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

In this work, we report the experimental readout results of a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$µ$</tex-math></inline-formula> 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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$µ$</tex-math></inline-formula> MUX readout, focusing on bolometric applications. Finally, the minimum demodulated flux noise was obtained through the optimally conditioned readout of a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$µ$</tex-math></inline-formula> MUX channel. The results obtained showed a minimum demodulated white flux noise of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\approx 4 \mu \Phi _{0}/\sqrt{Hz}$</tex-math></inline-formula> in both demodulation domains (i.e. amplitude <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\gamma$</tex-math></inline-formula> and resonator phase <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\theta$</tex-math></inline-formula>) and dominated by the cryogenic microwave chain noise temperature of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$T_{n}^{cryo}=4.6$</tex-math></inline-formula> 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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$µ$</tex-math></inline-formula> MUX devices.