  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.
