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The Planck Low Frequency Instrument (LFI)

The Low Frequency Instrument (LFI) on Planck uses state of the art microwave radiometers to map the sky in three frequency bands from 30 to 70 GHz. A radiometer measures radiation (microwave light) and produces a signal proportional to the temperature of the object being viewed. In a way, Planck is taking the temperature of the universe.

LFI feedhorns
LFI has 2 feedhorns at 30 GHz, 3 at 44 GHz and 6 at 70 GHz, as seen in this photograph taken during integration with HFI in early 2007. (Image credit: ESA)

The LFI uses highly sensitive microwave amplifiers. The devices in these amplifiers were designed and tested in a collaboration between the Jet Propulsion Laboratory and Northrup Grumman Space Technology. To measure the very faint signals from the cosmic microwave background (CMB), these amplifiers must be cryogenically cooled to 20 Kelvin (-253 C).

In order to achieve the extraordinary stability required by Planck, the LFI radiometers have a rather complex design called a pseudo-correlation continuous comparison receiver (Bersanelli 1995). This complicated structure effectively compares the temperature of the CMB to that of a thermally stable "reference load". This allows us to remove tiny changes in the gain of the cryogenic amplifiers and see the underlying signal from the sky.

Radiometer schematic
LFI "Continuous Comparison" Pseudo-correlation receiver radiometer schematic. One phase switch operates at 8192 Hz, providing alternately "Sky" and "Reference" outputs at each diode. Differencing these states effectively removes 1/f noise from the data.

The first prototype of the LFI radiometers was built and tested at the University of California, Santa Barbara in 1997.

UCSB Prototype Radiometer
This was the first prototype of an LFI radiometer. The cryogenic section is on the left. The layout follows the schematic in the previous figure. In later models, the phase switches and second hybrid coupler were included in the cryogenic section, forming the "Front End Module" or FEM.

Data from the 70 GHz engineering model
Data from the 70 GHz engineering model. Black curves show Sky and Reference state outputs, the red curve shows the difference. Offsets have been adjusted for clarity, all three lines are lowpass filtered at 10 Hz to highlight the 1/f noise.

Further refinement and development was carried out at JPL, Ylinen Electronics (Helsinki, Finland) and Jodrell Bank Observatory (Manchester, United Kingdom). After this, the flight units were built and put through a rigorous test campaign before being integrated with High Frequency Instrument (HFI) and the spacecraft bus.

70 GHz RCA photo
A flight model 70 GHz Radiometer Chain Assembly under test at Electro-Bit Microwave in Finland.

30 GHZ RCA photo
The 30 GHz Radiometer Chain Assembly being mounted in the RCA cryo testchamber at Alenia Spazio, Italy. The four waveguides are visible, the twisted copper section connected to the 20 K Front End Module, and the straight stainless steel section connecting to the 300 K Back-End Module. The horn feeds two radiometers, each carrying one of the linearly polarized components provided by an orthomode Transducer.

This plot shows the great reduction in 1/f noise achieved by the Planck radiometers. The raw radiometer output is shown in red, the difference data in black, and the average of the two output diodes in blue. The 1/f knee is reduced by nearly 3 orders of magnitude and the noise spectra are completely consistent with expectations.

Further Reading:
  • M. Bersanelli et al., Planck-LFI: Instrument Design and Ground Calibration Strategy, J. European Microwave Assoc., vol. 1, issue 3, 189-195 (1995)

  • L. Valenziano et al., The Los Frequency Instrument On-board the Planck Satellite: Characteristics and Performance, New Astron. Rev., 51, 287-297 (2007)
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