Progress on frequency domain multiplexed readout of TES bolometers with sub-kelvin SQUID operation

Digital Frequency Domain Multiplexing (DfMux) is a method of biasing and reading out many TES bolometers using superconducting MHz resonators and a SQUID amplifier. DfMux was recently deployed in two Cosmic Microwave Background receivers and is an alternative and baseline readout technology for CMB-S4 and LiteBIRD, respectively. The next generation of DfMux puts the SQUID,TES bias element, and resonators onto a common carrier at 100 mK in order to relax requirements on cryogenic wiring and allow for an increased multiplexing factor and improved TES performance. Characterization of a prototype system that is compatible with the POLARBEAR-2 receivers is presented.