Introduction
In the world of Ethernet circuit design, Bob Smith Termination (BST) has long been a topic of debate among engineers and signal integrity experts. Named after its inventor, Robert W. Smith, this termination scheme is widely used to reduce common-mode noise and electromagnetic interference (EMI) in Ethernet systems. However, its effectiveness and correctness have been questioned, with some arguing that it is suboptimal or even unnecessary. This article delves into the intricacies of Bob Smith Termination, examining its principles, applications, controversies, and alternatives. By exploring both sides of the debate, we aim to provide a comprehensive understanding of whether BST is the correct choice for Ethernet designs.
What is Bob Smith Termination?
Definition and Purpose
Bob Smith Termination is a circuit configuration designed to mitigate common-mode noise in Ethernet systems. It involves connecting the center taps of Ethernet transformers to ground through a network of resistors and capacitors. The primary goal is to provide a low-impedance path for common-mode currents, thereby reducing EMI and improving signal integrity .
Typical Configuration
The standard BST configuration includes:
- Four 75-ohm resistors: Two for the transmit (Tx) pair and two for the receive (Rx) pair.
- A capacitor: Typically ranging from 1 nF to 4.7 nF, depending on the system’s bandwidth.
- Ground connection: The resistors and capacitor are connected to a common ground point, often chassis ground or analog ground .
Historical Context
Bob Smith introduced this termination scheme in the 1990s, primarily for use in Ethernet systems with Category 5 (CAT5) cables. The design was based on the assumption that the common-mode impedance of CAT5 cables is approximately 145 ohms, which has since been challenged .
How Does Bob Smith Termination Work?
Common-Mode Noise Reduction
Common-mode noise arises when unwanted currents flow equally in both conductors of a differential pair, often due to external interference or imbalances in the circuit. BST provides a controlled path for these currents to dissipate, preventing them from radiating as EMI .
Impedance Matching
BST aims to match the common-mode impedance of the Ethernet cable, ensuring minimal signal reflection and maximum power transfer. However, the effectiveness of this matching depends on the accuracy of the assumed impedance values .
Role of the Capacitor
The capacitor in the BST network serves two purposes:
- DC blocking: It prevents DC currents from flowing through the resistors, ensuring that the termination network only affects AC signals.
- Frequency tuning: The capacitor’s value determines the frequency range over which the termination is effective .
Controversies and Criticisms
Disputed Impedance Values
One of the main criticisms of BST is its reliance on an assumed common-mode impedance of 145 ohms for CAT5 cables. Studies have shown that the actual common-mode impedance of CAT5 cables is closer to 70-100 ohms, depending on the configuration .
Alternative Termination Schemes
Jim Satterwhite, a prominent critic of BST, proposed an alternative termination scheme using 52.3-ohm resistors. His measurements demonstrated that this configuration provides better return loss and common-mode noise reduction compared to the traditional 75-ohm BST .
Mixed Results in Practice
Some designers report excellent results with BST, while others find it ineffective or even detrimental. For example, Royce Bohnert’s study found no significant difference in return loss between BST, Satterwhite’s scheme, and no termination at all .
Impact on Power over Ethernet (PoE)
In PoE systems, BST can interfere with the detection of powered devices (PDs). The 75-ohm resistors create a low-impedance path that can cause PD detection to fail. To address this, some designs incorporate a series capacitor to isolate the BST network during the detection phase .
Best Practices for Using Bob Smith Termination
When to Use BST
- Legacy Systems: BST is well-suited for older Ethernet systems using CAT5 cables, where the assumed impedance values are more applicable.
- EMI Reduction: In environments with high EMI, BST can provide additional noise suppression.
- Cost-Effective Solutions: BST is relatively simple and inexpensive to implement, making it a practical choice for budget-conscious designs .
When to Avoid BST
- Modern Cables: For CAT6 and higher cables, the common-mode impedance is significantly different, rendering BST less effective.
- PoE Applications: In PoE systems, BST may require modifications (e.g., adding a series capacitor) to avoid interference with PD detection.
- High-Speed Designs: For high-speed Ethernet (e.g., 10 Gbps), alternative termination schemes may offer better performance .
Simulation and Testing
Before implementing BST, it is crucial to simulate the circuit using tools like SPICE to evaluate its effectiveness. Physical testing, such as return loss measurements and EMI scans, can further validate the design .
Alternatives to Bob Smith Termination
Satterwhite’s Termination Scheme
Jim Satterwhite’s alternative uses 52.3-ohm resistors to better match the common-mode impedance of CAT5 cables. This scheme has been shown to improve return loss and reduce common-mode noise .
No Termination
In some cases, omitting the termination network altogether may yield acceptable results, especially in low-noise environments or with modern cables .
Custom Termination Networks
For specialized applications, custom termination networks can be designed to match the specific impedance and noise characteristics of the system. This approach requires detailed analysis and testing but can offer superior performance .
Case Studies
Case Study 1: Legacy Ethernet System
A manufacturer of industrial Ethernet switches implemented BST in their CAT5-based systems. Despite initial skepticism, the design achieved excellent EMI performance and passed all regulatory tests. The company attributed this success to careful simulation and testing during the design phase .
Case Study 2: PoE-Enabled Device
A developer of PoE-enabled security cameras encountered issues with PD detection when using BST. By adding a series capacitor to the termination network, they were able to isolate the BST during the detection phase, ensuring reliable operation .
Case Study 3: High-Speed Ethernet Router
A networking equipment manufacturer opted for Satterwhite’s termination scheme in their 10 Gbps Ethernet router. The design achieved superior signal integrity and EMI performance, validating the use of alternative termination methods in high-speed applications .
Conclusion
Bob Smith Termination has been a staple of Ethernet circuit design for decades, offering a simple and cost-effective solution for reducing common-mode noise and EMI. However, its effectiveness is highly dependent on the specific application, cable type, and design constraints. While BST remains a viable option for legacy systems and low-cost designs, modern Ethernet systems may benefit from alternative termination schemes or even the omission of termination altogether. Ultimately, the decision to use BST should be based on careful simulation, testing, and consideration of the system’s unique requirements. By understanding the strengths and limitations of BST, designers can make informed choices that optimize performance and reliability in their Ethernet designs.