Full-Scale Readout Electronics for the ECHo-100k Experiment

TL;DR

The paper tackles the challenge of scaling readout for large cryogenic MMC arrays by implementing a full-scale, room-temperature software-defined radio (SDR) readout for a muMUX multiplexing scheme. It describes a modular three-unit hardware architecture that divides the 4–8 GHz band into five subbands, with digital baseband processing including flux-ramp demodulation and event detection. Key findings show a frequency comb with IRR > 40 dB, digital channel separation > 55 dB, and a non-limiting SDR noise contribution, along with a measured system linearity of $194 \pm 0.002\,\mu\mathrm{A}/\Phi_{0}$ (validated against a VNA). Collectively, the results demonstrate the feasibility of scalable, high-precision readout for thousands of cryogenic detectors using muMUX and SDR, informing the deployment strategy for the ECHo experiment.

Abstract

Recent advances in the development of cryogenic particle detectors such as magnetic microcalorimeters (MMCs) allow the fabrication of sensor arrays with an increasing number of pixels. Since these detectors must be operated at the lowest temperatures, the readout of large detector arrays is still quite challenging. This is especially true for the ECHo experiment, which presently aims to simultaneously run 6,000 two-pixel detectors to investigate the electron neutrino mass. For this reason, we developed a readout system based on a microwave SQUID multiplexer ($μ$MUX) that is operated by a custom software-defined radio (SDR) at room-temperature. The SDR readout electronics consist of three distinct hardware units: a data processing board with a Xilinx ZynqUS+ MPSoC; a converter board that features DACs, ADCs, and a coherent clock distribution network; and a radio frequency front-end board to translate the signals between the baseband and the microwave domains. Here, we describe the characteristics of the full-scale SDR system. First, the generated frequency comb for driving the $μ$MUX was evaluated. Subsequently, by operating the SDR in direct loopback, the crosstalk of the individual channels after frequency demultiplexing was investigated. Finally, the system was used with a 16-channel $μ$MUX to evaluate the linearity of the SDR, and the noise contributed to the overall readout setup.

Figures (4)

• Figure 1: Full-stack ECHo DAQ SDR electronics: digital processing board (left); analog/digital conversion board (center); high-frequency analog heterodyne mixer board (right).
• Figure 2: Frequency comb containing 4 of 5 subbands with 80 tones each, where the IQ imbalance is corrected and the power is equalized for all tones. A selected tone is highlighted and zoomed in.
• Figure 3: Evaluation of the channel separation. First, the analog wideband comb is separated into 5 subbands with 800 MHz each (left). Then, a polyphase filter bank within the digital domain channelizes individual tones into a TDM scheme. For simplicity, only an extract of the digital filter bank for one data stream is shown (right).
• Figure 4: Full SDR system characterization connected to the cryogenic domain. Left: power spectral density (PSD) of the noise amplitude on a single demodulated channel. PSD is normalized to the carrier tone power to compensate for different input powers at the ADC in the two setups due to attenuation by cryostat electronics. Since the noise level of the RF loopback is below the full cryogenic setup, the SDR electronics do not significantly influence the measurement accuracy. Right: linearity on a single channel measured with a series of triangular input signals of increasing amplitude. The SDR response was calculated to $194 \pm 0.002 \mu A/ \Phi_{0}$.