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

A&A 520, A6 (2010)Front EndModulesWaveguidesBack EndModulesFig. 6. Bologna design of the RCA sky load calibrator. The overall dimensionsin mm are reported in the drawing on the left. The pyramidson the bottom of the skyload are clearly visible on the right picture.The smaller dimensions of the feedhorns and front end modulesand the decision to use two small dedicated sky loads directlyin front of the horns allowed the cold part of the two RCAsunder test to be contained in a volume similar to that of the 30and 44 GHz chamber. Temperature interfaces such as FEMs,sky load and reference load were coupled together by means ofcopper slabs and then connected to the 4 K and 20 K coolers.The FEMs were controlled at their nominal temperature of 20 K;sky and reference loads were controlled in the range 10–25 Kwith a stability better than 10 mK; the back end modules wereinsulated from the chamber envelope by means of a supportingstructure without temperature control, which was considered unnecessary.3.4. The skyloads at 70 GHzThe design for the 70 GHz RCA skyload was made by YlinenElectronics. The basic design is shown in Fig. 8.Theloadconfigurationis a single folded conical structure in Eccosorb mountedin an aluminum housing. It is attached to a brass back plate. Asingle waveguide input is mounted through the back plate, providingthe method of applying RF stimulus signals through theabsorber for the RCA_SPR test (see Sect. 4.5). Load performanceswere measured over the whole V-band showing a return loss betterthan −20 dB. Two sensors were placed on the sky load, oneat the controller stage, referred to as T ctrl and one inside the absorber,T sky .Althoughthetemperaturealongtheskyloadwasexpected to be uniform due to its small dimensions, this was notthe case: due to the cool down effects, the thermal junction betweenthe temperature control and the load was not efficient asexpected. A typical difference in temperature within 4−7 Kwasobserved between the two thermometers. This cool-down effectwas not predictable so that the sky load was considered as a relativeinstead of an absolute temperature reference.Fig. 7. Top:two70GHzRCAsintegratedintheYlinenElectronicscryofacility.The two BEMs (on the right)areconnectedtothewaveguides,here surrounded by aluminum mylar. On the left the shroud contains thetwo horns facing the “Ylinen” skyload at about 50 K. Bottom: detailofthe front end. The two FEMs and the pair of horns are facing the twoskyload containers.4. Methods and results4.1. Functional testsFunctional tests were performed at ambient and at cryo temperature.All the RCAs were biased with nominal values andthe power consumption was verified. In addition, each phaseswitch was operated in the nominal mode to check its functionality.As an example the functional test performed at cryogenictemperature on RCA26 is shown in Fig. 9. ThefigurereportsFig. 8. Ylinen design of the RCA sky load calibrator. This design produceda load with −20 dB of return loss over the whole bandwidth. Thetwo light blue shaded regions represent the horn mouth at the left and therectangular waveguide injector on the right.Theabsorberisenclosedinametallicboxexceptforthepartfacingthehorn,whichisclosedwithaTeflonR○plate.Page 6 of 14