Development of multiplexed bolometer readout electronics for mm-wave space astronomy

The polarization anisotropies in the Cosmic Microwave Background contain a rich set of physical signatures. These include known phenomena, such as weak gravitational lensing, which offers a chance to study structure formation at redshifts sparsely explored; and predicted phenomena, such as the propagation of primordial gravity waves through the surface of last scattering (``gravitational B-modes''). The detection and measurement of the latter would be evidence in support of inflationary cosmology theories, the search for which has been at the epicenter of CMB experimental development for nearly a decade. A space-based platform is the best place to conduct such a search: the signature of B-modes is strongest at large angular scales, and full-sky coverage presents a significant advantage over ground-based instruments. Additionally, access to more frequency bands otherwise absorbed by Earth's atmosphere, improves the ability to characterize and subtract galactic foregrounds.To achieve the sensitivities capable of making precision measurements of the CMB polarization, the next generation of instruments will employ focal planes containing $\sim10,000$ detector elements. This is an order of magnitude increase over the existing instruments. The detectors themselves are arrays of Transition Edge Sensing (TES) Bolometers -- photon-noise limited sensors, which have been favored for the last decade of ground-based CMB instruments, but have never flown on a satellite platform. To achieve the requisite focal-plane densities, it is essential to ``multiplex'' -- read out several bolometers simultaneously, over a single pair of wires. In this document, we present an electronics system capable of reading out, and controlling, the next generation of TES focal-planes: The Space-Flight Representative 64x Digital Frequency Multiplexing (DfMUX) readout system. These electronics were commissioned by the Canadian Space Agency, and built in partnership with COM-DEV.The Flight Representative 64x DfMUX system multiplexes together 64 readout channels, a factor of four increase over all current frequency-multiplexed CMB readout systems. We measure a total readout noise of less than $10\frac{pA}{\sqrt{Hz}}$, which is consistent with prediction, and remains substantially lower than the intrinsic background photon noise of the CMB. We do acknowledge some excess noise at high frequency, and demonstrate that it is due to non-idealities in our testing setup that are well described by our noise model, not the Flight Representative readout electronics. Additionally, the Flight Representative DfMUX system meets all Canadian Space Agency space-flight criteria. these include radiation-hardness; thermal optimization for radiative heat transfer; a per-channel power consumption of 49mW.

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