The X-ray Integral Field Unit (X-IFU) is the high resolution X-ray spectrometer of the ESA Athena X-ray observatory. Over a field of view of 5’ equivalent diameter, it will deliver X-ray spectra from 0.2 to 12 keV with a spectral resolution of 2.5 eV up to 7 keV on ∼ 5” pixels. The X-IFU is based on a large format array of super-conducting molybdenum-gold Transition Edge Sensors cooled at ∼ 90 mK, each coupled with an absorber made of gold and bismuth with a pitch of 249 μm. A cryogenic anti-coincidence detector located underneath the prime TES array enables the non X-ray background to be reduced. A bath temperature of ∼ 50 mK is obtained by a series of mechanical coolers combining 15K Pulse Tubes, 4K and 2K Joule-Thomson coolers which pre-cool a sub Kelvin cooler made of a 3He sorption cooler coupled with an Adiabatic Demagnetization Refrigerator. Frequency domain multiplexing enables to read out 40 pixels in one single channel. A photon interacting with an absorber leads to a current pulse, amplified by the readout electronics and whose shape is reconstructed on board to recover its energy with high accuracy. The defocusing capability offered by the Athena movable mirror assembly enables the X-IFU to observe the brightest X-ray sources of the sky (up to Crab-like intensities) by spreading the telescope point spread function over hundreds of pixels. Thus the X-IFU delivers low pile-up, high throughput (< 50%), and typically 10 eV spectral resolution at 1 Crab intensities, i.e. a factor of 10 or more better than Silicon based X-ray detectors. In this paper, the current X-IFU baseline is presented, together with an assessment of its anticipated performance in terms of spectral resolution, background, and count rate capability. The X-IFU baseline configuration will be subject to a preliminary requirement review that is scheduled at the end of 2018.
The X-IFU (X-rays Integral Field Unit), one of the two instruments of the Athena mission, is a cryogenic Xray spectrometer for high-spectral resolution imaging. The large array of 3840 detectors each composed of an absorber coupled to a Transition Edge Sensor (TES) will be operated with a bath temperature of 50 mK. This instrument is designed to provide a challenging energy resolution of 2.5 eV in the 0.2 to 7 keV range. The DRE (Digital Readout Electronics) drives the frequency multiplexed readout of the sensors and implements the feedback required to optimise the detection chain dynamic range. To comply with the instrument energy resolution requirement, the constraints on the detection chain sub-systems are very stringent (thermal stability, signal to noise ratio, linearity,...). This implies a strong optimisation effort during the design of the sub-system in order to both satisfy the performance requirements and to fit in the mass, volume and power allocations. We have developed a numerical simulator of the X-IFU detection chain in order to validate the architecture of the DRE. The simulator implements the contributions of the different detection chain elements in the overall instrument performance. The details of the DRE architecture are included in the simulator and we use it to validate the different design options.
The Digital Readout Electronics (DRE) of the X-ray Integral Field Unit (X-IFU) instrument onboard Athena is made of two main parts: the DRE demultiplexor (DRE-DEMUX) and the DRE event processor (DRE-EP). The DRE-DEMUX drives the frequency domain multiplexed readout of the X-IFU Focal Plane Assembly (FPA) and it linearises the readout chains to increase their dynamic range. The DRE-EP processes the pixels’s data in order to detect the events and to measure the X-ray photon energy and arrival time. We have developed a prototype of the DRE-DEMUX module. We used a modular architecture with several boards in order to validate the different key functionalities one by one with a short design-test-rework cycle. To test the functionalities and performances of the DRE-DEMUX breadboards in a representative environment we developed several test equipments. Although the prototype is not flight representative in many aspects (EMC, power supplies, components grade, . . . ) it is intended to demonstrate the DRE-DEMUX functionalities and to validate the numerous operating procedures of our electronics. The preliminary tests conducted on the DRE-DEMUX prototype coupled to the dedicated test equipments validated its functionalities but also demonstrated that it is compliant with the its energy resolution requirement, which is the most constraining for the DRE.
In the framework of the ESA Athena mission, the X-ray Integral Field Unit (X-IFU) micro-calorimeter will provide unprecedented spatially resolved high-resolution X-ray spectroscopy. For this purpose, the on-board Event Processor (EP) must initially trigger the current pulses induced by the X-ray photons hitting the detector to proceed with a reconstruction which provides the arrival time, spatial location and energy of each event. The current event triggering design is implemented in two stages: one initial trigger of the low-pass filtered derivative of the raw data to extract records containing pulses and a second stage performing a fine detection to look for all the pulses in the record. In order to establish the current baseline detection technique of the EP in the X-IFU instrument, an assessment of the capabilities of different triggering algorithms is required, both in terms of performance (detection efficiency) and computational load, as processing will take place on-board. We present a comparison of two detection algorithms, the Simplest Threshold Crossing (STC) and the model-dependent Adjusted Derivative (AD). The analysis also evaluates the (possible) negative effect of different instrumental scenarios as a reduced sampling rate. The evaluations point out that the simplest algorithm STC shows worse performance than AD for the smallest pulses separations and the lowest secondary energies. Nevertheless, checking the expected number of such pulses combinations in a typical bright source observation, we conclude that it does not have impact in the science. Moreover, the savings in the computational resources and calibration needs make STC a valuable option.
The 96 read-out chains which are foreseen in the X-ray Integral Field Unit (XIFU) on ESA's L2 mission Athena to receive the signals from the 3840 X-ray microcalorimeter transition-edge sensors (TES), are based on the principle of Frequency Domain Multiplexing (FDM) with closed-loop baseband feedback (BBFB) to match the dynamic range of the read-out to that of the detectors. The XIFU instrument concept currently undergoes a Phase-A assessment. The Digital-to-Analogue Converters (DACs) which generate the carrier signals of the FDM and the signals of the BBFB loops were identified as critical elements. In this presentation we formulate the dynamic range requirements for the DACs and assess to what extend a current state-of-the-art system, based on Analog Devices AD 9726, meets these requirements. In this context, the need to place resonance frequencies on an exact grid, possibly with the assistance of frequency tuning PID loops, or increased accuracy of the lithographic production of the LC bandpass filters used in FDM, is discussed. Finally, the impact of pulse shape, in particular electrical bandwidth, on DAC performance is assessed.
The X-ray Integral Field Unit (X-IFU) on board the Advanced Telescope for High-ENergy Astrophysics (Athena) will provide spatially resolved high-resolution X-ray spectroscopy from 0.2 to 12 keV, with ~ 5" pixels over a field of view of 5 arc minute equivalent diameter and a spectral resolution of 2.5 eV up to 7 keV. In this paper, we first review the core scientific objectives of Athena, driving the main performance parameters of the X-IFU, namely the spectral resolution, the field of view, the effective area, the count rate capabilities, the instrumental background. We also illustrate the breakthrough potential of the X-IFU for some observatory science goals. Then we brie y describe the X-IFU design as defined at the time of the mission consolidation review concluded in May 2016, and report on its predicted performance. Finally, we discuss some options to improve the instrument performance while not increasing its complexity and resource demands (e.g. count rate capability, spectral resolution).
IRAP is developing the warm electronic, so called Detector Control Unit" (DCU), in charge of the readout of the SPICA-SAFARI's TES type detectors. The architecture of the electronics used to readout the 3 500 sensors of the 3 focal plane arrays is based on the frequency domain multiplexing technique (FDM). In each of the 24 detection channels the data of up to 160 pixels are multiplexed in frequency domain between 1 and 3:3 MHz. The DCU provides the AC signals to voltage-bias the detectors; it demodulates the detectors data which are readout in the cold by a SQUID; and it computes a feedback signal for the SQUID to linearize the detection chain in order to optimize its dynamic range. The feedback is computed with a specific technique, so called baseband feedback (BBFB) which ensures that the loop is stable even with long propagation and processing delays (i.e. several µs) and with fast signals (i.e. frequency carriers at 3:3 MHz). This digital signal processing is complex and has to be done at the same time for the 3 500 pixels. It thus requires an optimisation of the power consumption. We took the advantage of the relatively reduced science signal bandwidth (i.e. 20 - 40 Hz) to decouple the signal sampling frequency (10 MHz) and the data processing rate. Thanks to this method we managed to reduce the total number of operations per second and thus the power consumption of the digital processing circuit by a factor of 10. Moreover we used time multiplexing techniques to share the resources of the circuit (e.g. a single BBFB module processes 32 pixels). The current version of the firmware is under validation in a Xilinx Virtex 5 FPGA, the final version will be developed in a space qualified digital ASIC. Beyond the firmware architecture the optimization of the instrument concerns the characterization routines and the definition of the optimal parameters. Indeed the operation of the detection and readout chains requires to properly define more than 17 500 parameters (about 5 parameters per pixel). Thus it is mandatory to work out an automatic procedure to set up these optimal values. We defined a fast algorithm which characterizes the phase correction to be applied by the BBFB firmware and the pixel resonance frequencies. We also defined a technique to define the AC-carrier initial phases in such a way that the amplitude of their sum is minimized (for a better use of the DAC dynamic range).
We report on the development and characterization of the low-noise, low power, mixed analog-digital SIRIUS ASICs for both the LAD and WFM X-ray instruments of LOFT. The ASICs we developed are reading out large area silicon drift detectors (SDD). Stringent requirements in terms of noise (ENC of 17 e- to achieve an energy resolution on the LAD of 200 eV FWHM at 6 keV) and power consumption (650 μW per channel) were basis for the ASICs design. These SIRIUS ASICs are developed to match SDD detectors characteristics: 16 channels ASICs adapted for the LAD (970 microns pitch) and 64 channels for the WFM (145 microns pitch) will be fabricated. The ASICs were developed with the 180nm mixed technology of TSMC.
The detector system of the X-Ray Integral Field Unit (X-IFU), one of the two ATHENA focal plane instruments will be an ambitious step forward in the field of astronomical X-ray detection. We describe its baseline configuration, consisting of 3840 Transition Edge Sensors (TES) microcalorimeters with an energy resolution of 2.5 eV FWHM, spanning a 5 arcminute field-of-view and allowing an imaging resolution of 5 arcsec. The detectors are read out in 96 channels of 40 pixels each, using frequency domain multiplexing (FDM). Each channel contains a dual-stage SQUID pre-amplifier and a low-noise amplifier (LNA). In order to enhance the dynamic range of the SQUIDs a specific technique, baseband feedback (BBFB), is applied. The generation of the carrier and feedback signals, and the signal processing are done in the digital domain. We review the requirements for the main elements of this system, needed to ensure the high performance of the detector system. From the resolution requirements for the detectors follows a budget for contributions to the energy resolution on top of the intrinsic detector resolution. This budget forms the basis for the assessment of the dynamic range requirements for the SQUID and the LNA and the DACs and the ADC. Requirements are also derived for the levels of crosstalk and non-linearity in the readout chain.
We are developing the digital readout electronics (DRE) of the X-Ray Integral Field Unit (X-IFU), one of the two Athena focal plane instruments. This subsystem is made of two main parts: the DRE-DEMUX and the DRE-EP. With a frequency domain multiplexing (FDM) the DRE-DEMUX makes the readout of the 3 840 Transition Edge Sensors (TES) in 96 channels of 40 pixels each. It provides the AC signals to voltage-bias the TES, it demodulates the detector's data which are readout by a SQUID and low noise amplifiers and it linearizes the detection chain to increase its dynamic range. The feedback is computed with a specific technique, so called baseband feedback (BBFB) which ensures that the loop is stable even with long propagation and processing delays (i.e. a few μs) and with high frequency AC-bias (up to 5 MHz). This processing is partly analogue (anti aliasing and reconstruction filters) but mostly digital. The digital firmware is simultaneously applied to all the pixels in digital integrated circuits. After the demultiplexing the interface between the DRE-DEMUX and the DRE-EP has to cope with a data rate of 61.44 Gbps to transmit the data of the individual pixels. Then, the DRE-EP detects the events and computes their energy and grade according to their spectral quality: low resolution, medium resolution and high resolution (i.e. if two consecutive events are too close the estimate of the energy is less accurate). This processing is done in LEON based processor boards. At its output the DRE-EP provides the control unit of the instrument with a list including for each event its time of arrival, its energy, its location on the focal plane and its grade.
Athena is designed to implement the Hot and Energetic Universe science theme selected by the European Space Agency for the second large mission of its Cosmic Vision program. The Athena science payload consists of a large aperture high angular resolution X-ray optics (2 m2 at 1 keV) and twelve meters away, two interchangeable focal plane instruments: the X-ray Integral Field Unit (X-IFU) and the Wide Field Imager. The X-IFU is a cryogenic X-ray spectrometer, based on a large array of Transition Edge Sensors (TES), offering 2:5 eV spectral resolution, with ~5" pixels, over a field of view of 50 in diameter. In this paper, we present the X-IFU detector and readout electronics principles, some elements of the current design for the focal plane assembly and the cooling chain. We describe the current performance estimates, in terms of spectral resolution, effective area, particle background rejection and count rate capability. Finally, we emphasize on the technology developments necessary to meet the demanding requirements of the X-IFU, both for the sensor, readout electronics and cooling chain.
Proc. SPIE. 8452, Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy VI
KEYWORDS: Telescopes, Sensors, Field programmable gate arrays, Space telescopes, Signal processing, Electronic filtering, Space operations, Infrared telescopes, Digital electronics, Filtering (signal processing)
The SpicA FAR infrared Instrument (SAFARI) is a European instrument for the infrared domain telescope SPICA, a
JAXA space mission. The SAFARI detectors are Transistor Edge Sensors (TES) arranged in 3 matrixes. The TES front
end electronic is based on Superconducting Quantum Interference Devices (SQUIDs) and it does the readout of the
3500 detectors with Frequency Division Multiplexing (FDM) type architecture. The Detector Control Unit (DCU),
contributed by IRAP, manages the readout of the TES by computing and providing the AC-bias signals (1 - 3 MHz) to
the TES and by computing the demodulation of the returning signals. The SQUID being highly non-linear, the DCU has
also to provide a feedback signal to increase the SQUID dynamic. Because of the propagation delay in the cables and the
processing time, a classic feedback will not be stable for AC-bias frequencies up to 3 MHz. The DCU uses a specific
to compensate for those delays: the BaseBand FeedBack (BBFB). This digital data processing is done for the 3500 pixels
in parallel. Thus, to keep the DCU power budget within its allocation we have to specifically optimize the architecture of
the digital circuit with respect to the power consumption. In this paper we will mainly present the DCU architecture. We
will particularly focus on the BBFB technique used to linearize the SQUID and on the optimization done to reduce the
power consumption of the digital processing circuit.
One of the instruments on the Advanced Telescope for High-Energy Astrophysics (Athena) which was one of the three
missions under study as one of the L-class missions of ESA, is the X-ray Microcalorimeter Spectrometer (XMS). This
instrument, which will provide high-spectral resolution images, is based on X-ray micro-calorimeters with Transition
Edge Sensor (TES) and absorbers that consist of metal and semi-metal layers and a multiplexed SQUID readout. The
array (32 x 32 pixels) provides an energy resolution of < 3 eV. Due to the large collection area of the Athena optics, the XMS instrument must be capable of processing high counting rates, while maintaining the spectral resolution and a low deadtime. In addition, an anti-coincidence detector is required to suppress the particle-induced background. Compared to the requirements for the same instrument on IXO, the performance requirements have been relaxed to fit into the much more restricted boundary conditions of Athena.
In this paper we illustrate some of the science achievable with the instrument. We describe the results of design studies for the focal plane assembly and the cooling systems. Also, the system and its required spacecraft resources will be given.
The High Time Resolution Spectrometer (HTRS) is one of the five focal plane instruments of the International
X-ray Observatory (IXO). The HTRS is the only instrument matching the top level mission requirement of
handling a one Crab X-ray source with an efficiency greater than 10%. It will provide IXO with the capability
of observing the brightest X-ray sources of the sky, with sub-millisecond time resolution, low deadtime, low
pile-up (less than 2% at 1 Crab), and CCD type energy resolution (goal of 150 eV FWHM at 6 keV). The HTRS
is a non-imaging instrument, based on a monolithic array of Silicon Drift Detectors (SDDs) with 31 cells in a
circular envelope and a X-ray sensitive volume of 4.5 cm<sup>2</sup> x 450 μm. As part of the assessment study carried
out by ESA on IXO, the HTRS is currently undergoing a phase A study, led by CNES and CESR. In this
paper, we present the current mechanical, thermal and electrical design of the HTRS, and describe the expected
performance assessed through Monte Carlo simulations.
The High Time Resolution Spectrometer (HTRS) is one of six scientific payload instruments of the International X-ray
Observatory (IXO). HTRS is dedicated to the physics of matter at extreme density and gravity and will observe the
X-rays generated in the inner accretion flows around the most compact massive objects, i.e. black holes and neutron
stars. The study of their timing signature and in addition the simultaneous spectroscopy of the gravitationally shifted and
broadened iron line allows for probing general relativity in the strong field regime and understanding the inner structure
of neutron stars. As the sources to be observed by HTRS are the brightest in the X-ray sky and the studies require good
photon statistics the instrument design is driven by the capability to operate at extremely high count rates.
The HTRS instrument is based on a monolithic array of Silicon Drift Detectors (SDDs) with 31 cells in a circular
envelope and a sensitive volume of 4.5 cm<sup>2</sup> × 450 μm. The SDD principle uses fast signal charge collection on an
integrated amplifier by a focusing internal electrical field. It combines a large sensitive area and a small capacitance, thus
facilitating good energy resolution and high count rate capability. The HTRS is specified to provide energy spectra with
a resolution of 150 eV (FWHM at 6 keV) at high time resolution of 10 μsec and with high count rate capability up to a
goal of 2·10<sup>6</sup> counts per second, corresponding to a 12 Crab equivalent source. As the HTRS is a non-imaging instrument
and will target only point sources it is placed on axis but out of focus so that the spot is spread over the array of 31 SDD
cells. The SDD array is logically organized in four independent 'quadrants', a dedicated 8-channel quadrant readout chip
is in development.
This paper describes the Heterodyne Instrument for the Far-Infrared (HIFI), to be launched onboard of ESA's Herschel Space Observatory, by 2008. It includes the first results from the instrument level tests. The instrument is designed to be electronically tuneable over a wide and continuous frequency range in the Far Infrared, with velocity resolutions better than 0.1 km/s with a high sensitivity. This will enable detailed investigations of a wide variety of astronomical sources, ranging from solar system objects, star formation regions to nuclei of galaxies.
The instrument comprises 5 frequency bands covering 480-1150 GHz with SIS mixers and a sixth dual frequency band, for the 1410-1910 GHz range, with Hot Electron Bolometer Mixers (HEB). The Local Oscillator (LO) subsystem consists of a dedicated Ka-band synthesizer followed by 7 times 2 chains of frequency multipliers, 2 chains for each frequency band. A pair of Auto-Correlators and a pair of Acousto-Optic spectrometers process the two IF signals from the dual-polarization front-ends to provide instantaneous frequency coverage of 4 GHz, with a set of resolutions (140 kHz to 1 MHz), better than < 0.1 km/s. After a successful qualification program, the flight instrument was delivered and entered the testing phase at satellite level. We will also report on the pre-flight test and calibration results together with the expected in-flight performance.
We present a versatile digital autocorrelation spectrometer designed to suit the needs of HIFI, the sub-millimeter heterodyne instrument of the ESA's FIRST satellite. This spectrometer will offer a set of three observation modes with different `on-line resolution/total band-width' combinations (82 kHz/500 MHz, 163 kHz/1 GHz and 325 kHz/2 GHz). An original architecture based on mixed Gallium Arsenide and Silicon technologies, allowed us to realize a 1024 channel, low power consumption and high speed correlation module: 4 mW per channel at 550 MHz clock frequency. A prototype spectrometer has been developed. It includes a 2 X 250 MHz Image Rejection Mixer, a 500 MHz clock frequency analogue to digital converter, and two 1024 channel digital correlators. This model has been integrated and tested (in laboratory and on telescope). We expose these test results.
We are developing a wide band hybrid digital autocorrelator spectrometer suitable for the FIRST-HIFI instrument. At this time we have a prototype which is able to analyze four bands of 175 MHz each with a two bit three level digitizer clocked at 400 MHz. The correlation begins in a 64 channel Gallium Arsenide ASIC (Application Specific Integrated Circuit) clocked at 400 MHz and ends in a classical CMOS circuit at a lower frequency. The Gallium Arsenide technology presents several advantages for space applications, among which one can cite: (1) the power does not vary with the frequency of the signal analyzed, (2) the technology is very little sensitive to radiation. Our circuits are cascadable so that if several circuits are associated, we can build a versatile spectrometer with variable bandwidth and resolution. This presentation consists in a full description of this prototype spectrometer and a review of the current test results. We will also present a design of a spectrometer using the same technology and which will fit the needs of the heterodyne instrument of FIRST.