Josephson Arbitrary Waveform Synthesizer for Flux-Ramp Modulation in Microwave SQUID Multiplexers

Microwave SQUID multiplexers (μMUX) have become a key technology for the readout of large arrays of low-temperature detectors, particularly in cosmic microwave background experiments. A critical element of this technique is the flux-ramp modulation signal, whose noise, especially at low frequencies, directly limits the readout performance. This work proposes and experimentally demonstrates the use of a Josephson Arbitrary Waveform Synthesizer (JAWS) as a low-noise, high precision source for generating the sawtooth-shaped flux-ramp signal required in μMUX readout systems. The JAWS, based on the AC Josephson effect, provides intrinsically quantized and spectrally pure waveforms with excellent signal-to-noise ratio. The operation of a 9000-junction JAWS array is characterized and integrated with a cryogenic μMUX device and a custom SDR-based readout system. With this setup, the noise floor level around the harmonic typically used for phase demodulation of the SQUID signal to recover the detector signal was −104dBc/Hz, and the signal-to-noise ratio of this harmonic was 73dB. These results demonstrate the feasibility of using JAWS-based waveform synthesis for μMUX flux-ramp generation.