  We describe an on-sky demonstration of a microwave-multiplexing readout
system in one of the receivers of the Keck Array, a polarimetry experiment
observing the cosmic microwave background at the South Pole. During the austral
summer of 2018-2019, we replaced the time-division multiplexing readout system
with microwave-multiplexing components including superconducting microwave
resonators coupled to radio-frequency superconducting quantum interference
devices at the sub-Kelvin focal plane, coaxial-cable plumbing and amplification
between room temperature and the cold stages, and a SLAC Microresonator Radio
Frequency system for the warm electronics. In the range 5-6 GHz, a single
coaxial cable reads out 528 channels. The readout system is coupled to
transition-edge sensors, which are in turn coupled to 150-GHz slot-dipole
phased-array antennas. Observations began in April 2019, and we report here on
an initial characterization of the system performance.
