Mitigation of Finite Bandwidth Effects in Time-Division-Multiplexed SQUID Readout of TES Arrays

Abstract

Time division multiplexing (TDM) is being developed as the readout technology of the X-ray integral field unit (X-IFU), a 3,168-pixel X-ray transition-edge sensor (TES) imaging spectrometer that is part of the European Space Agency's Athena satellite mission. Recent improvements in the low X-ray event count rate performance of TDM have been driven by increases in multiplexer bandwidth and the mitigation of settling transients. These methods and design changes have improved the 32-row multiplexed resolution of a NASA LPA 2.5a array from an initial (2.73 ± 0.03) eV to (1.97 ± 0.01) eV resolution at 5.9 keV. We discuss these recent advances in TDM readout, which have been implemented in an 8-column × 32-row spectrometer that will be deployed at the Lawrence Livermore National Laboratory electron beam ion trap (EBIT) facility, and present a model that will inform the design of future systems.

Performance of a Broad-Band, High-Resolution, Transition-Edge Sensor Spectrometer for X-ray Astrophysics

“…Thus, we would naively expect the readout noise to be reduced from the 25.6 pA/ÝHz that we reported in our previous 40-row experiments [7] to 22.9 pA/ÝHz for 32 rows. However, improved filtering of the cryostat feedthrough lines in combination with optimized SQUID design parameters (higher junction critical current and higher junction shunt resistance [4]) has enabled us to now achieve 20.2 pA/ÝHz in these new 32-row measurements. Further improvements in readout noise for X-IFU are likely when the SQ1 M in coupling is re-optimized from 40-row TDM to 34-row TDM.…”

“…The reduction in cross-column crosstalk is critical to scale the system beyond 8 TDM columns. Details of these TDM design optimizations are discussed in [4]. The new multiplexer chips we use here are referred to as 'mux19a' and have a measured M in = 264 pH, similar to 'mux18b'.…”

“…Due to the limitations on spacecraft mass and power, the X-IFU array requires a multiplexed readout scheme. The preferred readout approach for X-IFU is time division multiplexing (TDM) developed at NIST (Boulder, CO) [3], [4]. TDM is a mature readout scheme that has been fielded in many laboratory instruments [3] and flown on a sounding rocket experiment [5].…”

“…To linearize the SQUID output and increase the dynamic range, the circuit is operated in a digitally fed-back, flux locked loop (FLL). Further details of the TDM architecture can be found in [3], [4]. The current TDM baseline for X-IFU is 96 columns each with 34 rows (including 33 science pixels and 1 ohmic resistor for independently tracking the gain of the readout chain.…”

Conceptual Design of the Modular Detector and Readout System for the CMB-S4 survey experiment

“…25 To increase the multiplexing factor we will incorporate several low-risk technological improvements that have been developed by NIST Boulder for the readout of much faster X-ray TESs. [26][27][28] Planned improvements include adding a shunt resistor across the SSA to increase the system bandwidth, 29 a new faster SSA design, and a new SQ1 design with a higher input mutual inductance. Taken together, preliminary studies indicate these improvements may enable substantially lower row switching rates and thereby multiplexing factors in excess of 120 rows per column, but we baseline 80 rows in the conceptual design.…”