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

F. Villa et al.: Calibration of LFI flight model radiometersFig. 9. Functional test performed at cryogenic temperature on RCA26.The two lines (orange and cyan) refer to the sky and load signals, whenthe 4 KHz switching is activated. Outside the interval 310–350 s the twocurves are indistinguishable.the output voltage of the detector S-10 when the functional testis run: the BEM is switched on, the FEM is biased at nominalconditions and the fast 4 KHz switching is activated on the phaseswitches. It is evident that most of the change in signal is experiencedwhen the BEM is on and the phase switches are biasedcorrectly. These functional tests were also used as a referencefor further tests up to the satellite-level verification campaign,besides checking the functionality of the RCA to proceed withthe calibration.4.2. TuningBefore tuning the RCA, the DAE was set-up to read the voltagefrom each detector of the BEM, V BEM ,withappropriateresolution.The output signal from the DAE, V DAE ,isgivenbyV DAE = G DAE · (V BEM − O DAE ) . (14)The DAE gain, G DAE ,wassettoensurethatthenoiseinducedby the DAE did not influence the noise of the radiometric signalfrom the BEM detectors. The voltage offset, O DAE was adjustedto guarantee that the output voltage signal lay well within the[−2.5, +2.5] Volts range when the gain was set properly for theinput temperature range. G DAE and O DAE were set for each of thefour detectors and employed during all noise property tests.The aim of the RCA tuning procedure (RCA_TUN) wastochoose the best bias conditions for each FEM low noise amplifiers’(LNA) gate voltage and phase switch current. Each ofthe four LNAs in a 30 GHz FEM consists of four amplificationstages (five for the FEM at 44 GHz), each driven by the samedrain voltage, V d .ThegatevoltageV g1 biases the first amplificationstage, while V g2 biases the successive three (or four) stages.The phase switches are driven by two currents (I 1 and I 2 ), biasingeach diode. The currents determine the amount of attenuationby each diode and thus are adjusted toobtainthefinaloverallradiometerbalance.The phase switches between the LNA and the second hybriduse the interconnection of two hybrid rings to improve the bandwidthand the matching with two Shunt PIN diodes. Dependingon the polarization of the diodes the signal travels into a circuit,which can be λ/2 longer,sothatitisshiftedby180 ◦ .Detailscan be found in Hoyland (2004) andinCuttaia et al. (2009). InFig. 10. Conceptual scheme of the phase switch integrated into the radiometers.Each phase switch is composed of two diodes commandedby the currents I1 and I2. They act as a on/off switch. Depending onthe polarization state of the diodes the signal follows the magenta pathor the cyan λ/2 shiftedpath.Thetwocurrentsatwhichthediodesaretuned determine the attenuation of each path, represented here by thedifferent thickness. While the phase matching depends on the particularRF design, the amplitude matching depends on the (I 1 , I 2 )biassupplyof the diodes which is the goal of the phase switch tuning.Fig. 10 we report the conceptual schematic diagram of the phaseswitch.At 30 and 44 GHz the phase switches were tuned withone radiometer leg switched off. Intheseconditionsthesignalentering each phase switch diode is the same, and the output signalat the DAE can be used directly to precisely balance the twostates of the phase switch. Any differences in the sky and refsignal are caused only by the phase switches and not by othernon-idealities of the radiometers, nor by different input targettemperatures. The two currents were chosen to minimize thequadratic differences, δ PSW ,betweenoddandevensamplesofthe signal (corresponding in Fig. 10 to the magenta and cyanpath respectively). If for example the phase switch was tuned onthe same leg as the amplifier M1, the following expression wasminimized:δ PSWM1=√ (So00− S e 00) 2+(So01− S e 01) 2, (15)where S 00 , S 01 are the two DAE outputs related to the “M” halfFEM. In this case o and e refer to odd and even signal samples.The same differences for the other phase switches, δ PSWM2 ,δ PSWS1,andδ PSWS2were calculated in the same way. The I 1 andI 2 were varied around the best value obtained during the FEMstand-alone tests (Davis et al. 2009). The phase switches of the70 GHz RCAs were not tuned. To reduce the transient, the phaseswitches were always biased at the maximum allowable current.The front-end LNAs were tuned for noise temperature performance,T n ,andisolation,I. ForeachchannelT n and I weremeasured as a function of the gate voltages V g1 and V g2 .Firstly the minimum noise conditions were found by varyingV g1 while keeping V g2 and V d fixed. The noise temperature wasmeasured with the Y-factor method (see Appendix A for the detailsof this method). Because only relative estimates of Tn arerelevant for tuning purposes, we did not correct for the effect ofnon-linearity in the 30 and 44 GHz RCAs.Once the optimum V g1 was determined, the optimum V g2 wasfound by maximizing the isolation, I,I =∆V sky − G 0 · ∆T sky(∆Vref − G 0 · ∆T sky)+∆Vsky, (16)Page 7 of 14