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Description
Cryogenic detector arrays demand scalable, low-noise readout electronics to preserve their superior energy resolution. Frequency-division multiplexing enables large channel counts but leads to high amplifier power and potential signal distortion when using static carrier tones.
Continuously tracking the resonance frequencies is mitigating this problem, but requires the readout electronics to dynamically update the stimulation frequencies. This work presents an FPGA-based implementation based on a polyphase synthesizer and numerically controlled oscillators to solve this challenge. The system additionally improves frequency resolution and reduces memory usage compared to the current implementation, establishing a foundation for tone tracking in high-bandwidth detector systems.
Summary (500 words)
Cryogenic detectors significantly outperform conventional semiconducting sensors in energy resolution, making them an attractive option for next-generation particle physics experiments. However, scaling these systems to large detector arrays introduces substantial challenges in readout complexity and thermal management. To limit heat load from room-temperature readout electronics, multiplexing schemes like time-division multiplexing (TDM) or frequency-division multiplexing (FDM) are widely employed. FDM couples multiple detectors via resonators to a common transmission line, with each detector signal encoded as a resonance frequency modulation of its associated circuit.
Traditional FDM readout schemes rely on static carrier tones placed near the resonances. Detector signals are inferred from amplitude modulation caused by shifts in resonance frequency. While straightforward to implement, this method becomes increasingly inefficient as the frequency offset between carrier and resonance grows. In such cases, the total power seen by the cryogenic low-noise amplifier increases, limiting the achievable multiplexing factor as the amplifier needs to be operated below its P1 dB point to avoid saturation and signal distortion.
To address this limitation and enable efficient scaling the channel count, tone tracking has emerged as a promising approach. By dynamically adjusting carrier frequencies to follow resonance shifts in real time, the system maintains optimal alignment, thereby minimizing total power and improving linearity. Implementing tone tracking, however, requires significant extensions to the digital signal processing capabilities of the room-temperature readout electronics, particularly in terms of frequency-comb generation with low latency and high frequency resolution.
This work presents an FPGA-based system for adaptive tone generation using a polyphase filter bank to generate a wideband frequency comb based on multiple baseband signals. These baseband signals are calculated online by a numerically controller oscillator (NCO), which enables adjusting the frequency of individual carriers at run-time.
The resulting architecture performs bidirectional conversion between baseband and a wideband multiplexed spectrum, effectively forming the backbone of the digital front-end. In addition to enabling adaptive carrier generation, the system improves frequency resolution and incorporates mechanisms for automatic calibration of the frequency demodulation stage. These features are critical for maintaining signal fidelity in high-density multiplexing scenarios. A major design focus was placed on minimizing FPGA resource utilization and processing latency, both of which are critical constraints for real-time tone tracking, especially in systems with high detector bandwidth.
Performance evaluation shows that the system meets key requirements for future integration into a full closed-loop tone tracking scheme. The next steps involve implementing feedback control to continuously adjust carrier frequencies based on detected resonance shifts, completing the tone tracking loop. This progression will enable higher multiplexing factors needed for future large-scale cryogenic detector arrays.