Design and Performance of 30/40 GHz Diplexed Focal Plane for BICEP Array

• Published in: arXiv.org 2024-05
• Main Authors: Corwin Shiu, Soliman, Ahmed, O'Brient, Roger, Steinbach, Bryan, Bock, James J, Frez, Clifford F, Jones, William C, Megerian, Krikor G, Moncelsi, Lorenzo, Schillaci, Alessandro, Turner, Anthony D, Weber, Alexis C, Zhang, Cheng, Zhang, Silvia
• Format: Article
• Language: English
• Subjects: Antenna arrays; Antennas; Bandpass filters; Bandwidths; Butterworth filters; Cosmic microwave background; Diplexers; Ellipticity; Focal plane; Impedance; Linear polarization; Optical properties; Phased arrays; Pixels
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.2405.03767

We demonstrate a wide-band diplexed focal plane suitable for observing low-frequency foregrounds that are important for cosmic microwave background polarimetry. The antenna elements are composed of slotted bowtie antennas with 60% bandwidth that can be partitioned into two bands. Each pixel is composed of two interleaved 12\(\times\)12 pairs of linearly polarized antenna elements forming a phased array, designed to synthesize a symmetric beam with no need for focusing optics. The signal from each antenna element is captured in-phase and uniformly weighted by a microstrip summing tree. The antenna signal is diplexed into two bands through the use of two complementary, six-pole Butterworth filters. This filter architecture ensures a contiguous impedance match at all frequencies, and thereby achieves minimal reflection loss between both bands. Subsequently, out-of-band rejection is increased with a bandpass filter and the signal is then deposited on a transition-edge sensor bolometer island. We demonstrate the performance of this focal plane with two distinct bands, 30 and 40 GHz, each with a bandwidth of \(\sim\)20 and 15 GHz, respectively. The unequal bandwidths between the two bands are caused by an unintentional shift in diplexer frequency from its design values. The end-to-end optical efficiency of these detectors are relatively modest, at 20-30%, with an efficiency loss due to an unknown impedance mismatch in the summing tree. Far-field beam maps show good optical characteristics with edge pixels having no more than \(\sim\) 5% ellipticity and \(\sim\)10-15% peak-to-peak differences for A-B polarization pairs.