Improved Performance of TES Bolometers using Digital Feedback

TL;DR

This paper introduces digital feedback techniques, including Digital Active Nulling and Digitally Enhanced Voltage Bias, to improve the stability, linearity, and multiplexing capabilities of TES bolometers in astrophysics applications.

Contribution

It presents novel digital feedback methods implemented on FPGA to overcome impedance and dynamic range limitations in TES bolometers, enhancing their performance and stability.

Findings

Digital Active Nulling increases linearity and reduces input impedance.

Implementation of DAN and DEVB improves stability and multiplexing.

Methods are tested and ready for integration in current experiments.

Abstract

Voltage biased, frequency multiplexed TES bolometers have become a widespread tool in mm-wave astrophysics. However, parasitic impedance and dynamic range issues can limit stability, performance, and multiplexing factors. Here, we present novel methods of overcoming these challenges, achieved through digital feedback, implemented on a Field-Programmable Gate Array (FPGA). In the first method, known as Digital Active Nulling (DAN), the current sensor (e.g. SQUID) is nulled in a separate digital feedback loop for each bolometer frequency. This nulling removes the dynamic range limitation on the current sensor, increases its linearity, and reduces its effective input impedance. Additionally, DAN removes constraints on wiring lengths and maximum multiplexing frequency. DAN has been fully implemented and tested. Integration for current experiments, including the South Pole Telescope, will be discussed. We also present a digital mechanism for strongly increasing stability in the presence of large series impedances, known as Digitally Enhanced Voltage Bias (DEVB).