  A technological milestone for experiments employing Transition Edge Sensor
(TES) bolometers operating at sub-kelvin temperature is the deployment of
detector arrays with 100s--1000s of bolometers. One key technology for such
arrays is readout multiplexing: the ability to read out many sensors
simultaneously on the same set of wires. This paper describes a
frequency-domain multiplexed readout system which has been developed for and
deployed on the APEX-SZ and South Pole Telescope millimeter wavelength
receivers. In this system, the detector array is divided into modules of seven
detectors, and each bolometer within the module is biased with a unique ~MHz
sinusoidal carrier such that the individual bolometer signals are well
separated in frequency space. The currents from all bolometers in a module are
summed together and pre-amplified with Superconducting Quantum Interference
Devices (SQUIDs) operating at 4 K. Room-temperature electronics demodulate the
carriers to recover the bolometer signals, which are digitized separately and
stored to disk. This readout system contributes little noise relative to the
detectors themselves, is remarkably insensitive to unwanted microphonic
excitations, and provides a technology pathway to multiplexing larger numbers
of sensors.
