SO utilizes microwave multiplexing (μMux) to readout the detector channels. μMux operates via phase modulated frequency division multiplexing, illustrated in the circuit diagram. The electronics that couple to the cameras and form the room temperature component of the detector electronics have been developed by the SLAC National Accelerator Laboratory (SLAC).
TES detectors couple to RF SQUID resonant circuits, each with a unique resonant frequency, driven by tones from a frequency comb generated as part of the tone-tracking feedback. 200 TESs are DC voltage biased on a single bias line. As incident optical power modulates the resistance of the TES the current through an inductor is modulated varying the flux in the RF SQUID. The inductance of the RF SQUID is periodic with flux thus the resonant frequency shifts as a function of input flux and the SLAC Microwave RF (SMuRF ) Electronics tracks this frequency. The output comb is amplified before leaving the cryostat where the signal is further processed by warm readout electronics before being recorded to disks and transmitted back to the United States for analysis. SO is targeting a multiplexing factor of 2000.