## 2 Materials and Methods: The Original Version of the Ex-PdBI W-Band Receiver
### Need for a New Dedicated Local Oscillator: The ALMA Band 3 Local Oscillator  
As previously described, the W-band receiver builds on a down-conversion system that uses an SIS mixer, which was originally pumped with a traditional Gunn diode located at room temperature outside the cryostat. For the installation on the SRT, a new dedicated local oscillator that operates at high frequencies, also placed at room temperature, was adopted. This local oscillator was developed by the National Radio Astronomy Observatory (NRAO) for the ALMA Band 3 (84-116 GHz) project [39]. It was expected to meet the main requirements of frequency range tuneability and delivered output power. The ALMA Band 3 LO module consists of an active multiplier chain (AMC) in cascade with a power amplifier (PA). In our arrangement for the SRT, we injected a signal at the input of the AMC from an external signal generator (Rohde and Schwarz signal generator, model SMF 100A [40]). The generator synthesises a single-tone continuous signal tuneable across the 15.333-18.000 GHz frequency range, with a power level between 10 dBm and 15 dBm (in the ALMA system, the 15.333-18.000 GHz signal is provided by a commercial YIG oscillator). In the AMC, this signal is frequency multiplied by 6 (first by 2, then by 3), filtered, and amplified in order to achieve the desired W-band local oscillator frequency (i.e., 92-108 GHz) with an output power level in the range 1-10 mW. The AMC module incorporates two monolithic microwave integrated circuit (MMIC) multipliers (double and triple multipliers) and an MMIC W-band amplifier. Since the ALMA receivers are equipped with two orthogonal polarisation channels, the PA module splits the signal from the AMC module in two and further amplifies these new signals thanks to a user-controlled amplitude system, with a step of 0.2 dB, in order to achieve a power value between \(-4\) dBm and 2 dBm. Only one of the two PA output channels is used for the ex-PdBI receiver, whereas the other channel is terminated to 50 Ohm load. The LO output of the PA is available from a standard WR10 waveguide with UG385 flange. A section of interconnecting WR10 waveguide must be used between the PA and the WR10 waveguide vacuum feedthrough located at the LO input interface of the receiver cryostat.  
The W-band receiver bandwidth 84-116 GHz is the same as that of the ALMA Band 3. However, the IF range of the ex-PdBI receiver falls between 1.25 and 1.75 GHz (500 MHz bandwidth), as opposed to the 4-8 GHz bandwidth of the ALMA Band 3 [39]. Consequently, the local oscillator bandwidth required to cover the desired radio frequency band for SRT is 85.75-114.25 GHz, which is larger than the bandwidth of the ALMA Band 3. Some preliminary tests were necessary to test the ALMA Band 3 local oscillator in the extended frequency range of the ex-PdBI receiver with the goal of deriving the optimal power level for achieving high performances. Figure 3 shows a photo of the ALMA Band 3 local oscillator. The results of these tests are presented in Section 5.  
Figure 3: Photo of the ALMA Band 3 local oscillator showing the AMC (left module) and PA (right module) connected to each other.