CCAT: Mod-Cam Readout Overview and Flexible Stripline Performance

Abstract

The CCAT Observatory's primary science instrument, Prime-Cam, is nearing readiness for deployment to the Fred Young Submillimeter Telescope (FYST) in the Atacama Desert in northern Chile. When fully deployed, Prime-Cam will field approximately 100,000 kinetic inductance detectors (KIDs) across seven instrument modules making both broadband and polarimetric measurements. Meanwhile, in-lab characterization of the first CCAT instrument module, a 280 GHz broadband camera fielding over 10,000 KIDs, is currently underway in the testbed instrument Mod-Cam. Both Mod-Cam and Prime-Cam will employ 46 cm long low-thermal-conductivity flexible circuits ("stripline") between 4 K and 300 K to connect large-format arrays of multiplexed KIDs in each instrument module to readout electronics. The 280 GHz camera currently installed in Mod-Cam uses six of these striplines to read out its over 10,000 detectors. We have examined the thermal and electrical performance of the stripline installed in Mod-Cam. We begin by characterizing the OFHC copper in the stripline traces, allowing for the estimation of thermal loading through these flexible circuits in their configurations in both Mod-Cam and Prime-Cam. We then directly measure the thermal conductivity of the stripline, finding it is best described by $kA = 22\pm6~T^{0.84\pm0.09}~\mathrm{μ~W~m~K^{-1}}$ for temperature ranges of 6 K < T < 20 K and $kA~=~0.6\pm0.3~T^{-0.4\pm0.1}~\mathrm{mW~m~K^{-1}}$ for ranges from 20 K < T < 80 K. Following our thermal characterizations, we report on the transmission and crosstalk properties of the Mod-Cam readout chain, isolating elevated crosstalk to SMP-SMA transition printed circuit boards (PCBs) that interface with the stripline. This finding validates the stripline circuit as a viable high-density cabling option for large-format array readout.

Figures (6)

• Figure 1: Top: A picture of Mod-Cam from the side. The white arrow denotes the "back" of Mod-Cam. Middle: The back of Mod-Cam when the cryostat is open, after the instrument module has been installed. The mount for the six stripline assemblies is highlighted in the white box, along with the purple SMP-SMA transition PCBs and coaxial cable interface to the 4 K low noise amplifiers (LNAs). Bottom: A preliminary rendering of the larger Prime-Cam readout harness with 18 stripline assemblies and accompanying transition PCBs. The red, green, and yellow plates in the rendering are mounted to the Prime-Cam 300, 40, and 4 K shells respectively. Temporary installation rods shown between plates in the rendering are removed after the readout harness is inserted in Prime-Cam.
• Figure 2: Top Left: The connectorized end of one stripline circuit. The numbers denote the labeling scheme used between channels for the "stripline-only" and "stripline+PCB" crosstalk measurements shown in Fig. \ref{['fig:crosstalk']}. Top Right: A photograph of one SMP-SMA transition PCB. Two PCBs plug into ganged triple-SMP connectors on each end of the stripline. The numbers overlaid denote the labeling scheme used in the "PCB-only" crosstalk measurement shown in Fig. \ref{['fig:crosstalk']}. Bottom: A diagram demonstrating the full stripline and SMP-SMA transition used in Mod-Cam. Four identical transition PCBs are used to connect to coaxial cables at 300 K and 4 K. Thermal clamping points where the ground plane is extended to improve heatsinking are shown on the stripline; these physically coincide with the location of the 40 K and 4 K shells.
• Figure 3: Profilometry of a ground trace on a stripline assembly showing the trace thickness to be approximately $22~\mu \mathrm{m}$ thick and $0.43~\mathrm{mm}$ wide.
• Figure 4: A photograph of the testing setup used for 4 K thermal loading measurements is shown (left) along with a schematic diagram of the testing setup (right). Hot and cold side temperatures, $T_h$ and $T_c$ respectively, are each measured using a Cernox cryogenic temperature sensor. Thermal loads are applied across the stripline with a $1~\mathrm{k} \Omega$ resistor sunk to the warm side clamp.
• Figure 5: Top: Data points relating the power applied to the stripline to the difference in temperature between the hot and cold sides are shown with best fit lines overlaid. The data is split according to $T_h$ values occurring before and after the turnaround in OFHC copper $k(T)$ given by nist_copper. The full dataset, containing $T_c$ and $T_h$ temperatures spanning from $3.5-84~K$ yields a best fit $kA~=~0.3\pm0.2~T^{-0.3\pm0.1}~\mathrm{mW~m~K^{-1}}$. Data points with $\alpha = 0$ corresponding to $T_h - T_c$ are shown to demonstrate the effect of rescaling. Bottom: The ranges of $T_h$ for each half of the split dataset are overlaid on the functional form of OFHC copper thermal conductivity given by nist_copper.