Microwave Multiplexing Readout Development

As NASA telescopes push for ever greater sensitivity, one of the primary hurdles is reading out the large arrays of sensors. Many of the next generation of space telescopes, including the Origins Space Telescope and Lynx, will require reading out thousands of superconducting sensors on a single line. Microwave multiplexing is a technology that has already begun enabling high-density readout. Microwave multiplexers rely on superconducting resonators that shift frequency due to input signals. The resonator frequencies are measured with warm electronics by sending and measuring probe tones. A team centered at SLAC and Stanford University designed and deployed a microwave multiplexing system named SMuRF. Microwave multiplexing requires fast digital signal processing, which is currently implemented on FPGAs. The FPGAs communicate with ADC and DACs via a high speed serial link called JESD. The JESD layer has high power demands. Devices called Radio Frequency Systems-on-Chip (RFSoC) have recently been introduced to the market. The RFSoC combines the FPGA, ADC, and DACs onto one device, eliminating the JESD layer. We propose to replace the current FPGAs, ADCs, and DACs with the RFSoC, dropping the energy requirements of the microwave multiplexer. A key technical hurdle for microwave multiplexing is the development of tone tracking, an algorithm designed to track resonator frequencies and ensure that probe tones always lie at the resonator minimum. This reduces the power transmitted to amplifiers. Nonlinearities in the amplifiers generate large numbers of third-order harmonics which act as a pseudo-noise floor. By lowering the input power to the amplifier, tone tracking enables multiplexing factors in the thousands. SMuRF is the only microwave multiplexing system to successfully demonstrate tone tracking. Both the current SMuRF FPGA and new RFSoC are built on the Xilinx Ultrascale+ firmware architecture. This will make it simpler and less expensive to port the existing tone tracking code to the new devices. In this proposal, we outline a plan to develop the RFSoC boards, integrate the new multiplexer with X-ray sensors, measure X-rays, and test the radiation hardness of the system. This elevates the SMuRF microwave multiplexing electronics from TRL 3 to TRL 5. The schedule is designed to align with the Lynx development schedule. The SLAC and Stanford team is well positioned to successfully develop an RFSoC-based microwave multiplexer. The engineers have experience building the hardware and firmware. SMuRF is currently on a CMB telescope at the South Pole, which will provide a unique venue to test for subtle systematic effects. The RFSoC-based SMuRF microwave multiplexer will help unlock the next generation of telescopes used to study our cosmic origins. Whether through an X-ray telescope or infrared spectrometer, the future of observational astrophysics relies on ever increasing sensor counts. SMuRF will provide the critical technology to read them out.

The Strategic Astrophysics Technology program (SAT) supports focused development efforts for key technologies to the point at which they are ready to feed into major missions in the three science themes of the Astrophysics Division: Exoplanet Exploration, Cosmic Origins, and the Physics of the Cosmos. This program is specifically designed to address middle technology readiness level (TRL) "gaps" between levels 3 and 6: the maturation of technologies that have been established as feasible, but which are not yet sufficiently mature to incorporate into flight missions without introducing an unacceptable level of risk. NASA does not require a data management plan for proposals to SAT.

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