Morgan K M, Becker D T, Bennett D A, Gard J D, Imrek J, Mates J A B, Pappas C G, Reintsema C D, Schmidt D R, Ullom J N, Weber J, Wessels A, Swetz D S
Department of Physics, University of Colorado Boulder, Boulder, CO 80309, USA.
National Institute of Standards and Technology, Boulder, CO 80305, USA.
J Low Temp Phys. 2019;199. doi: 10.1007/s10909-019-02250-2.
Microwave SQUID multiplexing has become a key technology for reading out large arrays of X-ray and gamma-ray microcalorimeters with mux factors of 100 or more. The desire for fast X-ray pulses that accommodate photon counting rates of hundreds or thousands of counts per second per sensor drives system design toward high sensor current slew rate. Typically, readout of high current slew rate events is accomplished by increasing the sampling rate, such that rates of order 1MHz may be necessary for some experiments. In our microwave multiplexed readout scheme, the effective sampling rate is set by the frequency of the flux-ramp modulation ( ) used to linearize the SQUID response. The maximum current slew rate between samples is then nominally /2 (where is the input coupling) because it is generally not possible to distinguish phase shifts of > from negative phase shifts of < -. However, during a pulse, we know which direction the current ought to be slewing, and this makes it possible to reconstruct a pulse where the magnitude of the phase shift between samples is > . We describe a practical algorithm to identify and reconstruct pulses that exceed this nominal slew rate limit on the rising edge. Using pulses produced by X-ray transition-edge sensors, we find that the pulse reconstruction has a negligible impact on energy resolution compared to arrival time effects induced by under-sampling the rising edge. This technique can increase the effective slew rate limit by more than a factor of two, thereby either reducing the resonator bandwidth required or extending the energy range of measurable photons. The extra margin could also be used to improve crosstalk or to decrease readout noise.
微波超导量子干涉器件（SQUID）复用技术已成为读出大量X射线和伽马射线微热量计阵列的关键技术，复用系数可达100或更高。对能够适应每个传感器每秒数百或数千计数的光子计数率的快速X射线脉冲的需求，推动系统设计朝着高传感器电流转换速率发展。通常，通过提高采样率来实现对高电流转换速率事件的读出，以至于某些实验可能需要1MHz量级的速率。在我们的微波复用读出方案中，有效采样率由用于使SQUID响应线性化的磁通斜坡调制（ ）频率设定。那么样本之间的最大电流转换速率标称值为 /2 （其中 是输入耦合），因为通常无法区分大于 的相移和小于 - 的负相移。然而，在脉冲期间，我们知道电流应该向哪个方向转换，这使得有可能重建样本之间相移幅度大于 的脉冲。我们描述了一种实用算法，用于识别和重建上升沿超过此标称转换速率限制的脉冲。使用由X射线过渡边缘传感器产生的脉冲，我们发现与上升沿欠采样引起的到达时间效应相比，脉冲重建对能量分辨率的影响可以忽略不计。该技术可以将有效转换速率限制提高两倍以上，从而要么降低所需的谐振器带宽，要么扩展可测量光子的能量范围。额外的余量还可用于改善串扰或降低读出噪声。
