## 2 Materials and Methods: The Original Version of the Ex-PdBI W-Band Receiver
A side-wall coupler, with a coupling of about 20 dB and an insertion loss of about 0.2 dB, is used to inject the local oscillator into the signal path [28]. The coupler is connected to the horn using a brass bracket, which also acts as a support for the mixer and flange between the mixer and coupler. Figure 1: **(a)** Schematic of the signal acquisition chain of millimetre-wave and RF components of the original version of the ex-PdBI receiver. The mixer and the LO coupler are thermally cooled at 4 K inside a cryostat with 70 K, 15 K, and 4 K cryogenic stages. **(b)** Photo of the signal acquisition chain in the Dewar. The IF output from the SIS mixer is connected to an isolator (see Figure 1b), with a stainless-steel coaxial cable. The down-converted IF signal is amplified by a low noise amplifier (LNA) [35] that delivers an instantaneous bandwidth of 600 MHz, based on high-electron-mobility transistor (HEMT) technology, with a gain of about 30 dB and a noise temperature of about 5 K.  
All of these components are installed on a hybrid cryostat (HDV10 from Infrared Laboratories) [36], or Dewar (see Figures 1b and 2a), in which there are three cryogenic stages: two of them (i.e., 70 K and 20 K) are achieved thanks to a CTI model 350 CP cold head [37] and the other (i.e., 4 K) is provided by a 10-inch-diameter high thermal conductivity plate in thermal contact with a vessel of liquid helium. The mixer, local oscillator coupler, circular horn, polarising grid, and optical lenses are mounted on the 4 K stage. The isolator and the LNA are mounted on the 20 K screen, connected to a stainless-steel inner and outer conductor coaxial cable. The SSB SIS receiver, including the SIS mixer, the IF LNA, the feed, and optics, deliver a low receiver noise temperature of less than 50 K when measured in front of the cryostat vacuum window. The receiver chain in the Dewar is followed by a chain section operating at room temperature. It provides another amplification stage, with a gain of about 60 dB, and a programmable step attenuator from 0 to 20 dB, which is useful for controlling the amplitude level of the signals that are fed into the back end for sampling and later processing. The noise temperature of radio astronomy receivers is one of the most important performance parameters that directly impact the sensitivity achieved during radio astronomy observation. The ex-PdBI W-band receiver is equipped with a classical calibration system that performs noise temperature measurements using the cold and ambient loads method. The noise temperature of the receiver is evaluated at its input vacuum window and positioned on the surface of the Dewar. The receiver calibration system is composed of two loads at room temperature and cryogenic temperature, respectively. The load at room temperature is an absorbing panel of a squared shape made of Emerson & Cuming AN72 material [38], which can be positioned in the receiver signal path with a dedicated direct current (DC) motor. The cold load consists of an absorbing panel suited to use at 4 K, positioned into the Dewar, which can be activated by moving a rooftop mirror with normal angle of reflection, located on the outside of the cryostat at room temperature. The rooftop mirror can be moved along the receiver signal path using a dedicated DC motor. Figure 2: (**a**) Photo of the ex-PdBI receiver mounted on a laboratory test bench. (**b**) Photo of the single side band (SSB) superconductor–insulator–superconductor (SIS) mixer. This rooftop mirror allows the rotation of the polarisation angle of the incoming beam by 90 degrees and the reversal of its direction towards the cryogenic load. The original receiver control system permits the control of all receiver functions through a VME-based microprocessor. All communications with the control computer are possible thanks to an Ethernet link. Structural and Logistical Adaptation of the Ex-PdBI W-Band Receiver for the Installation on the Sardinia Radio Telescope  
As mentioned above, the receiver was originally designed to be installed on a PdBI antenna.