How to Tackle EMI/EMC and Signal Integrity Issues in HF PCB Design

High-frequency PCB design is challenging, with electromagnetic interference (EMI) and signal integrity (SI) issues among the most critical obstacles. These issues can degrade circuit performance, impact reliability, and result in failed compliance testing. To address them, careful planning and execution are essential throughout the design process. Below are strategies to overcome these challenges.

For networks with complex signal directions, the impact of topology varies for unidirectional, bidirectional, and different types of signal levels. It is challenging to determine which topology is optimal for signal quality. Simulations must be performed beforehand, and selecting the appropriate topology requires engineers to have a thorough understanding of circuit principles, signal types, and even the difficulty of routing. And to know more about PCBs, see our ultimate guide on PCB manufacturing.

Tips to Optimize PCB Layout for Signal Integrity:

The layout is a significant factor in ensuring signal integrity. For high-frequency designs, pay attention to:

• Trace Lengths: Keep signal traces as short as possible to minimize propagation delay and reflections.

• Impedance Control: Match the trace impedance to the source and load using controlled-impedance traces, especially for high-speed signals.

• Via Placement: Minimize the use of vias in high-speed signal paths as they can introduce impedance discontinuities and reflections.

• Differential Pair Routing: Use tightly coupled differential pairs for high-speed signals to reduce crosstalk and maintain signal integrity.

How is Impedance Matching Performed in Layout Design of HF PCBs?

Impedance matching is one of the critical factors in designing high-speed PCBs. Impedance is directly related to the trace layout, such as whether the trace is on the surface layer (microstrip) or an internal layer (stripline/double stripline), its distance from the reference plane (power or ground), trace width, and PCB material, all of which affect the trace’s characteristic impedance. Therefore, impedance can only be determined after routing.

Most simulation software cannot account for impedance discontinuities in routing due to limitations in circuit models or the mathematical algorithms used. In such cases, the schematic can only reserve some terminators, such as series resistors, to mitigate the effects of impedance discontinuities. The fundamental solution is to avoid impedance discontinuities as much as possible during routing.

Tips for Designing High-Speed (>100MHz) High-Density PCBs?

When designing high-speed, high-density PCBs, crosstalk interference must be given particular attention, as it significantly impacts timing and signal integrity. Here are some points to note:  

Spacing: Pay attention to trace spacing. A common practice is to maintain a spacing of at least twice the trace width. Simulation can be used to determine the impact of trace spacing on timing and signal integrity, and to identify acceptable minimum spacing. The results may vary for different chip signals.  

Cross-talk: Avoid having the routing directions of adjacent layers be the same, and especially avoid exact overlaps between traces on adjacent layers, as such configurations produce more crosstalk than neighboring traces on the same layer. Crosstalk occurs when signals on adjacent traces interfere with each other. Mitigation techniques include:

• Trace Spacing: Increase the spacing between adjacent high-speed traces to reduce capacitive and inductive coupling.

• Shielding: Add grounded guard traces between sensitive signal traces to provide isolation.

Via's: Use blind and buried vias to increase routing space, although this increases PCB manufacturing costs. While achieving completely parallel and equal-length traces in practice is challenging, efforts should be made to approximate this as closely as possible.  

Additionally, consider reserving differential and common-mode terminations to mitigate the impact on timing and signal integrity.

Selection of Filters at High Frequency:

Selecting inductance and capacitance values in an LC filter involves balancing noise suppression and performance. Inductance must suit the noise frequency and allow transient currents without excessive ripple, while capacitance depends on ripple noise requirements, with ESR and ESL affecting performance.

LC filters v/s RC filters: For low-frequency noise, LC filters may underperform if inductance is insufficient, making RC filters more effective despite energy loss in the resistor. In switching power supplies, LC placement impacts stability due to poles and zeros in feedback loops. Resistor power dissipation is also crucial in RC filtering. The components can be mounted as SMT or through hole, see the blog for assembly related info.

How to manage Ground and Power Planes in HF PCBs:

A well-designed ground and power distribution system is critical:

• Dedicated Ground Planes: Use continuous, unbroken ground planes to act as a return path and reduce EMI.

• Power Planes with Decoupling Capacitors: Strategically place decoupling capacitors near ICs to stabilize power supply fluctuations and suppress noise.

• Stack-Up Design: Implement a multilayer stack-up with proper layer arrangement. High-frequency signals should be routed on internal layers sandwiched between ground planes to reduce EMI.

What are the Types of Terminations?  

In digital circuits, timing is the most critical aspect. The purpose of adding matching is to improve signal quality so that a definitive signal is obtained at the moment of decision. Termination, also known as matching, is categorized by location into source termination and end termination.

• For level-sensitive signals, ensure that the signal quality is stable while meeting setup and hold time requirements.

• For edge-sensitive signals, ensure that the signal edge is monotonic and that the edge rate meets requirements.  

Typically achieved by parallel resistance matching, including Pull-up, Pull-down resistors, Thévenin matching, AC matching and Schottky diode matching.

What is EMI in PCB design?

EMI (Electromagnetic Interference) in PCB design refers to unwanted electromagnetic signals that can disrupt a circuit's performance. For example, in a high-speed digital PCB, poorly routed high-frequency traces can radiate interference, affecting sensitive analog components nearby. To mitigate EMI, designers often use techniques like proper grounding, ensuring signal integrity through impedance matching, and incorporating shielding. For instance, placing high-frequency components away from analog circuits and using ground planes can significantly reduce EMI, improving the overall reliability and functionality of the PCB.

What is EMC in PCB design?

EMC (Electromagnetic Compatibility) in PCB design ensures that a device operates as intended without causing or being affected by electromagnetic interference (EMI). For example, in a high-frequency PCB, poor layout or insufficient shielding can cause EMI, leading to signal distortion in nearby circuits. To address this, designers might add ground planes for shielding, optimize signal routing to reduce crosstalk, and use decoupling capacitors near power pins to stabilize voltage. Proper EMC design minimizes interference, ensuring compliance with industry standards and reliable system performance.

What rules should designers follow to address EMC/EMI in high-speed PCB design?  

A good EMI/EMC design must account for component placement, PCB layer stack-up, critical connection routing, and component selection from the layout stage. Without adequate planning, addressing these issues later would be much more costly and time-consuming. Complying with EMI/EMC regulations, such as FCC or CE standards, is vital for product success. These standards provide guidelines on PCB design and testing to ensure electromagnetic compatibility.

• Place the clock generator away from external connectors.

• Route high-speed signals through internal layers, ensuring impedance matching and continuity with the reference plane to reduce reflections.

• Use components with slower signal slew rates to minimize high-frequency noise.

• Select decoupling or bypass capacitors with suitable frequency responses to reduce power plane noise.

• Minimize loop areas and optimize the return path of high-frequency signals to reduce radiation.

• Use split ground planes to contain high-frequency noise.

• Carefully select PCB-to-chassis ground connection points to enhance EMC performance.

• Use 45° or curved corners instead of sharp 90° angles to minimize signal reflections and impedance changes.

• Use termination resistors to reduce signal reflections and noise.

How to meet EMC requirements without significantly increasing costs?  

The cost increase on a PCB due to EMC requirements often comes from adding ground plane layers to enhance shielding effects and including ferrite beads, chokes, and other components to suppress high-frequency harmonics. Apart from this, it is usually necessary to use additional shielding structures to ensure that the entire system meets EMC requirements.

Use of Ferrite Beads in HF PCBs:

A ferrite bead is a passive component used in PCBs to suppress high-frequency noise in electronic circuits. It acts as a low-pass filter, allowing DC and low-frequency signals to pass while attenuating high-frequency noise. Key Uses of Ferrite Beads:

1. Noise Suppression

• Blocks high-frequency noise from propagating in power or signal lines.
• Commonly used to eliminate EMI in circuits.

2. Compliance with EMC Standards

• Prevents noise generated by circuits from interfering with other devices.
• Helps achieve regulatory compliance for electromagnetic emissions.

What is a "Signal Return Path"?

The signal return path, also known as return current, refers to the path through which the current flows back to the driver. In high-speed digital signal transmission, the signal travels from the driver along the PCB transmission line to the load, and then returns from the load through the shortest path via the ground or power layer to the driver. This returning signal is referred to as the signal return path.

Dr. Johnson explained in his book that high-frequency signal transmission is essentially a process of charging the dielectric capacitance between the transmission line and the DC layer. Signal Integrity (SI) analysis examines the electromagnetic properties of this field and their coupling.

Suitable EDA Software For High-Speed Signal Processing PCBs:

Before finalizing the design, use simulation tools to analyze EMI and SI. Tools like HyperLynx or Ansys HFSS can predict issues and suggest optimizations for trace routing, impedance matching, and noise reduction

Additionally, the EasyEDA Professional Edition (EasyEDA Pro) is recommended, combining ease of use and powerful features, making it particularly suitable for complex PCB design needs. Its advantages include:

1. Enhanced performance: Based on the WebGL engine, it supports PCB designs with tens of thousands of pads while maintaining smooth operation in complex projects with numerous components.

2. Improved rule management: Provides more robust rule constraint and auto-routing features, significantly improving routing efficiency.

3. Multi-board design support: Allows multiple single-board designs within one project, ideal for complex projects.

4. Enhanced component management: Integrates symbols, packages, 3D models, etc., through the "component concept," making component selection easier and reducing switching between the marketplace and design interface.

5. Efficient output and compatibility: Offers powerful DXF import and export functionality, enhanced PDF output, and meets diverse design delivery needs.

Conclusion

EMI and signal integrity challenges in high-frequency PCB design require a comprehensive approach involving careful layout, stack-up design, and the use of advanced simulation tools. By following these best practices, designers can enhance performance, reduce noise, and achieve compliance with regulatory standards, paving the way for reliable and efficient high-frequency PCB designs.

FAQ:

1. If a circuit consists of several PCBs, should they share the same ground?

If a circuit consists of several PCBs, they are generally required to share the same ground. Using multiple power supplies within a single circuit is typically impractical. However, if specific conditions allow, using separate power supplies could reduce interference.

2. How should layout and routing be handled to ensure the stability of signals above 50 MHz?

High-speed digital signal routing focuses on reducing the impact of transmission lines on signal quality. For high-speed signals above 100 MHz, the layout requires the signal routing to be as short as possible.

In digital circuits, high-speed signals are defined by their signal rise time. Additionally, different types of signals (such as TTL, GTL, and LVTTL) require different methods to ensure signal quality.

3. What is ESD:

ESD (Electrostatic Discharge) is the sudden flow of electricity between two objects due to static buildup. It can damage sensitive components.

4. When designing a PCB to reduce interference, should the ground line form a closed-loop structure?

When designing a PCB, the loop area should generally be minimized to reduce interference. The ground line should not form a closed loop; instead, a tree-branch structure is preferable. Additionally, the ground area should be maximized as much as possible.

5. What do FCC and EMC specifically mean in terms of safety standards?

FCC: Federal Communications Commission (United States Communication Commission).

EMC: Electromagnetic Compatibility.

FCC is a standards organization, while EMC is a standard. Each standard has its specific purpose, associated reasons for its establishment, and defined testing methods.

6. What are the rules for adopting termination (matching)?  

In digital circuits, timing is the most critical aspect. The purpose of adding matching is to improve signal quality so that a definitive signal is obtained at the moment of decision.