The Simons Observatory: Studies of Phase Drift in RF Transmission Lines from the First Large-Scale Deployment of Microwave Frequency Multiplexing for Cosmology

TL;DR

This work assesses whether diurnal temperature-driven phase drift in room-temperature RF transmission lines can contaminate SMuRF-based microwave frequency multiplexing for the Simons Observatory LAT. The authors deploy off-resonance tones inside the SMuRF bands to directly track phase drift, converting measured phase changes via $\tau=\frac{\theta}{2\pi f}$ and $\delta Q=I_{\text{res}}^{\text{offset}}\tan\theta$ to a TES current-equivalent noise using $\langle df/dI_{\text{TES}}\rangle_{\Phi_0}=5.71\times10^{-2}$ Hz pA$^{-1}$. They find diurnal phase drift at the level of a few picoseconds, with the inferred detector-noise contribution lying within the readout budget and below the on-sky detector noise, implying no degradation in mapping speed. The results validate the practicality of large-scale microwave multiplexing in a field deployment and provide quantitative guidance for phase stability and tone-tracking considerations in future CMB experiments.

Abstract

Fulfilling the science goals of the Simons Observatory, a state-of-the-art cosmic microwave background (CMB) experiment, has required deploying tens of thousands of superconducting bolometers. Reading out data from the observatory's more than 67,000 transition-edge sensor (TES) detectors while maintaining cryogenic conditions requires an effective multiplexing scheme. The SLAC microresonator radio frequency (SMuRF) electronics have been developed to provide the warm electronics for a high-density microwave frequency multiplexing readout system, and this system has been shown to achieve multiplexing factors on the order of 1,000. SMuRF has recently been deployed to the Simons Observatory, which is located at 5,200 m on Cerro Toco in Chile's Atacama Desert. As the SMuRF system is exposed to the desert's diurnal temperature swings, resulting phase drift in RF transmission lines may introduce a systematic signal contamination. We present studies of phase drift in the room-temperature RF lines of the Simons Observatory's 6 m large-aperture telescope, which hosts the largest deployment to date of TES microwave frequency multiplexing to a single telescope. We show that these phase drifts occur on time scales which are significantly longer than sky scanning, and that their contribution to on-sky in-transition detector noise is within the readout noise budget.

Figures (7)

• Figure 1: Two ATCA crates containing a total of seven SMuRF systems installed to the LATR. RF coaxial cables and DC cables, which transmit signals into the cryostat via the URH, are shown.
• Figure 2: Time delay ($\tau$) of off-resonance tones compared with ambient temperature in the LATR cabin during one observing day. Each off-resonance tone is centered to $\tau=0\;\text{ps}$ at the start of the plot.
• Figure 3: In-phase $\left(I=\text{Re}\left[S_{21}\right]\right)$ and quadrature $\left(Q=\text{Im}\left[S_{21}\right]\right)$ components of microwave resonators as tracked by SMuRF before (blue) and after (green) calibration, which corresponds to a rotation in the $IQ$-plane. Resonant frequency locations marked with lilac dots. Distance along the $I$-axis from the resonant frequency to the origin after calibration is labeled as $I_\text{res}^\text{offset}$.
• Figure 4: Distribution of $I_\text{res}^\text{offset}$, as defined in Figure \ref{['fig:eta-rotation']}. Measured using SMuRF for microwave resonators deployed to one UFM installed to the LATR. Error bars show $1\sigma$ Poisson counting uncertainties.
• Figure 5: Amplitude spectral densities of the inferred TES noise-equivalent currents from the measured phase drifts (green) during a single 40-minute-long constant-elevation scan with one UFM installed to the LAT. Shown in comparison to a representative 150 GHz on-sky in-transition detector (blue) and a representative open SQUID channel (lilac).