Planck Pre-Launch Status Papers - APC - Université Paris Diderot ...

J.-M. Lamarre et al.: Planck pre-launch status: the HFI instrumentFig. 15. Flight model of a six-channel JFETs board mounted on the 50 Kstage. (Photo courtesy of Galileo Avionica.)Fig. 14. The REU (top) consistsof12belts(bottom) ofsixchannelseach.The PAU & REU electronics are made of about 58 000 electroniccomponents, with a total mass of 46 kg and a power supplyof only 88 W.4.2.3. Performance of the electronicsThe HFI bolometers provide an electrical signal in the nV rangeunder impedance of about 10 MΩ. Anypick-upfromexternalsignals can heat the bolometers and add noise (Yvon et al. 2008).An early analysis showed that it wasessentialtodesignthehardwarewith a high level of EMI immunity. This design was difficultbecause of the need for thermal isolation between warm andcold parts. Thus nanoVolt signals have to travel across more thanseven metres before significant voltage amplification takes placein PAU. This was achieved by designing the FPU as a closedbox and by using EM gaskets, special shielded wires and harnesses.To ensure continuity of the Faraday cage in between the4Kstage,18Kstage,JFETbox(locatedat50Kontheframeofthe telescope baffle) and the PAU on the service module (SVM),stainless steel bellows were used as a shield containing all thewires and harnesses.The harness between the PAU and REU units representsanother highly critical section of the readout chain sensitiveto EMI/EMC. Double shielding with aluminium and frequentgrounding of the harness provide a susceptibility of 5 V/masrequiredfor space equipment. In addition, a numerical simulationof the EMI susceptibility of the system and its grounding schemewas developped. As simulations predicted, the careful shieldingdescribed above was not satisfactory on its own: ground currentsflowing in the mechanical structure and in shields contaminatedthe readout through capacitive coupling with the high impedancereadout lines. We had to use materials (mainly Sapphire) showingexcellent thermal conduction and electrical isolation propertiesto open contamination paths at critical places in the mechanicalstructure surrounding the HFI instrument. This design wasvalidated by a series of specific tests.During the EMC tests performed in the ToulouseINTESPACE EMC laboratory, the conducted and radiated emissionas well as the susceptibility of the full readout electronicswere measured and found to match all spacecraft specifications.The system test at satellite level performed at CSL confirmedthe validity of the design. Nevertheless, as anticipated, we foundlines on the scientific signal produced by the periodic currentdriving the compressors of the 4 K cooler. These lines can beeasily removed from the signal (see the description of anomalousnoise and systematic effects in Sect. 3). The design of theelectronics also provided a low level of cross-talk between channels,with rejection ratios of typically −70 dB for the JFET boxand −110 dB for the PAU & REU. This was confirmed by testsat instrument level. The thermal stability of all critical componentswas chosen to provide a stability of the gains with temperaturebetter than 80 ppm/K. With other refinements of the designthat will be reported in a future paper, this thermal stabilitywas essential to ensure readout electronics free of low frequencynoise down to 0.016 Hz, which is the frequency at which theCMB dipole is measured (Fig. 16).Page 13 of 20