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Error and feedback signals vs time for two multiplexed pixels. Pixel A shows the rising edge of a pulse from an x-ray impact at time 0. The feedback signal and the row address signal (not pictured) for pixel A are turned on at the beginning of the frame. After a settling time, in this example , the error signal, is averaged over the sampling time , and then pixel A is turned off and B is turned on. The pixels have different quiescent signal levels due to slight differences in the sensor and first-stage-SQUID operating conditions.
Measured TES resolution vs for two multiplexed pixels. The top axis gives as calculated using Eq. (1) with . The poorer resolution to the left of the plot is due to being too short to allow switching transients to decay fully before sampling. The degradation to the right is due to taking too few samples on the pulse rise, where the signal changes the fastest.
Histogrammed pulse-energy spectra with overlaid fits to the complex for four multiplexed TES pixels. The spectra are offset vertically by . Nonmultiplexed resolution is (FWHM); the slight degradation to an average resolution of is well explained by the and effects, and can be significantly reduced by straightforward upgrades to the multiplexer.
Energy resolution vs number of pixels for the current multiplexer (solid) and various modifications (dashed). The curves show the predicted resolution based on effects measured in Fig. 2 and also on SQUID noise aliasing. Data (circles) are for , 3, 4, and 8 (the data point for 8 pixels uses four blank pixel periods, and thus simulates the timing of multiplexing eight pixels with the four available sensors). The dashed curves are predictions for various open-loop system bandwidths, and also include twice the pulse , SQUID amplifier noise half that in the current multiplexer chip, data-acquisition firmware correction for propagation delays, and optimized coupling between the TES and first-stage SQUID. More involved upgrades, including transmission-line optimization of the couplings among the cold amplifier stages and a substantial upgrade in the clock rate of the digital-feedback electronics, could allow a usable of up to .
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