## 1 Introduction  
Microwave kinetic inductance detectors (MKIDs) represent an emerging type of detector that has been proven to be useful in a range of applications [1, 2]. In the field of astronomy, there are many applications [3, 4, 5] and instruments, such as the NIKA2 large-field-of-view millimetre continuum camera in the IRAM telescope [6], the MISTRAL instrument in the Sardinia Radio Telescope (SRT) [7], the ToITEC camera in the Large Millimetre Telescope (LMT) [PERSON] [8] or the updated design of the SPT-3G+ camera for the South Pole Telescope (SPT) [9]. MKIDs consist of a chain of resonators made of superconducting materials operated at sub-Kelvin (K) temperatures that are used for high-sensitivity astronomical applications. The working principle of these detectors is based on the principle that the surface impedance of a superconductor is modified by incident photons. An incident photon with an energy level of \(h\
u>2\Delta\), where \(\Delta\) is the superconducting gap energy in a superconducting film, causes the breaking of Cooper pairs and creates a number of excitations, known as quasi-particles [1, 10, 11]. A general and open-source configuration for performing the readout from these devices has already been proposed [12, 13, 14, 15, 16]. In these systems, a control and signal processing unit generates a comb of probe frequencies for each resonator, which are then digitised and sent to a filtering and mixing stage. This signal is then sent to the component on which this work focuses: an attenuator. The main reason for placing an attenuator before the MKID is that a high signal level (for a high signal-to-noise ratio) will increase the latter's temperature and so must be attenuated before reaching the MKID. A schematic of the proposed readout configuration of this work is depicted in Figure 1. The MKID is placed on the coldest stage of the cryostat, and the attenuator is placed just before the MKID on a different stage at a temperature of around 80 K. This is because the signal attenuation is such that the resistive noise emitted from the attenuator is now significant compared to the input signal. At room temperature (RT = 300 K), this thermally generated noise is sufficiently high as to dominate the phase readout of the detector. Cooling the attenuator to <4 K virtually removes this added signal, and the phase readout is then dominated by other parameters. Even around 80 K, the noise from the attenuator is improved by a factor of 4 with respect to 300 K.  
In the field of astrophysical instrumentation, there are a large number of electronic components that are subjected to cryogenic temperatures [17; 18; 19; 20; 21; 22; 23; 24; 25]. However, most of the electronic components available on the market are not usually prepared or characterised to work at cryogenic temperatures. In most cases, the minimum working temperature given by the manufacturer does not drop below 0, \(-40\) or \(-55\)\({}^{\circ}\)C for commercial, industrial and military electronics, respectively. These minimum temperatures are well above the ranges needed in cryogenics, which are below 120 K [26; 27; 23]. In certain instances, the minimum working temperature provided by the manufacturer does not imply that the component cannot work at lower temperatures, but rather that it is only characterised for commercial, industrial or military ranges. Using this type of electronics for cryogenic applications can result in huge savings in component costs, as the price of electronics specially designed for cryogenics is extremely high. The AV850M1-00 [28] monolithic microwave integrated circuit (MMIC) is a commercial voltage variable attenuator that exhibits a maximum working frequency of 40 GHz and an attenuation range of 40 dB. The chip was designed using commercial 0.25 \(\upmu\)m technology, and it was characterised for temperatures between \(-55\)\({}^{\circ}\)C and 90 \({}^{\circ}\)C. Figure 2 shows the schematic of the attenuator.