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We describe the newest generation of the SLAC Microresonator RF (SMuRF) electronics, a warm digital control and readout system for microwave-frequency resonator-based cryogenic detector and multiplexer systems such as microwave SQUID multiplexers ($\mu$mux) or microwave kinetic inductance detectors (MKIDs). Ultra-sensitive measurements in particle physics and astronomy increasingly rely on large arrays of cryogenic sensors, which in turn necessitate highly multiplexed readout and accompanying room-temperature electronics. Microwave-frequency resonators are a popular tool for cryogenic multiplexing, with the potential to multiplex thousands of detector channels on one readout line. The SMuRF system provides the capability for reading out up to 3328 channels across a 4-8 GHz bandwidth. Notably, the SMuRF system is unique in its implementation of a closed-loop tone-tracking algorithm that minimizes RF power transmitted to the cold amplifier, substantially relaxing system linearity requirements and effective noise from intermodulation products. Here we present a description of the hardware, firmware, and software systems of the SMuRF electronics, comparing achieved performance with science-driven design requirements. We focus in particular on the case of large channel count, low bandwidth applications, but the system has been easily reconfigured for high bandwidth applications. The system described here has been successfully deployed in lab settings and field sites around the world and is baselined for use on upcoming large-scale observatories., Comment: 28 pages, 25 figures, + references. Comments welcome!

The next generation of cryogenic CMB and submillimeter cameras under development require densely instrumented sensor arrays to meet their science goals. The readout of large numbers ($\sim$10,000--100,000 per camera) of sub-Kelvin sensors, for instance as proposed for the CMB-S4 experiment, will require substantial improvements in cold and warm readout techniques. To reduce the readout cost per sensor and integration complexity, efforts are presently focused on achieving higher multiplexing density while maintaining readout noise subdominant to intrinsic detector noise. Highly-multiplexed cold readout technologies in active development include Microwave Kinetic Inductance Sensors (MKIDs) and microwave rf-SQUIDs. Both exploit the high quality factors of superconducting microwave resonators to densely channelize sub-Kelvin sensors into the bandwidth of a microwave transmission line. We present advancements in the development of a new warm readout system for microwave SQUID multiplexing, the SLAC Superconducting Microresonator RF electronics, or SMuRF. The SMuRF system is unique in its ability to track each tone, minimizing the total RF power required to read out each resonator, thereby significantly reducing the linearity requirements on the cold and warm readout. Here, we present measurements of the readout noise and linearity of the first full SMuRF system, including a demonstration of closed-loop tone tracking on a 528 channel cryogenic microwave SQUID multiplexer. SMuRF is being explored as a potential readout solution for several future CMB projects including Simons Observatory, BICEP Array, CCAT-prime, Ali-CPT, and CMB-S4. Parallel development of the platform is underway to adapt SMuRF to read out both MKID and fast X-ray TES calorimeter arrays., Comment: 16 pages, 5 figures