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

The third generation South Pole Telescope camera (SPT-3G) improves over its predecessor (SPTpol) by an order of magnitude increase in detector number. 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 system depart in significant ways from the characterization and models drawn from the previous generation of cameras. We present an updated derivation for electrical crosstalk in higher-bandwidth DfMUX systems, and identify two previously uncharacterized contributions to readout noise. The updated crosstalk and noise models successfully describe the measured crosstalk and readout noise performance of SPT-3G, and suggest improvements to the readout system for future experiments using DfMUX, such as the LiteBIRD satellite.