LFI design, qualification and performance
This section provides an overview of the Low-Frequency Instrument and of its different sub-systems.
- Instrument description
- Ground tests
- In-flight calibration
- Performance summary
- Systematic effects
- Sorption cooler
The Planck-LFI(Planck) Low Frequency Instrument instrument is an array of 11 radiometric receivers in the Ka, Q, and V bands, with centre frequencies close to 30, 44, and 70 GHz. The exact central frequencies for each receiver are reported in Planck-Early-III; for simplicity, here we will refer to the three channels using their nominal centre frequencies. A detailed description of the LFI(Planck) Low Frequency Instrument instrument is given in Planck-PreLaunch-IV, and references therein.
The heart of the LFI(Planck) Low Frequency Instrument instrument is a compact, multi-frequency array of differential receivers with ultra-low-noise amplifiers based on cryogenic indium phosphide (InP) high-electron-mobility transistors (HEMTs). To minimize power dissipation in the focal plane unit, which is cooled to 20 K, the radiometers are split into two subassemblies connected by a set of waveguides, as shown in Fig. 1, left panel.
The LFI(Planck) Low Frequency Instrument horns are situated in a ring around the HFI(Planck) High Frequency Instrument (see Fig. 1 – right panel). Each horn collects radiation from the telescope and feeds it to one or more detectors. The radiometer design is driven by the need to suppress 1/f-type noise induced by gain and noise temperature fluctuations in the amplifiers, which would be unacceptably high for a simple total power system. A differential pseudo-correlation scheme is adopted, in which signals from the sky and from a blackbody reference load are combined by a hybrid coupler, amplified in two independent amplifier chains, and separated out by a second hybrid (Fig. 2). The sky and the reference load power can then be measured and differenced. Since the reference signal has been subject to the same gain variations in the two amplifier chains as the sky signal, the true sky power can be recovered. Insensitivity to fluctuations in the back-end amplifiers and detectors is realised by switching phase shifters at about 8 kHz synchronously in each amplifier chain. The rejection of 1/f noise, as well as the immunity to other systematic effects, is optimized if the two input signals are nearly equal. For this reason the reference loads are cooled to approximately 4 K by mounting them on the 4K structure of the HFI(Planck) High Frequency Instrument. In addition, the effect of the residual offset (< 2 K in nominal conditions) is reduced by introducing a gain modulation factor in the on-board processing to balance the output signal . The differencing receiver greatly improves the stability of the measured signal.
- Planck early results. III. First assessment of the Low Frequency Instrument in-flight performance, A. Mennella, R. C. Butler, A. Curto, et al. , A&A, 536, A3, (2011).
- Planck pre-launch status: Design and description of the Low Frequency Instrument, M. Bersanelli, N. Mandolesi, R. C. Butler, et al. , A&A, 520, A4+, (2010).
- 1/f noise and other systematic effects in the Planck-LFI radiometers, M. Seiffert, A. Mennella, C. Burigana, N. Mandolesi, M. Bersanelli, P. Meinhold, P. Lubin, A&A, 391, 1185-1197, (2002).