Transformer-Coupled TES Frequency Domain Readout Prototype

Frequency-domain multiplexing (fMUX) is a mature readout scheme for transition edge sensor (TES) detectors in the millimetre and sub-millimetre bands. It is implemented at MHz carrier frequencies for the South Pole Telescope, POLARBEAR, and Simons Array and is planned for deployment on the LiteBIRD space polarimeter. The existing implementations couple to the detectors with low-noise, low-input-impedance superconducting quantum interference device (SQUID) transimpedance amplifiers and rely on complex arrangements to achieve sufficient linearity and dynamic range. We introduce a new cryogenic amplification scheme that couples the multiplexed TES devices to a traditional, high-linearity, high-dynamic-range, field-effect transistor amplifier using a novel high-turns-ratio, wide-band cryogenic transformer. We characterize the bandwidth, transimpedance, input impedance, and system noise of the transformer-coupled fMUX system to demonstrate that it is a promising candidate for MHz frequency-domain multiplexing without the use of SQUIDs. For the initial prototype, we demonstrate a bandwidth of 1.5–6 MHz, transimpedance of $$267 \pm 2 \,{\Omega}$$, input impedance of $$0.2\,{\Omega}$$, and system noise $$16 \pm 3\,{\mathrm {pA}}/\sqrt{{\mathrm {Hz}}}$$. We also present an optimized design which will have amplifier noise contributions of approximately $$5 \,{\mathrm {pA}}/\sqrt{{\mathrm {Hz}}}$$, comparable to the SQUID contribution in the existing systems.