# On-sky performance of the SPT-3G frequency-domain multiplexed readout

## TL;DR

This paper reports on the successful on-sky performance of the SPT-3G frequency-domain multiplexed readout system, demonstrating low noise and effective operation of a large TES bolometer array on the South Pole Telescope.

## Contribution

It presents the first on-sky characterization of the SPT-3G fMux readout, confirming low noise levels and photon-noise-limited performance for large-scale TES arrays.

## Key findings

- Median 1/f knee frequency of 33 mHz indicates low low-frequency noise.
- Readout white noise is below photon noise levels, confirming photon-noise-limited operation.
- Successful implementation of 68x fMux system on a large TES array.

## Abstract

Frequency-domain multiplexing (fMux) is an established technique for the readout of large arrays of transition edge sensor (TES) bolometers. Each TES in a multiplexing module has a unique AC voltage bias that is selected by a resonant filter. This scheme enables the operation and readout of multiple bolometers on a single pair of wires, reducing thermal loading onto sub-Kelvin stages. The current receiver on the South Pole Telescope, SPT-3G, uses a 68x fMux system to operate its large-format camera of $\sim$16,000 TES bolometers. We present here the successful implementation and performance of the SPT-3G readout as measured on-sky. Characterization of the noise reveals a median pair-differenced 1/f knee frequency of 33 mHz, indicating that low-frequency noise in the readout will not limit SPT-3G's measurements of sky power on large angular scales. Measurements also show that the median readout white noise level in each of the SPT-3G observing bands is below the expectation for photon noise, demonstrating that SPT-3G is operating in the photon-noise-dominated regime.