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

M. Bersanelli et al.: Planck pre-launch status: Design and description of the Low Frequency InstrumentFig. 4. The LFI instrument in the configuration for instrument level testcryogenic campaign.Fig. 3. LFI RAA. Top:drawingoftheintegratedinstrumentshowingthe focal plane unit, waveguide bundle and back-end unit. The elementsthat are not part of LFI hardware (HFI front-end, cooler pipes, thermalshields) are shown in light grey. Bottom: moredetailsarevisibleintheexploded view, as indicated in the labels.the payload module after integrating the HFI 4 K box. Figure 5shows the LFI within the Planck satellite.3.3. Receiver designThe LFI receivers are based on an additive correlation concept,or pseudo-correlation, analogous to schemes used in previousapplications in early works (Blum 1959), as well as inrecent CMB experiments (Staggs et al. 1996; Jarosik et al. 2003).The LFI design introduces new features that optimise stabilityand immunity to systematics within the constraints imposed bycryogenic operation and by integration into a complex payloadsuch as Planck.TheFEMcontainsthemostsensitivepartofthereceiver, where the pseudo-correlation scheme is implemented,while the BEM provides further RF amplification and detection.In each radiometer (Fig. 6), after the OMT, the voltagesof the signal from the sky horn, x(t), and from the referenceload, y(t), are coupled to a 180 ◦ hybrid that yields the mixedsignals (x + y)/ √ 2and(x − y)/ √ 2atitstwooutputports.Thesesignals are then amplified by the cryogenic low-noise amplifiers(LNAs) characterised by noise voltage, gain, and phase n F1 , g F1 ,φ F1 and n F2 , g F2 , φ F2 .Oneofthetwosignalsthenrunsthroughaswitchthatshiftsthephasebetween0and180 ◦ at a frequencyof 4096 Hz. A second phase switch is mounted for symmetry andredundancy on the other radiometer leg, but it does not introduceany switching phase shift. The signals are then recombined by asecond 180 ◦ hybrid coupler, thus producing an output, which isasequenceofsignalsproportionaltox(t) andy(t) alternatingattwice the phase switch frequency.In the back-end modules (Fig. 6), the RF signals are furtheramplified in the two legs of the radiometers by room temperatureamplifiers characterised by noise voltage, gain and phase n B1 ,g B1 , φ B1 and n B2 , g B2 , φ B2 .Thesignalsarefilteredandthendetectedby square-law detector diodes. A DC amplifier then booststhe signal output, which is connected to the data acquisition electronics.The sky and reference load DC signals are integrated,digitised, and then transmitted tothegroundastwoseparatedstreams of sky and reference load data.The sky and reference load signals recombined after the secondhybrid in the FEM have highly correlated 1/ f fluctuations.This is because, in each radiometer leg, both the sky and thereference signals undergo the same instantaneous fluctuationsdue to LNAs intrinsic instability. Furthermore,thefastmodulationdrastically reduces the impact of 1/ f fluctuations comingfrom the back-end amplifiers and detector diodes, since theswitch rate ∼4 kHzismuchhigherthanthe1/ f knee frequencyof the BEM components. By taking the difference between thePage 7 of 21