  The SLAC Microresonator Radio Frequency (SMuRF) electronics is being deployed
as the readout for the Cosmic Microwave Background (CMB) telescopes of the
Simons Observatory (SO). A Radio Frequency System-on-Chip (RFSoC) based readout
of microwave frequency resonator based cryogenic sensors is under development
at SLAC as an upgrade path for SMuRF with simplified RF hardware, a more
compact footprint, and lower total power consumption. The high-speed integrated
data converters and digital data path in RFSoC enable direct RF sampling
without analog up and down conversion for RF frequencies up to 6 GHz. A
comprehensive optimization and characterization study has been performed for
direct RF sampling for microwave SQUID multiplexers, which covers noise level,
RF dynamic range, and linearity using a prototype implementation. The SMuRF
firmware, including the implementation of closed-loop tone tracking, has been
ported to the RFSoC platform and interfaced with the quadrature mixers for
digital up and down conversion in the data converter data path to realize a
full microwave SQUID multiplexer readout. In this paper, a selection of the
performance characterization results of direct RF sampling for microwave SQUID
multiplexer readout will be summarized and compared with science-driven
requirements. Preliminary results demonstrating the read out of cryogenic
sensors using the prototype system will also be presented here. We anticipate
our new RFSoC-based SMuRF system will be an enabling readout for on-going and
future experiments in astronomy and cosmology, which rely on large arrays of
cryogenic sensors to achieve their science goals.
