Design and Assembly of SPT-3G Cold Readout Hardware

The third-generation upgrade to the receiver on the South Pole Telescope, SPT-3G, was installed at the South Pole during the 2016–2017 austral summer to measure the polarization of the cosmic microwave background. Increasing the number of detectors by a factor of 10 to $$\sim 16,000$$∼16,000 required the multiplexing factor to increase to 68 and the bandwidth of the frequency-division readout electronics to span 1.6–5.2 MHz. This increase necessitates low-thermal conductance, low-inductance cryogenic wiring. Our cold readout system consists of planar thin-film aluminum inductive–capacitive resonators, wired in series with the detectors, summed together, and connected to 4K SQUIDs by $$10-\upmu \hbox {m}$$10-μm-thick niobium–titanium (NbTi) broadside-coupled striplines. Here, we present an overview of the cold readout electronics for SPT-3G, including assembly details and characterization of electrical and thermal properties of the system. We report, for the NbTi striplines, values of $$R \le 10^{-4} \Omega $$R≤10-4Ω, $$L = 21 \pm 1~\hbox {nH}$$L=21±1nH, and $$C = 1.47\pm .02~\hbox {nF}$$C=1.47±.02nF. Additionally, the striplines’ thermal conductivity is described by $$kA = 6.0\pm 0.3 \ T^{0.92 \pm 0.04}~\upmu \hbox {W}~\hbox {mm}~\hbox {K}^{-1}$$kA=6.0±0.3T0.92±0.04μWmmK-1. Finally, we provide projections for cross talk induced by parasitic impedances from the stripline and find that the median value of percentage cross talk from leakage current is 0.22 and $$0.09\%$$0.09% from wiring impedance.