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.
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 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.
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.