Performance and characterization of the SPT-3G digital frequency-domain multiplexed readout system using an improved noise and crosstalk model

TL;DR

This paper evaluates the SPT-3G readout system, updates noise and crosstalk models for higher bandwidth, and suggests improvements for future large-scale detector readouts in cosmology experiments.

Contribution

It provides an improved noise and crosstalk model for high-bandwidth DfMUX systems, validated against SPT-3G measurements, and proposes design modifications for future applications.

Findings

Updated crosstalk model accurately predicts measured performance.

Identified two new dominant noise contributions at high bias frequencies.

Proposed specific hardware adjustments to enhance future readout systems.

Abstract

The third generation South Pole Telescope camera (SPT-3G) improves upon its predecessor (SPTpol) by an order of magnitude increase in detectors on the focal plane. The technology used to read out and control these detectors, digital frequency-domain multiplexing (DfMUX), is conceptually the same as used for SPTpol, but extended to accommodate more detectors. A nearly 5x expansion in the readout operating bandwidth has enabled the use of this large focal plane, and SPT-3G performance meets the forecasting targets relevant to its science objectives. However, the electrical dynamics of the higher-bandwidth readout differ from predictions based on models of the SPTpol system due to the higher frequencies used, and parasitic impedances associated with new cryogenic electronic architecture. To address this, we present an updated derivation for electrical crosstalk in higher-bandwidth DfMUX systems, and identify two previously uncharacterized contributions to readout noise, which become dominant at high bias frequency. The updated crosstalk and noise models successfully describe the measured crosstalk and readout noise performance of SPT-3G. These results also suggest specific changes to warm electronics component values, wire-harness properties, and SQUID parameters, to improve the readout system for future experiments using DfMUX, such as the LiteBIRD space telescope.