Design and Performance of 220 and 270 GHz Bandpass Filters for BICEP Array

The BICEP Array (BA) is the latest in the BICEP/Keck series of experiments that aim to measure the polarization of the cosmic microwave background (CMB) with small aperture polarimeters located at the South Pole. To constrain the frequency response of these receivers, each detector is serially coupled to a bandpass filter (BPF). The electric circuits of these BPFs utilize series and shunt capacitors as well as series inductors, but critically do not include shunt inductors, which simplifies fabrication. The filters are designed and simulated with Sonnet, and optimized for noise by considering loading from the atmosphere and the CMB. Multiple 220 GHz detector modules have had their frequency response measured at the South Pole. The 270 GHz detector modules have recently begun testing in a lab setting, and their performance in a BA receiver will be measured this winter.