Online Demodulation and Trigger for Flux-ramp Modulated SQUID Signals

TL;DR

This paper presents an FPGA-based implementation of real-time flux-ramp demodulation and an online trigger system for SQUID signals, enabling efficient multi-channel processing and event detection in cryogenic microcalorimeter readouts.

Contribution

It introduces a novel FPGA-based system for online flux-ramp demodulation and triggering, improving data processing speed and reducing data volume for SQUID-based measurements.

Findings

Successful demonstration with 32 channels at 500 MHz

Effective event detection in microcalorimeter experiments

Significant data reduction achieved during measurement

Abstract

Due to the periodic characteristics of SQUIDs, a suitable linearization technique is required for SQUID-based readout. Flux-ramp modulation is a common linearization technique and is typically applied for the readout of a microwave-SQUID-multiplexer as well as since recently also for dc-SQUIDs. Flux-ramp modulation requires another stage in the signal processing chain to demodulate the SQUID output signal before further processing. For cryogenic microcalorimenters, these events are given by fast exponentially rising and slowly exponentially decaying pulses which shall be detected by a trigger engine and recorded by a storage logic. Since the data rate can be decreased significantly by demodulation and event detection, it is desirable to do both steps on the deployed fast FPGA logic during measurement before passing the data to a general-purpose processor. In this contribution, we show the implementation of efficient multi-channel flux-ramp demodulation computed at run-time on a SoC-FPGA. Furthermore, a concept and implementation for an online trigger and buffer mechanism with its theoretical trigger loss rates depending on buffer size is presented. Both FPGA modules can be operated with up to 500 MHz clock frequency and can efficiently process 32 channels. Correct functionality and data reduction capability of the modules are demonstrated in measurements utilizing magnetic microcalorimeter irradiated with an Iron-55 source for event generation and read out by a microwave SQUID multiplexer.