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Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy XI

The FDM readout for the LSPE/SWIPE TES bolometers

Davide Vaccaro, Alessandro Massimo Baldini, F. Cei, Luca Galli, Marco Grassi, Donato Nicolò, M. Piendibene, Franco Spinella, A. Tartari, G. Signorelli

Published Jul 13, 2018• 2 citations
Publisher

Abstract

We present the design and experimental demonstration of a 16-channel frequency domain multiplexing (FDM) readout for transition-edge sensor (TES) bolometers. This readout system is going to be implemented on the LSPE/SWIPE balloon-borne experiment, whose goal is to detect the polarization of cosmic microwave background (CMB) at large angular scales and whose launch is scheduled for December 2019. We describe the fabrication process of the Niobium superconducting inductors and the qualification tests performed in our 300 mK cryogenic facility in INFN Pisa of the boomerang shaped PCBs hosting the LC chains and the gradiometric SQUIDs, which are going to be mounted on the back of the SWIPE focal planes. The development of the warm readout electronics is presented, together with the firmware for the generation and readout of the biasing frequency comb.

DOI
10.1117/12.2310148

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We present the design and experimental demonstration of a 16-channel frequency domain multiplexing (FDM) readout for transition-edge sensor (TES) bolometers. This readout system is going to be implemented on the LSPE/SWIPE balloon-borne experiment, whose goal is to detect the polarization of cosmic microwave background (CMB) at large angular scales and whose launch is scheduled for December 2019. We describe the fabrication process of the Niobium superconducting inductors and the qualification tests performed in our 300 mK cryogenic facility in INFN Pisa of the boomerang shaped PCBs hosting the LC chains and the gradiometric SQUIDs, which are going to be mounted on the back of the SWIPE focal planes. The development of the warm readout electronics is presented, together with the firmware for the generation and readout of the biasing frequency comb.
LiteBIRD is a satellite mission designed to map the polarization of the Cosmic Microwave Background (CMB) at degree and larger scales from 40 to 402 GHz. LiteBIRD will use 4,600 Transition-Edge Sensor (TES) bolometers biased and read out using Digital Frequency Domain Multiplexing (DfMux). The DfMux implementation for LiteBIRD uses sub-kelvin Superconducting Quantum Interference Device (SQUID) at the same 0.1 K thermal stage as the detectors, this allows for reduced parasitic impedances within the mK circuit and improved SQUID performance. Additionally it must work in the integrated system with the spacecraft’s wiring harnesses, which will be longer than is typical on similar ground based experiments, and therefore have more significant parasitic impedances which will impact readout performance. The properties of SQUID candidates at millikelvin temperatures and effects of the spacecraft-like meter scale wiring harness are investigated. Additionally, the possibility of inductively rather than resistively biasing our bolometers at the 0.1K stage, to reduce power dissipation in the bias element, is investigated. We will report progress on validating the cryogenic components of this readout system.
The new generation of Cosmic Microwave Background (CMB) experiments exploits the properties of low-temperature detectors, particularly superconducting Transition Edge Sensor (TES) bolometers, whose characteristics make them ideal for present and future CMB measurements. Frequency Domain Multiplexing (FDM) configuration is performed by connecting an LC filter in series with each TES. The presence of the LC filters and the SQUID input coil and wiring in series with all the LC channels cause systematic electrical crosstalk.In this work we present the tests and characterization performed on an LC chip produced by SRON (Space Research Organization Netherlands) for the experiment SWIPE. These LC chips are composed of lithographically fabricated inductors and by parallel plate capacitors. The measurements were focused on retrieving the resonance frequencies to be compared to the expected design values, the quality factor and the parasitic resistance. The results were consistent with previous measurements performed on similar setups, showing Q-factors above104\mathbf {10^{4}}104and parasitic resistance on the order of10−2  Ω\mathbf {10^{-2}\;\Omega }10−2Ω. The dependence of the properties of the LC filters and the SQUID on the temperature and the bias settings was studied. Measurements to define the level and the major contributors to electrical crosstalk were performed. Both modeling and experimental results indicated an electrical crosstalk level on the order of 10-4.
For the next generation of Cosmic Microwave Background (CMB) experiments, kilopixel arrays of Transition Edge Sensor (TES) bolometers are necessary to achieve the required sensitivity and their science goals. We are developing read-out electronics for POLARBEAR-2 CMB experiment, which multiplexes 32-TES bolometers through a single superconducting quantum interface device (SQUID). To increase both the bandwidth of the SQUID electronics and the multiplexing factor, we are modifying cold wiring and developing LC filters, and a low-inductance superconducting cable. Using these components, we will show frequency domain multiplexing up to 3 MHz.
To enable the next-generation of bolometric cameras, we are developing the microwave SQUID multiplexer (μMUX). Upcoming receivers such as Simons Observatory, CCAT-prime, BICEP array, Ali-CPT, and CMB-S4 plan to instrument focal planes with 50,000-500,000 sensors. Sensor count is achieved by tiling many 150 mm-diameter densely packed detector arrays into these focal planes. The fabrication and quality of large-format bolometer arrays has been demonstrated and is now mature. In contrast, the readout technology required for next-generation receivers needs development. The sensitivity, low cross-talk, high multiplexing density, and small component size make the μMUX well-suited for this goal. In this approach, the TES signal modulates the inductance of an rf-SQUID that loads a high-Q microwave resonator. The coupled signal therefore modulates the microwave resonance frequency, which may be read out using homodyne techniques. By coupling each resonator to the same microwave feedline, many detectors can be read out on a single coaxial cable pair. The multiplexing density is in practice limited by signal bandwidth, allowable cross-talk, and the digitization bandwidth of room-temperature readout electronics. We present the design and performance of a scalable 64-channel multiplexer chip optimized for bolometric applications. We utilize a new quarter wave resonator design that increases the physical linear density by a factor of two, therefore achieving a smaller footprint for simplified detector packaging. Measurements of this design show 100 kHz resonator bandwidth, uniform 1.8 MHz frequency spacing, and an input referred current noise of 35 pA/√Hz that is well below the level of an optimized, background-limited TES bolometer. Using 8 daisy-chained and frequency scaled chips, we create a 512-channel multiplexer and use it to readout a 512 TES-bolometer array. We present the results of this large-scale μMUX demonstration including system yield, signal cross-talk, and an analysis of noise in various TES bias configurations. The result demonstrates the multiplexing density required to read out 2,000 sensors between 4-8 GHz.
We describe the development of a frequency-domain multiplexer (MUX) to read out arrays of superconducting transition-edge sensors (TES). Fabrication of large-format arrays of these sensors is becoming practical; however, reading out each sensor in the array is a major instrumental challenge that is possibly solved by frequency-domain multiplexing. Each sensor is AC biased at a different frequency, ranging from 380 kHz to 1 MHz. The sensor signal amplitude-modulates its respective AC bias frequency. An LC filter associated with each sensor suppresses Johnson noise from the other sensors. The signals are combined at a current summing node and measured by a single superconducting quantum interference device (SQUID). The individual signals from each sensor are then lock-in detected by room temperature electronics. Test chips with fully lithographed LC filters for up to 32 channels have been designed and fabricated. The capacitance and inductance values have been measured and are close to the design goals. We discuss the basic principles of frequency-domain multiplexing, the design and testing of the test chips, and the implementation of a practical system.
In contemporary cosmic microwave background experiments, bolometric detectors are often background limited, and in this case the sensitivity of instruments can only be improved by increasing the number of background-limited detectors, and so contemporary TES receivers contain as many pixels as possible. Frequency-domain multiplexing (fMUX) is one strategy for reading out many detectors with one SQUID. For any readout system, it is important to carefully evaluate the thermal design of detector, in conjunction with the readout bandwidth, in order to ensure stable electro-thermal feedback (ETF). We demonstrate a novel technique for characterizing the thermal circuit of our detectors, using am AC-bias and the fMUX electronics. This technique is used to study the internal thermal coupling of a TES bolometer. We illustrate how the insights gathered by this technique have been instrumental in improving the stability of our multiplexed detectors for the south pole telescope (SPT).