Simons Observatory.

Findings

Bias step and impedance methods yield comparable time constants.

Impedance measurements provide a more comprehensive TES model.

Bias steps are faster but less detailed.

Abstract

The Simons Observatory (SO) will perform ground-based observations of the cosmic microwave background (CMB) with several small and large aperture telescopes, each outfitted with thousands to tens of thousands of superconducting aluminum manganese (AlMn) transition-edge sensor bolometers (TESs). In-situ characterization of TES responsivities and effective time constants will be required multiple times each observing-day for calibrating time-streams during CMB map-making. Effective time constants are typically estimated in the field by briefly applying small amplitude square-waves on top of the TES DC biases, and fitting exponential decays in the bolometer response. These so-called "bias step" measurements can be rapidly implemented across entire arrays and therefore are attractive because they take up little observing time. However, individual detector complex impedance measurements, while too slow to implement during observations, can provide a fuller picture of the TES model and a better understanding of its temporal response. Here, we present the results of dark TES characterization of many prototype SO bolometers and compare the effective thermal time constants measured via bias steps to those derived from complex impedance data.