Tone Tracking for Operation of Microwave-SQUID Multiplexers

by Benedikt Lukas Keppner

Wednesday Sep 23, 2026, 9:00 AM → 10:00 AM Europe/Berlin

Seminarraum 4. OG (IPE)

Final presentation of Benedikt Keppner's master thesis on the Implementation of Tone Tracking functionality for operation of microwave-SQUID multiplexed cryogenic detector arrays

Abstract (EN)

Research fields such as particle and astrophysics require detectors with exceptionally high energy resolution. Superconducting sensors operated in cryogenic environments offer such fidelity. Magnetic Microcalorimeters (MMCs) in combination with superconducting quantum interference devices (SQUIDs) are a solution for measuring energies with sub-electron-volt precision. To operate large arrays of such sensors, multiplexing schemes are mandatory. The microwave-SQUID-multiplexer (μMUX) employs frequency-division multiplexing by inductively coupling detector pixels and SQUIDs to resonators with individual resonance frequencies. A flux-ramp modulation technique is applied to linearize the periodic SQUID characteristic.
The room temperature electronics for μMUX readout developed at IPE is a versatile and modular software-defined radio (SDR) solution. Its core is a multi-processor system-on-chip (MPSoC) with an embedded field-programmable-gate-array (FPGA). The readout electronics enable data acquisition of frequency-multiplexed cryogenic detector arrays.
Currently, the multiplexing factor is limited by the 1-dB compression point of the cryogenic amplifier, which distorts the measurement result if driven at excessive input power. In this thesis, the development and implementation of a novel readout approach to overcome this limitation is described:
The tone tracking system is an extension to the current electronic stack. It is expected to improve the scalability of sensor multiplexing by tracking the generated carrier tone to the modulated resonance frequency in real-time and thus continuously operating the resonator in its S₂₁ transmission power minimum. Analyzing the received (RX) signal and correcting the carrier generation (TX) in a closed loop poses challenges and demands a specialized digital signal processing concept.
The development, implementation, and characterization of the FPGA-based tone tracking system are presented in this work. The system is capable of multi-channel readout of cryogenic μMUX using carrier frequencies continuously adjusted to the effective resonance frequency of the respective channel. This new approach for μMUX readout is established and experimentally evaluated. Noise, linearity, and bandwidth measurements are performed to characterize the implementation and allow a comparison to the conventional readout based on static carrier signals. Depending on the characteristics of the sensors, the tone tracking system reduces the input power to the cryogenic amplifier stage. A power reduction of 12 dB was achieved in the presented setup while preserving the noise level of the conventional readout. The functionality of the system is demonstrated using different measurement setups.