Schematic of the LEKID test system. The LEKID readout is based on the ROACH 2 board. The 100–200 MHz resonance frequencies of the LEKIDs allow us to use the ADC/DAC at the baseband frequencies. For arrays with higher resonance frequency KIDs, we use IQ mixers which are not shown here (Johnson et al. 2016). The entire array is read out on a single pair of coaxial lines. The MMW sources are also shown. A HWP mounted at 2.7 K and rotated with a cryogenic stepper motor modulates the MMW radiation that enters the cryostat through a WR6 waveguide. The radiation (140–165 GHz) can either be single frequency (coherent) or broadband (incoherent) (Flanigan et al. 2016). Additionally, there is a blackbody source made from Eccosorb, the temperature of which can be controlled via a heater resistor.