A TDMA Hybrid SQUID Multiplexer

We have developed a multiplexed read-out for transition-edge sensors (TES) based on a hybrid time- and frequency-domain basis set, similar to that used in time-division multiple-access (TDMA) mobile phones. The hybrid basis set uses bandwidth more efficiently than microwave frequency-division SQUID multiplexing, making it possible to multiplex more detectors in each output line. The high open-loop bandwidth provided by our SQUID TDMA system also makes it possible to multiplex large arrays of fast, high dynamic range detectors such as fast x-ray calorimeters. In this approach, we embed the second-stage SQUID amplifier of our standard time-division multiplexer in an impedance matching circuit coupled to a broadband cryogenic high electron mobility transistor (HEMT) in a microwave reflectometer configuration. The input signals are flux coupled into the first-stage SQUID amplifiers whose signals are time-division multiplexed into the second-stage SQUID. At room temperature, the signal from the HEMT is mixed down to dc for analysis and further signal processing.