Kavli Affiliate: James J. Bock
| First 5 Authors: Corwin Shiu, Ahmed Soliman, Roger O’Brient, Bryan Steinbach, James J. Bock
| Summary:
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.
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