The use of non-cryogenic certified commercial electronics for cryogenic applications may be attractive due to their cost and availability, but it also carries risks related to reliability, performance and thermal compatibility. The decision to use commercial components that are not certified for cryogenics instead of components specifically designed for such applications must be carefully weighed based on specific project needs and risk tolerances. This work presents the characterisation of an attenuator circuit at cryogenic temperatures used in a microwave kinetic inductance detector (MKID) readout system. In order to characterise the operation of the attenuator at cryogenic temperatures and because the circuit works at frequencies up to 40 GHz, a specific microwave PCB has been designed. The cooling system used consists of a cryostat, all the connectors, cables, a vacuum pump, a compressor, pressure and temperature sensors, a temperature control system and a cold head operating in a closed helium gas cycle according to the Gifford–McMahon principle. The circuit was tested and characterised at temperatures ranging from 296.5 K to 83.6 K.