[Paper Review] An Analog Trigger System for Atmospheric Cherenkov Telescopes

M. Barceló, J. A. Barrio+7 more|arXiv (Cornell University)|Jul 11, 2013

Astrophysics and Cosmic Phenomena3 references3 citations

TL;DR

This paper presents an analog trigger system for the Cherenkov Telescope Array (CTA) that uses clustered pixel signals in Medium and Large Size Telescopes to enable high-efficiency gamma-ray detection with low background rates. By combining analog sum and majority trigger schemes via modular L0 and L1 stages, the system achieves a 190 ns total latency, <1% dead time, and noise below 0.2 phe, meeting CTA’s performance requirements with full compatibility for integration into telescope cameras.

ABSTRACT

Arrays of Cherenkov telescopes typically use multi-level trigger schemes to keep the rate of random triggers from the night sky background low. At a first stage, individual telescopes produce a trigger signal from the pixel information in the telescope camera. The final event trigger is then formed by combining trigger signals from several telescopes. In this poster, we present a possible scheme for the Cherenkov Telescope Array telescope trigger, which is based on the analog pulse information of the pixels in a telescope camera. Advanced versions of all components of the system have been produced and working prototypes have been tested, showing a performance that meets the original specifications. Finally, issues related to integrating the trigger system in a telescope camera and in the whole array will be dealt with.

Motivation & Objective

• To develop a low-latency, high-efficiency analog trigger system for CTA's Medium and Large Size Telescopes to reduce background trigger rates while maintaining high gamma-ray detection efficiency.
• To implement a modular, scalable trigger architecture compatible with the camera's 7-pixel hexagonal cluster design and future ASIC integration.
• To support dual trigger schemes—sum and majority—offering improved sensitivity for low-energy gamma rays (<200 GeV).
• To ensure compatibility with existing readout systems (NECTAR and DRAGON) and enable seamless integration into telescope camera electronics.
• To minimize power consumption, weight, and system complexity through analog signal processing and optimized hardware design.

Proposed method

• The system uses a three-stage architecture: Level 0 (L0) processes individual pixel signals within a 7-pixel hexagonal cluster, applying analog sum or majority logic based on threshold-crossing.
• Level 1 (L1) evaluates all compact regions formed by a cluster and its neighbors, combining L0 outputs to detect spatial-temporal signal excesses above a threshold.
• The L1 distribution system ensures synchronized trigger signal delivery to all clusters with a 40 ns dead time, enabling fast readout and low latency.
• L0 and L1 functions are implemented on mezzanine boards for testing, with final integration into front-end and backplane boards of the telescope camera.
• The system employs analog adders, fast comparators, LVDS signaling, and adjustable attenuators and clippers to equalize gains and suppress after-pulses.
• Analog trigger signals are fanned out via dedicated fan-out circuits and distributed through the ATB (Analog Trigger Board), which also supplies power, Ethernet, and clock signals.

Experimental results

Research questions

• RQ1 How can an analog trigger system be designed to achieve low latency and high efficiency in detecting atmospheric Cherenkov showers in CTA telescopes?
• RQ2 What are the performance trade-offs between sum and majority trigger schemes in terms of sensitivity to low-energy gamma rays and background suppression?
• RQ3 Can a modular analog trigger architecture be developed that integrates seamlessly with existing readout systems like NECTAR and DRAGON?
• RQ4 What level of electronic noise and dynamic range is achievable in an analog trigger system to ensure reliable signal detection above night sky background?
• RQ5 To what extent can power consumption, weight, and system complexity be reduced through analog processing and future ASIC integration?

Key findings

• The analog trigger system achieves a total latency of 190 ns, with 20 ns from L0, 170 ns from L1 distribution, and negligible dead time (<1%) due to a 40 ns trigger recovery window.
• Electronic noise is measured at below 2 mV per cluster, equivalent to 0.2 phe, well below the typical night sky background of >2 phe per cluster.
• The system supports trigger rates up to 100 MHz, with a maximum differential output voltage of 2 V for 200 phe, ensuring high dynamic range and calibration compatibility.
• Power consumption is below 590 mW per channel (with clipping circuits contributing ~1/3), and could be reduced to 400 mW if clipping is eliminated via PMT selection.
• Weight is approximately 25 g per channel, with 4 g for L0/L1 and 21 g for the ATB, demonstrating compactness suitable for large-scale deployment.
• Integrated testing with NECTAR and DRAGON readout boards confirmed full mechanical, electrical, and logical compatibility, validating system readiness for mass production.