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arXiv:2509.23569 · IEEE Transactions on Applied Superconductivity

Microwave SQUID Multiplexer Readout Performance Using a Direct-RF RFSoC-Based Software-Defined Radio

Manuel Elías García Redondo, J. Bonilla-Neira, Nahuel Agustín Müller, Luciano Pablo Ferreyro, J.M. Geria, Timo Muscheid, R. Gartmann, Alejandro Almela, Matías Rolf Hampel, L.E. Ardila-Perez, Mathias Wegner, M. Platino, O. Sander, Sebastian Kempf, M. Weber

Published Mar 31, 2026• 0 citations
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Abstract

In this work, we report the experimental readout results of a$µ$MUX device using a Direct-RF Software-Defined Radio (SDR) prototype based on the ZCU216 Radio-Frequency System-on-Chip (RFSoC) evaluation board. First, the Direct-RF SDR architecture is described. Then, the analog performance of the SDR system was characterized both in loopback and when coupled to the cryogenic multiplexing system to evaluate the noise contributions of the SDR readout system. Later, the SDR system performance was optimized for$µ$MUX readout, focusing on bolometric applications. Finally, the minimum demodulated flux noise was obtained through the optimally conditioned readout of a$µ$MUX channel. The results obtained showed a minimum demodulated white flux noise of$\approx 4 \mu \Phi _{0}/\sqrt{Hz}$in both demodulation domains (i.e. amplitude$\gamma$and resonator phase$\theta$) and dominated by the cryogenic microwave chain noise temperature of$T_{n}^{cryo}=4.6$K. These results are comparable to those obtained with traditional SDR architectures and demonstrate that the Direct-RF SDR prototype is suitable for the readout of$µ$MUX devices.

DOI
10.1109/tasc.2026.3679166

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The Microwave SQUID Multiplexer (μMUX) is the device of choice for the readout of a large number of low-temperature detectors in a wide variety of experiments within the fields of astronomy and particle physics. While it offers large multiplexing factors, the system noise performance is highly dependent on the cold- and warm-readout electronic systems used to read it out, as well as the demodulation domain and parameters chosen. In order to understand the impact of the readout systems in the overall detection system noise performance, first, we extended the available μMUX simulation frameworks, including additive and multiplicative noise sources in the probing tones (i.e., phase and amplitude noise), along with the capability of demodulating the scientific data, either in the resonator’s phase or the scattering amplitude. Then, considering the additive noise as a dominant noise source, the optimum readout parameters to achieve minimum system noise were found for both open-loop and flux-ramp demodulation schemes in the aforementioned domains. Later, we evaluated the system noise sensitivity to multiplicative noise sources under the optimum readout parameters. Finally, as a case study, we evaluated the optimal demodulation domain and the expected system noise level for a typical software-defined radio readout system. This work leads to an improved system performance prediction and noise engineering based on the available readout electronics and the selected demodulation domain.
Microwave SQUID multiplexing (μμμMUX) is a widely used readout technique for large-scale transition-edge sensor (TES) arrays. It uses radio-frequency (RF) probe tones to interrogate cryogenic resonators, requiring frequency conversion between the baseband electronics and the cryogenic RF signal chain. This work describes the RF Board, a room-temperature frequency-conversion board deployed in the AliCPT μμμMUX readout system. The board up-converts 0-4 GHz baseband I/Q signals to the 4-8 GHz RF band for injection into the cryogenic chain and down-converts returned RF signals to baseband I/Q for ADC digitization. For the current 1000-tone operation, with a DAC output tone power of -30 dBm/tone, the required power windows are -35 to -25 dBm/tone for the RF tones transmitted to the cryostat and -45 to -35 dBm/tone at the ADC input for the returned tones. The RF Board is characterized using swept single-tone measurements covering up-conversion, down-conversion, and RF loopback. Based on these measurements, the RF output power is calculated to be -31.06 to -25.53 dBm, satisfying the RF output window. Assuming a representative cryogenic-chain transmission of -40 dB, the loopback result gives an estimated returned power of -45 to -35 dBm, within the target range. These results show that the RF Board meets the wideband frequency-conversion and tone-power requirements for the μμμMUX readout system.
Transition Edge Sensors (TESs) are highly sensitive cryogenic detectors widely used in astronomy, high-energy physics and nuclear radiation detection. As TES is increasingly deployed in large-scale arrays to improve sensitivity and effective collecting area, multiplexed readout becomes essential for reducing system complexity. Microwave SQUID multiplexing (μMUX) technology combining superconducting resonators with radio-frequency superconducting quantum interference devices (RF-SQUIDs), represents a state-of-the-art solution. In this work, a room-temperature readout prototype for TES μMUX readout was designed. The system features a scalable two-stage architecture, including a highly integrated, chassis-independent front-end signal processing module and back-end event-reconstruction module. A high-frequency-resolution frequency-comb signal generation method with real-time, per-tone frequency and phase tunability was developed. In addition, optimization of the firmware implementation effectively overcomes the computational-resource bottleneck of the Field Programmable Gate Array (FPGA). The prototype supports autonomous frequency sweeping, parameter diagnostics, and online event reconstruction. Self-test shows S 21 phase noise better than −110 dBc/Hz at −30 dBm, remaining stable with increasing channel count. Joint tests with a μMUX cryogenic system successfully captured the RF-SQUID response and achieved an equivalent noise current of 44 pA/√Hz referred to the TES, validating the prototype as a practical solution of the room-temperature system for TES μMUX readout.