🔭 astrophysics

CCAT: Mod-Cam Readout Overview and Flexible Stripline Performance

This paper characterizes the thermal and electrical performance of the flexible stripline cables used to read out over 10,000 kinetic inductance detectors in the CCAT Observatory's Mod-Cam testbed, confirming their viability for large-format arrays while identifying specific transition boards as the source of elevated crosstalk.

Original authors: Ben Keller, Rodrigo Freundt, James R. Burgoyne, Scott Chapman, Steve Choi, Cody J. Duell, Christopher Groppi, Caleb Humphreys, Lawrence T. Lin, Alicia Middleton, Michael D. Niemack, Darshan Patel, Eve

Published 2026-03-02

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CC BY 4.0

Original authors: Ben Keller, Rodrigo Freundt, James R. Burgoyne, Scott Chapman, Steve Choi, Cody J. Duell, Christopher Groppi, Caleb Humphreys, Lawrence T. Lin, Alicia Middleton, Michael D. Niemack, Darshan Patel, Eve Vavagiakis, Samantha Walker, Yuhan Wang, Ruixuan, Xie

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you are trying to listen to a massive choir of 100,000 tiny singers (the detectors) in a freezing cold room, but you need to send their songs to a warm control room where the recording equipment lives. The challenge? You can't just run a thick, warm wire between the two rooms, or the heat from the wire would melt the ice and ruin the singers' performance. You need a "thermal bridge" that carries the sound perfectly but lets almost no heat through.

This paper is about building and testing that bridge for a giant new telescope called FYST (Fred Young Submillimeter Telescope), which will sit on a high mountain in Chile to look at the early universe.

Here is the story of how they solved this problem, explained simply:

1. The Problem: Too Many Singers, Too Few Wires

The telescope's main instrument, Prime-Cam, will have about 100,000 tiny sensors (called Kinetic Inductance Detectors or KIDs). To read the data from all of them, you usually need thousands of individual coaxial cables (like thick, insulated wires).

The Analogy: Imagine trying to connect 100,000 microphones to a mixing board using standard cables. You would need a bundle of cables so thick it wouldn't fit through the door, and the heat traveling down those cables would be like opening a furnace door in a freezer. It would break the equipment.

2. The Solution: The "Flat Ribbon" (Stripline)

Instead of thick round cables, the team designed a flexible circuit board that looks like a flat ribbon. They call this a "stripline."

The Analogy: Think of a standard coaxial cable as a thick, insulated garden hose. The new stripline is like a flat, flexible ribbon of copper sandwiched between layers of plastic (polyimide). It's thin, flexible, and acts as a "thermal choke," letting the electrical signal pass easily but blocking the heat from traveling up the line.

3. Testing the "Ribbon" (Thermal Performance)

Before putting this ribbon on the real telescope, they tested it in a lab using a smaller version called Mod-Cam. They needed to know two things:

How much heat leaks through?
- They measured the purity of the copper inside the ribbon. High-purity copper conducts electricity well but, when made very thin, conducts heat poorly.
- The Result: They found the ribbon is excellent at its job. They calculated that even with many ribbons, the heat leaking into the cold part of the telescope is small enough to be manageable. They even redesigned the ribbon for the main telescope (Prime-Cam) to be even thinner and longer, making it an even better "heat blocker."
How does the heat flow?
- They literally heated one end of the ribbon and measured the temperature at the other end. They found that the heat flow changes depending on how cold it is, but they now have a perfect mathematical formula to predict exactly how much heat will leak in any situation.

4. The "Static" Problem (Crosstalk)

Once they knew the ribbon didn't leak heat, they had to check if it leaked signals.

The Analogy: Imagine you are in a room with 100 people talking on walkie-talkies. If the walkie-talkies are too close together or the antennas are bad, Person A might hear Person B's conversation. This is called crosstalk.
The Discovery: When they tested the system, they heard too much "static" (crosstalk). They traced the noise and found the culprit wasn't the ribbon itself (which was quiet), but the adapter boards (PCBs) that connect the flat ribbon to the round cables.
- The Fix: It was like finding that the microphone cables were touching each other at the plug. The team redesigned these adapter boards to bury the signal deeper inside the board, shielding it from its neighbors. This reduced the "static" by a huge amount (about 20 decibels), making the connection crystal clear.

5. Why This Matters

This paper proves that using these flat, flexible ribbons is a viable way to read out massive arrays of sensors.

The Big Picture: Without this technology, building a telescope with 100,000 sensors would be impossible because the heat from the wires would destroy the experiment.
The Outcome: They have validated that this "flat ribbon" technology works. It keeps the telescope cold, keeps the signals clear, and paves the way for Prime-Cam to be deployed in 2026 to help us understand how the universe began.

In short: They built a super-thin, heat-blocking "information highway" that can carry data from 100,000 sensors without melting the ice, and they fixed the "traffic noise" at the exit ramp so the data arrives perfectly clean.

1. Problem Statement

The CCAT Collaboration is developing the Prime-Cam instrument for the Fred Young Submillimeter Telescope (FYST), which will house approximately 100,000 Kinetic Inductance Detectors (KIDs) across seven modules. A critical engineering challenge in reading out such large-format arrays is managing the thermal load and physical space constraints at cryogenic temperatures (specifically the 4 K stage).

Thermal Constraint: Traditional readout methods using individual coaxial cables for thousands of detectors would generate excessive heat, exceeding the cooling capacity of the cryostat's 4 K stage.
Space Constraint: The sheer volume of coaxial cabling required would physically overwhelm the instrument's mechanical design.
Signal Integrity: High-density cabling solutions must maintain low crosstalk between channels to ensure accurate data acquisition.

To address this, the team is utilizing flexible stripline circuits (46 cm long, low-thermal-conductivity circuits) to connect detector arrays at 4 K to warm readout electronics at 300 K. The paper focuses on characterizing the stripline currently installed in the testbed instrument Mod-Cam to validate its use for the full-scale Prime-Cam deployment.

2. Methodology

The authors employed a multi-faceted approach involving thermal characterization and electrical performance testing:

Thermal Characterization:
- Residual Resistance Ratio (RRR): A 5 cm section of the stripline was measured at 300 K and 4 K to determine the purity of the OFHC (Oxygen-Free High-Conductivity) copper traces.
- Thermal Conductivity Measurement: Using a custom setup in a Bluefors SD250 Dilution Refrigerator, the team applied heat to one end of a suspended stripline while measuring temperature gradients ( $T_h$ and $T_c$ ) across the circuit. They modeled the thermal conductance ($kA$) as a power law function of temperature ( $kA = A_0 T^\alpha$ ).
- Modeling: They used the measured RRR and thermal conductivity data to estimate thermal loading for both the current Mod-Cam configuration and the redesigned Prime-Cam configuration (which features thinner copper traces and longer distances between temperature stages).
Electrical Characterization (Crosstalk):
- Warm S21 Measurements: Using a Vector Network Analyzer (VNA), the team measured transmission and crosstalk ( $S_{21}$ ) across the readout bandwidth (300 MHz – 1.5 GHz).
- Component Isolation: To identify the source of signal interference, they tested three configurations separately:
  1. Stripline only.
  2. Stripline + SMP-SMA transition Printed Circuit Boards (PCBs).
  3. Transition PCBs only.

3. Key Contributions

Thermal Conductivity Modeling: The paper provides the first direct measurement of the thermal conductivity of the specific flexible stripline design used for CCAT. The data is split into two temperature regimes to account for the non-linear behavior of OFHC copper:
- 6 K < T < 20 K: $kA = 22 \pm 6 \, T^{0.84 \pm 0.09} \, \mu\text{W m K}^{-1}$
- 20 K < T < 80 K: $kA = 0.6 \pm 0.3 \, T^{-0.4 \pm 0.1} \, \text{mW m K}^{-1}$
Identification of Crosstalk Source: The study successfully isolated the source of elevated crosstalk in the readout chain. It was determined that the stripline itself was not the culprit (crosstalk >50 dB below transmission); rather, the SMP-SMA transition PCBs were the primary source of interference.
Design Optimization for Prime-Cam: Based on thermal modeling, the authors validated a redesign for Prime-Cam striplines that reduces copper thickness (9 µm vs. original 18 µm) and increases the distance between thermal stages.

4. Results

Thermal Performance:
- The RRR measurement yielded a value of ~120, confirming the use of high-purity OFHC copper.
- Mod-Cam Loading: The estimated thermal loading for the current Mod-Cam setup is 0.64 W per readout harness at the 4 K stage.
- Prime-Cam Prediction: Without modification, a fully populated Prime-Cam (3 harnesses) would load ~5–6 W at 4 K, which is unsustainable. However, the redesigned Prime-Cam stripline reduces this predicted load to ~1.5 W total (0.53 W per harness), bringing it within the available thermal budget.
Electrical Performance:
- Stripline: Inter-channel crosstalk on the stripline alone was >50 dB below the signal, meeting requirements.
- Transition PCBs: The original transition PCBs caused crosstalk levels up to 30 dB higher than non-shared configurations. Specific traces showed crosstalk of -40 dB at 1.4 GHz.
- Redesign Validation: Simulations of the new PCB design (using impedance-matched vias to bury signal traces between ground planes) predict crosstalk levels of -60 dB (single structure) and -57 dB (full PCB), representing a ~20 dB improvement and satisfying project requirements.

5. Significance

This work validates flexible stripline circuits as a viable, scalable solution for reading out large-format KID arrays (100,000+ detectors) in cryogenic environments.

Feasibility: It demonstrates that the thermal load of these circuits can be managed within the strict cooling limits of the FYST telescope, enabling the deployment of Prime-Cam.
Engineering Solution: By identifying the transition PCBs as the bottleneck for signal integrity, the authors provided a clear path for redesign, ensuring that the high-density readout will not suffer from signal degradation.
Future Impact: The successful characterization and optimization of these circuits are critical for the success of the CCAT/FYST project, which aims to conduct unprecedented sub-millimeter observations of the early universe (Epoch of Reionization) and cosmology (Sunyaev-Zel'dovich effect). The results directly inform the fabrication of the Prime-Cam readout harnesses currently under construction.