Digital Frequency-Domain Multiplexing (DfMux) is a technique that uses MHz superconducting resonators and Superconducting Quantum Interference Device (SQUID) arrays to read out sets of transition edge sensors. DfMux has been used by several Cosmic Microwave Background experiments, including most recently POLARBEAR-2 and SPT-3 G with multiplexing factors as high as 68, and is the baseline readout technology for the planned satellite mission LiteBIRD. Here, we present recent work focused on improving DfMux readout noise, reducing parasitic impedance, and improving sensor operation. We have achieved a substantial reduction in stray impedance by integrating the sensors, resonators, and SQUID array onto a single-carrier board operated at 250 mK. This also drastically simplifies the packaging of the cryogenic components and leads to better-controlled crosstalk. We demonstrate a low readout noise level of 8.6 \(\mathrm{pA/Hz}1^{1/2}\), which was made possible by operating the SQUID array at a reduced temperature and with a low dynamic impedance. This is a factor of two improvement compared to the achieved readout noise level in currently operating Cosmic Microwave Background experiments using DfMux and represents a critical step toward maturation of the technology for the next generation of instruments.

数字频域复用 (DfMux) 是一种使用 MHz 超导谐振器和超导量子干涉装置 (SQUID) 阵列来读取过渡边缘传感器组的技术。DfMux 已被多个宇宙微波背景实验使用，包括最近的 POLARBEAR-2 和 SPT-3 G，复用因子高达 68，是计划中的卫星任务LiteBIRD的基线读出技术. 在这里，我们介绍了最近的工作，重点是改善 DfMux 读出噪声、降低寄生阻抗和改善传感器操作。我们通过将传感器、谐振器和 SQUID 阵列集成到工作频率为 250 mK 的单载板上，显着降低了杂散阻抗。这也大大简化了低温组件的封装，并导致更好地控制串扰。我们展示了 8.6 \(\mathrm{pA/Hz}1^{1/2}\)的低读出噪声水平，这是通过在降低的温度和低动态阻抗下操作 SQUID 阵列来实现的。与当前使用 DfMux 进行的宇宙微波背景实验中所达到的读出噪声水平相比，这是一个两倍的改进，代表了朝着下一代仪器技术成熟的关键一步。