Maintaining signal integrity (SI) and preventing connectors from ruining high-speed signals requires careful attention to design details like impedance matching, minimizing reflections, reducing crosstalk, and ensuring proper terminations. It begins by defining the specific signal speed and bandwidth requirements and selecting the optimal connector.
The definition of high-speed signals is relative to the application, and in many instances, it’s increasing over time. For example, in today’s digital electronics applications, high-speed signals are defined as ≥ 10 Gbps. In some applications, the line of demarcation is moving toward 100 Gbps or higher.
What’s the best high-speed connector?
The identification of the best high-speed connector depends on application factors like the operating environment, required bandwidth, installation requirements, and more. Many connectors are designed to meet specific industry standards. Some standard connectors include (Figure 1):
- Fiber optics delivers the maximum speed, often >1 petabit per second, and great immunity from electromagnetic interference (EMI). Two examples of the numerous options include LC connectors for high-density networking and MPO for high-bandwidth data center installations.
- Backplanes prioritize low insertion loss and high signal integrity (SI). Examples include Amphenol’s Paladin HD and EXAMAX2, TE Connectivity’s IMPACT, MULTIGIG RT, and Samtec’s ExaMAX.
- Mezzanine connectors are specialized board-to-board interconnects designed for stacking parallel PCBs in high-density, high-performance applications. Some options include Samtec’s SEARAY™, Molex’s HD Mezz, and TE Connectivity’s Mezalok.
- RF/Coaxial connectors come in a variety of industry standards like SMA, SMPM, and BNC, optimized for mm-wave systems and RF testing.
Connector selection, integration, and system testing of high-speed signals focuses on optimizing SI. The optimal SI is crucial for maintaining data quality and timing. Key SI metrics for high-speed signals include eye diagram metrics (height, width, jitter), S-parameters (also called scattering parameters) like insertion loss and return loss, and impedance control.
For example, matching the connector’s differential impedance (often 100 Ω or 85 Ω) to the system helps to minimize insertion loss and crosstalk and ensure sufficient bandwidth. Connector differential impedance measures the impairment of high-speed electrical signals travelling through a pair of connector pins/contacts driven with equal, opposite polarity signals.
Optimization is essential for preventing signal reflections in high-speed systems like USB, HDMI, and PCIe, etc. by matching the impedance of the PCB traces. There can be some leeway in how precisely the matching needs to be.
PCIe example
Differential impedance is only one factor to consider. Figure 2 is a time domain reflectometry (TDR) graph for a differential channel. It plots impedance over time as the signal travels between PCBs using mating connectors and a 12” cable, showing how different channel impedances (85 Ω, 92.5 Ω, and 100 Ω) affect signal integrity within a PCIe 5.0 reference system.
This test uses a signal rise time of 12 ps, and the test setup includes 2-inch PCB sections and a 12-inch cable, with a 93 Ω mating connector. Three different total channel impedances are shown: 85 Ω (red line), 92.5 Ω (green line), and 100 Ω (blue line).
The figure shows the effect of impedance matching and signal reflections at different points in the channel. The cable impedance choice depends on the priorities for a specific design. For example, if insertion loss is the most important consideration, then 93 or 100 Ω cable can be the best choice.
Eye on performance
Designers use eye diagrams to visualize the performance of high-speed interconnects. Eye diagrams show how inter-symbol interference (ISI), jitter, and noise distort high-speed signals by simulating or measuring the channel’s impulse response from S-parameter. The resulting eye diagram shows the cumulative effects of insertion loss (closing the eye vertically) and jitter (closing it horizontally).
Common uses for eye diagrams include verifying signal quality and confirming standards compliance. S-parameters, like S21 for insertion loss and S11 for return loss, are used to describe the channel’s frequency response. A sampling oscilloscope or a high-bandwidth real-time digital oscilloscope can be used for measuring and analyzing eye diagrams. Design simulation tools can convert S-parameters to the time domain using inverse fast Fourier transform (IFFT) to determine how a digital pulse is distorted (Figure 3).
Summary
Proper specification, selection, and design of high-speed interconnects for advanced digital and communication systems relies on connector performance. Specific types of connectors are available that deliver optimal bandwidth for a range of applications. Simulation and testing are important considerations to ensure that the designed performance is the delivered performance.
References
10 Best High-Speed PCB Routing Practices, Sierra Circuits
Connectors are fighting! Causes and countermeasures for poor connector connection, IRISO
FEA Simulation Aids Signal Integrity in High-Speed Connector Designs, Greenconn
High-speed Connector Design with Modeling and Simulation, Dassault Systèmes
How Different Connector Types Affect Network Performance, Cable Wholesale
How RF Trends in Electronics Are Impacting Connectors, EPEC Engineered Technologies
How to select a High-Speed Connector for Your Design Application, Air Electro
Multi-Board PCB Signal Integrity: A Complete Guide, Altium
PCI Express: Is 85 Ohms Really Needed?, Samtec
The Benefits of Overmolded Electrical Connectors for Communication Applications, Bead
Tips to Prevent EMI in High-Speed Signals, AllElectroHub
What Is an S-Parameter?, MathWorks
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