  Large arrays of cryogenic sensors for various imaging applications ranging
across x-ray, gamma-ray, Cosmic Microwave Background (CMB), mm/sub-mm, as well
as particle detection increasingly rely on superconducting microresonators for
high multiplexing factors. These microresonators take the form of microwave
SQUIDs that couple to Transition-Edge Sensors (TES) or Microwave Kinetic
Inductance Detectors (MKIDs). In principle, such arrays can be read out with
vastly scalable software-defined radio using suitable FPGAs, ADCs and DACs. In
this work, we share plans and show initial results for SLAC Microresonator
Radio Frequency (SMuRF) electronics, a next-generation control and readout
system for superconducting microresonators. SMuRF electronics are unique in
their implementation of specialized algorithms for closed-loop tone tracking,
which consists of fast feedback and feedforward to each resonator's excitation
parameters based on transmission measurements. Closed-loop tone tracking
enables improved system linearity, a significant increase in sensor count per
readout line, and the possibility of overcoupled resonator designs for enhanced
dynamic range. Low-bandwidth prototype electronics were used to demonstrate
closed-loop tone tracking on twelve 300-kHz-wide microwave SQUID resonators,
spaced at $\sim$6 MHz with center frequencies $\sim$5-6 GHz. We achieve
multi-kHz tracking bandwidth and demonstrate that the noise floor of the
electronics is subdominant to the noise intrinsic in the multiplexer.
