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

A&A 520, A4 (2010)Fig. 19. Picture of the lower part of the LFI RAA during an advancedphase of the instrument flight-model integration showing the LFI backendunit. The two lateral trays hosting the radiometer BEMs are symmetricallydisposed to the left and right sides of the DAE-BEU box. Thelower part of the straight stainless steel waveguides are shown.trend analysis and systematic error tests. These data are used extensivelyduring the functionality checks of the radiometers.The two DAE “lateral trays” contain the circuitry needed toprovide the power supply to the RCAs, divided into four powergroups (Fig. 9). These power supplies are independent of eachother to minimise crosstalk and interference. The bias of eachFEM and the controls for the phase switches are regulated to selectablevoltage levels and filtered to achieve a minimum levelof conducted noise. Particularly critical for the instrument performanceis the optimal biasing of the FEM amplifiers. As describedin Sect. 4.1.5, thebiasvoltagesofthefirstLNAstageand of the following stages are programmable separately.Fig. 18. Pictures of the LFI waveguides mounted on the RAA during theintegration of the LFI flight model. Top:aviewofthestraightSSsections,black-painted on the outside and arranged in groups of four. Onthe upper part the Cu sections are connectedthroughmultipleflanges.The upper and lower mechanical support structures are also visible.The three interface levels corresponding to the three V-grooves are alsoshown. Bottom:backviewoftheLFIfront-endunitshowingthetwistedCu sections connecting to the FEMs. The waveguide routing and centralhole in the main frame are designed to interface with the HFI front-end4Kbox.exercised by the radiometer noise varies from 10 to 450, dependingon channel. Acquired data are converted into serial streamsand automatically transferred to the signal-processing unit in theREBA through synchronous serial links for processing and compression(Sect. 4.5).The DAE is also in charge of collecting and storing housekeepingdata in a dedicated RAM. This information is retrievedby the REBA and organised into two dedicated packets with periodsof 1 and 32 s, depending on the needed monitoring frequency.Housekeeping parameters include current consumptionsin the FEMs and temperature sensors, which are essential for4.5. Radiometer electronics box (REBA)Downstream of the DAE, the LFI signals are digitally processedby the REBA (radiometer electronics box assembly), which alsocontains the power supply for LFI and the interface with thesatellite SVM. The electronics hardware and on-board softwareare discussed by (Herreros et al. 2009). The REBA is a fully redundantunit, and it is internally separated into different subunitsas shown schematically in Fig. 9.The signal processing unit (SPU) receives the raw digital sciencedata from the DAE and performs on-board signal averaging,data compression, and science telemetry packetisation. Theneed to reject 1/ f noise led to raw data sampling at 8192 Hz(122µs/sample), the LFI internal clock generator frequency. Theclock synchronously drives the phase switches in the FEMs, theADCs, and the on-board processor, which reconstructs the orderingof the acquired signals and synchronises it with the onboardtime. Taking housekeeping and ancillary information intoaccount, this corresponds to a data rate of ∼5.7 Mbps, or a factorof 100 higher than the allocated data rate for the instrument,53.5 Kbps. Averaging the samples from sky and reference-loadsignals to within the Nyquist rate on the sky (3 bins per HPBWat each frequency) drastically reduces the data volume, leavinga compression requirement of a factor 2.4 (see Sect. 6).The adopted algorithm implemented in the SPU relies on threestepprocessing of nearly loss-less compression that requires5-parameter tuning to be optimised. The details of the LFI datacompression strategy and end-to-end test results are discussedby Maris et al. (2009).Page 16 of 21