[Paper Review] A method to adjust the impedance of the transmission line in a Multi-Strip Multi-Gap Resistive Plate Counter

D. Bartoş, M. Petriş+3 more|arXiv (Cornell University)|Aug 9, 2017

Electrical and Bioimpedance Tomography6 citations

TL;DR

This paper presents an innovative method to independently tune the transmission line impedance in Multi-Strip Multi-Gap Resistive Plate Counters (MSMGRPCs) by adjusting the readout strip width, enabling impedance matching to front-end electronics without compromising detector granularity. The prototype with 7.2 mm pitch and 1.32 mm readout strips achieved 50 Ω characteristic impedance, confirmed by lab tests showing no signal reflections, validating the design for trigger-less high-rate experiments like CBM at FAIR.

ABSTRACT

While in a triggered experiment the matching of the RPC transmission line impedance with the one of the front-end electronics is less critical, for a trigger-less data recording this becomes mandatory. As expected, impedance matching is not straightforward when other requirements in terms of time and position resolutions, efficiency and granularity, have to be fulfilled in the same time. A method and the very first results obtained with a RPC prototype built based on it, presented in this paper, show that the impedance matching, independent of its granularity, can be achieved using an innovative architecture of the RPC.

Motivation & Objective

• To solve the challenge of impedance matching in trigger-less, high-rate RPC systems like CBM at FAIR, where signal reflections degrade performance.
• To maintain high time resolution (<80 ps), efficiency (>95%), and spatial resolution (400 μm across strips) while enabling impedance tuning.
• To decouple impedance matching from detector granularity by using a novel architecture with variable readout strip width.
• To validate the method experimentally using a prototype with 7.2 mm pitch and tunable impedance.
• To reduce the number of front-end electronics channels by enabling larger strip pitches (e.g., 7.4 mm) without impedance mismatch.

Proposed method

• The method uses a two-layer stack of resistive electrodes with 50 Ω transmission line formed by the readout strips, whose impedance is tuned by varying the readout strip width independently of the high-voltage (HV) strip width.
• The HV strips (5.6 mm wide) are centered under the readout strips (1.32 mm wide), and signal induction occurs first on the HV strips, then on the readout strips.
• The transmission line impedance is calculated using APLAC simulations, which model the geometry and dielectric properties of the FR4 and glass layers.
• The prototype uses a 7.2 mm pitch, 5.6 mm HV strip width, and 1.32 mm readout strip width to achieve a 50 Ω characteristic impedance.
• Signal transmission is implemented via 30 cm twisted pair cables with 100 Ω impedance, terminated at both ends with 50 Ω resistors to simulate real FEE conditions.
• The system uses differential signal readout: time-of-flight is derived from the mean of two-end times, and position is obtained from time differences and charge sharing.

Experimental results

Research questions

• RQ1 Can the transmission line impedance in an MSMGRPC be tuned independently of the detector's spatial granularity?
• RQ2 Does adjusting the readout strip width alone allow precise control of the characteristic impedance to match 50 Ω or 100 Ω front-end electronics?
• RQ3 Can the proposed architecture maintain sub-80 ps time resolution and >95% efficiency while achieving impedance matching?
• RQ4 Is the impedance matching robust under high-rate conditions, as required for trigger-less data acquisition in CBM?
• RQ5 Can the method reduce the number of required front-end electronics channels by enabling larger strip pitches?

Key findings

• The prototype with 7.2 mm pitch and 1.32 mm readout strips achieved a characteristic impedance of 50 Ω, matching the target for 50 Ω front-end electronics.
• No signal reflections were observed in cosmic ray measurements, indicating successful impedance matching at the connectors and along the 30 cm twisted pair cables.
• The rise time of induced signals was less than 1 ns, and signal width was below 1 ns, confirming fast signal propagation without distortion.
• The time resolution was measured to be approximately 50 ps, meeting the CBM requirement of <80 ps.
• Efficiency exceeded 95% even at local particle fluxes up to 10^5 particles/(cm²·s), demonstrating robust performance under high-rate conditions.
• The method enables a 3× reduction in front-end electronics channels compared to narrow-strip solutions (e.g., 2.54 mm pitch), by allowing larger strip pitches without impedance mismatch.