  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.

Instrumentation and Methods for Astrophysics;Cosmology and Nongalactic Astrophysics