Spectral Characterization of a 90 GHz CLASS Pixel

The cosmology large angular scale surveyor (CLASS) is an experiment designed to measure the polarization of the cosmic microwave background on large angular scales to probe cosmic reionization and search for the inflationary <inline-formula><tex-math notation="LaTeX">$B$</tex-math></inline-formula>-mode signal. CLASS is a multifrequency ensemble of telescopes with bands centered at 40, 90, 150, and 220 GHz. Each telescope has arrays of feedhorn-coupled transition edge sensor bolometers at the focal plane. The frequency response is primarily defined by the on-chip bandpass filter (BPF) with additional contributions coming from the feedhorn, orthomode transducer, and 180° hybrid. In this study, we compare simulations and measurements of the frequency response of single pixel witness devices in the 90 GHz band with and without the BPF. For the first time, we can separate the effects of the BPF from the other microwave components using Fourier transform spectroscopy and design splits of the pixel. The results show that the <inline-formula><tex-math notation="LaTeX">$-$</tex-math></inline-formula>3-dB band edges are 80 and 108 GHz. The measurements demonstrate a robust method for characterizing the spectral response of individual components, which is crucial for optimizing the performance of future detector arrays.