[Paper Review] Upgrade of the MAGIC Telescope with a Multiplexed Fiber-Optic 2 GSamples/s FADC Data Acquisition system

MAGIC Collaboration, F. Göebel+7 more|ArXiv.org|Sep 14, 2007

Particle Detector Development and Performance2 references3 citations

TL;DR

This paper presents a cost-effective, fiber-optic multiplexed 2 GSamples/s FADC data acquisition system for the MAGIC Cherenkov telescope, enabling high-speed digitization of 16 PMT channels using a single FADC via optical signal delay. The system achieves a sustainable 1 kHz trigger rate with 390 ps time resolution and 800 dynamic range, significantly improving gamma-ray sensitivity and background suppression.

ABSTRACT

In February 2007 the MAGIC Air Cherenkov Telescope for gamma ray astronomy was fully upgraded with a fast 2 GSamples/s digitization system. The upgraded readout system uses a novel fiber-optic multiplexing technique. It consists of 10-bit 2 GSamples/s FADCs to digitize 16 channels consecutively and optical fibers to delay the analog signals. A distributed data acquisition system using GBit Ethernet and FiberChannel technology allows to read out the 100 kByte events with a continuous rate of up to 1 kHz.

Motivation & Objective

• Address the need for faster, lower-noise data acquisition in high-energy gamma-ray astronomy to improve sensitivity and background suppression.
• Reduce the cost and power consumption of digitizing 576 PMT channels in the MAGIC telescope by replacing individual FADCs with a multiplexed system.
• Enable high-time-resolution sampling of sub-ns Cherenkov pulses to enhance reconstruction of gamma-ray shower profiles and reduce hadronic background.
• Achieve a sustainable data acquisition rate of 1 kHz with minimal dead time and high data integrity using Ethernet and FiberChannel networks.
• Develop a scalable, modular DAQ system supporting real-time analysis and long-term storage of high-volume data (up to 1 TB/night).

Proposed method

• Implement a fiber-optic multiplexing technique using graded-index multimode fibers to delay analog PMT signals by 40 ns per channel across 16 inputs.
• Use VCSEL diodes at 850 nm to transmit and delay signals through optical fibers with 2.3 dB/km attenuation, enabling precise signal timing.
• Employ optical splitters with 50/50 splitting ratio (±3%) to route a fraction of light for trigger generation, ensuring low-jitter (1.25 ns) signal decision.
• Convert optical signals back to electrical using PIN diodes and use high-bandwidth MOSFET switches (clocked at 800 MHz) to sequentially route 16 channels to a single FADC.
• Digitize signals with DC282 10-bit, 2 GSamples/s FADCs (1 V input range, 700 MHz bandwidth), mapping 10-bit data to 16-bit for non-linearity correction.
• Store data in on-board 512 kByte RAM per channel, with asynchronous readout via cPCI bus to a central Linux-based server (MUXDAQ), using GBit Ethernet and FiberChannel for high-throughput transfer.

Experimental results

Research questions

• RQ1 Can a fiber-optic multiplexing technique reduce FADC cost and power consumption while maintaining high temporal resolution for Cherenkov telescope applications?
• RQ2 What is the achievable time resolution of the full signal chain using the new 2 GSamples/s FADC system with multiplexed PMT signals?
• RQ3 How does the system perform in terms of dynamic range and linearity when digitizing signals from 1 to 800 photoelectrons?
• RQ4 Can the system sustain a continuous 1 kHz trigger rate with minimal dead time and high data integrity?
• RQ5 To what extent does the improved timing resolution enhance background suppression and sensitivity in gamma-ray astronomy?

Key findings

• The system achieves a sustainable data acquisition rate of 1 kHz with a maximum transfer rate of 100 MB/s over GBit Ethernet, enabling continuous operation without additional dead time.
• The time resolution of the full signal chain is 390 ps (RMS of arrival time difference between two pixels), measured using uniform calibration pulses.
• The FADC system exhibits a linear dynamic range of 800 photoelectrons, with saturation effects becoming noticeable only beyond this level.
• The system resolves sub-ns pulses with a FWHM of 2.3 ns for a ~5 photoelectron signal, demonstrating excellent temporal fidelity.
• The multiplexer introduces noise peaks of ~10 ns duration at channel transitions, which are removed in analysis software without affecting signal integrity.
• Calibrated data from the MUXANA server is available within 24 hours post-observation, enabling rapid source detection, while raw data (up to 1 TB/night) is archived on LTO3 tapes for offline processing.