QUIJOTE instrumentation
The Multi-Frequency Instrument (MFI)
The first QUIJOTE Instrument (or Multi-Frequency Instrument) has four polarimeters (horns), two with receivers at 11-13GHz, and two more at 17-19 GHz. Originally, it was designed with an additional receiver at 30 GHz inside a horn at the center of the focal plane, but this horn was later removed. In total, there are 8 channels in each one of the four frequencies 11, 13, 17 and 19GHz, making 32 channels in total. The approximate angular resolution, given in terms of the full width at half-maximum, is 52 arcmin for the low-frequency bands (11 and 13 GHz), and 38 arcmin for the 17 and 19 GHz channels. The typical white noise level in timelines is 700-800 μK s1/2.
MFI operated between November 2012 and October 2018 mounted on the first QUIJOTE telescope. During the lifetime of the instrument, we had basically two instrumental configurations. The main difference of the second configuration with respect to the first one is the integration of 90º hybrid couplers in each polarimeter, giving correlated outputs in all four detectors. A more detailed (technical) description of the MFI can be found in Hoyland et al. (2012) and Pérez-de-Taoro et al. (2016) .
The Thirty-GHz Instrument (TGI)
The second QUIJOTE Instrument (or 30 GHz Instrument) is mainly devoted to primordial B-mode science. TGI is fitted with 31 polarimeters working in the range of 26-36GHz. The design of a single polarimeter for the TGI is presented in the figure. It includes a fixed polarizer and 90 deg and 180 deg phase switches to generate four polarisation states to minimize the different systematics in the receiver. The instrument is described in detail in Hoyland et al. (2014) , Sánchez-de-la-Rosa et al. (2016) , Gómez-Reñasco et al. (2016) and Artal et al. (2020) .
The Forty-GHz Instrument (FGI)
Also devoted to primordial B-mode science, the third QUIJOTE Instrument (or 40 GHz Instrument) consists of 31 polarimeters working at 40GHz. The conceptual design of a polarimeter chain for the FGI is identical to the one used for the TGI. Funding for the FGI was provided by the Consolider-Ingenio project CSD2010-00064 (EPI:Exploring the Physics of Inflation). The FGI receivers are described in Artal et al. (2020) . The status of the project is summarised in Perez de Taoro et al. (2016) and Rubiño-Martin et al. (2017) .
Since 2018, the QT2 is operating with a combination of the TGI and FGI instruments in a single cryostat. We name this hybrid instrument as TFGI, and contains half of the detectors at 30GHz, and the other half at 40GHz.
The Second Multi-Frequency Instrument (MFI2)
The importance of having observations in the 10-20 GHz frequency range as a legacy for future B-mode missions has been recognised by the CMB community. Because of this reason, from the QUIJOTE collaboration we are finalising a new version of the MFI, to be installed in the first QUIJOTE telescope. The new instrument, MFI2, consists of 5 polarimeters, three of them operating in the subband 10–15 GHz, and two in the subband 15–20 GHz. It is expected to be a full 2–3 times more sensitive than the former MFI. The microwave complex correlator design has been replaced by a simple correlator design with a digital back-end based on the latest Xilinx FPGAs (ZCU111). The cryogenic front-end of MFI2 was assembled at the end of 2023, and installed in the focal plane of QT1 using the old DAS from MFI. It is expected that the new FPGA acquisition and observing system will be completed during 2024. This digital system has been employed to be more robust against stray ground-based and satellite interference, having a frequency resolution of 1 MHz. A detailed description of the MFI2 can be found in Hoyland et al. (2022) .