[Paper Review] Clock Distributing for BaF2 Readout Electronics at CSNS-WNS

Bing He, Ping Cao+5 more|arXiv (Cornell University)|Feb 22, 2016

Nuclear Physics and Applications3 citations

TL;DR

This paper presents a high-precision clock distribution system for BaF2 scintillator readout electronics at the CSNS-WNS facility, using a PXIe-based twin-stage tree topology with coaxial cables for long-distance distribution and a differential star bus for module-level synchronization. The system achieves sub-20 ps jitter, enabling accurate pulse shape discrimination in 92-channel waveform digitization for precise (n,γ) cross-section measurements.

ABSTRACT

aF2 (Barium Fluoride) detector array is designed for the measurement of (n,γ) cross section precisely at CSNS-WNS (white neutron source at China Spallation Neutron Source). It is a 4πsolid angle-shaped detector array consisting of 92 BaF2 crystal elements. To discriminate signals from BaF2 detector, pulse shape discrimination methodology is used, which is supported by waveform digitization technique. There are total 92 channels for digitizing. The precision and synchronization of clock distribution restricts the performance of waveform digitizing. In this paper, the clock prototype for BaF2 readout electronics at CSNS-WNS is introduced. It is based on PXIe platform and has a twin-stage tree topology. In the first stage, clock is distributed from the tree root to each PXIe crate synchronously through coaxial cable over long distance, while in the second stage, clock is further distributed to each electronic module through PXIe dedicated differential star bus. With the help of this topology, each tree node can fan out up to 20 clocks with 3U size. Test result shows the clock jitter is less than 20ps, which can meet the requirement of BaF2 readout electronics. Besides, this clock system has advantages of high density, simplicity, scalability and cost saving, which makes it can be used in other applications of clock distributing preciously.

Motivation & Objective

• To enable precise, synchronized waveform digitization across 92 channels of BaF2 scintillator detectors at CSNS-WNS.
• To overcome timing jitter and synchronization limitations in high-channel-count readout electronics for pulse shape discrimination.
• To design a scalable, cost-effective, and high-density clock distribution system suitable for large-scale neutron detector arrays.
• To support accurate (n,γ) cross-section measurements by ensuring sub-20 ps timing stability across all channels.

Proposed method

• A PXIe-based clock distribution system is implemented with a twin-stage tree topology for hierarchical signal routing.
• In the first stage, a master clock is distributed from the root to each PXIe crate over long distances using coaxial cables.
• In the second stage, the clock is further distributed to individual electronic modules via a dedicated differential star bus.
• Each node supports up to 20 clock outputs within a 3U form factor, enabling high channel density.
• The system uses phase-locked loops and impedance-matched transmission lines to minimize jitter and ensure signal integrity.
• The design emphasizes scalability, simplicity, and cost efficiency for deployment in large detector systems.

Experimental results

Research questions

• RQ1 How can a clock distribution system achieve sub-20 ps jitter across 92 synchronized readout channels in a large BaF2 detector array?
• RQ2 What topology and transmission method ensure stable, low-jitter clock signals over long distances in a high-channel-count system?
• RQ3 Can a PXIe-based architecture support scalable, high-density clock distribution with minimal hardware overhead?
• RQ4 How does the twin-stage tree topology improve synchronization and reduce jitter compared to conventional methods?

Key findings

• The clock distribution system achieves a measured jitter of less than 20 ps, meeting the stringent timing requirements for BaF2 pulse shape discrimination.
• The twin-stage tree topology enables reliable clock distribution to 92 channels with up to 20 outputs per node using 3U-sized modules.
• The system demonstrates high scalability, simplicity, and cost efficiency due to standardized PXIe components and modular design.
• The use of coaxial cables for long-distance distribution and differential star buses for local distribution ensures signal integrity and low jitter.