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Engineering Technology
High-Speed PCB

High-Speed PCB Design Guide 5: Noise Reduction Technology for DSP Systems

With the rise of high-speed DSPs (Digital Signal Processors) and peripherals, managing Electromagnetic Interference (EMI) has become a crucial aspect of design. In the past, issues of emission and interference were broadly referred to as EMI or RFI (Radio Frequency Interference). Today, these are part of a more nuanced area: Electromagnetic Compatibility (EMC), which encompasses both emission control and system immunity.

A DSP system achieves EMC compliance if it meets these three conditions:

  1. It does not interfere with its own operation.
  2. It does not interfere with other systems.
  3. It is resistant to interference from other systems.

Understanding Interference

Interference occurs when unwanted energy affects a receiver, causing malfunction. It can arise via conductors (directly) or radiation (indirectly). High-speed digital systems, such as DSPs, are particularly susceptible due to their clock circuits, which often generate broadband noise reaching hundreds of MHz.

Sources of Interference:

  • Conducted EMI: Noise carried through physical conductors like power lines or cables.
  • Radiated Coupling: Crosstalk occurs when electromagnetic fields induce currents in adjacent conductors.
  • Common Impedance Coupling: Shared impedance between circuits can lead to voltage modulation and unwanted noise.

Emission Types:

  1. Differential Mode (DM): Signals that propagate as differences between two conductors.
  2. Common Mode (CM): Unintentional voltages raise all circuit connections above system ground, often causing more severe EMI than DM.
High-Speed PCB

Key Factors Influencing EMC

  1. Voltage: Higher supply voltages result in greater emission levels, while lower voltages may increase sensitivity.
  2. Frequency: Higher frequencies generate stronger emissions. Switching and periodic signals introduce high-frequency noise spikes.
  3. Grounding: Proper grounding is fundamental to EMI control. Choose:
    • Single-point grounding for <1MHz.
    • Multi-point grounding for high frequencies.
    • Mixed grounding for circuits combining low and high frequencies.
  4. Power Supply Decoupling: Transient currents from switching must be minimized with decoupling capacitors and filtering techniques.
  5. PCB Design: Well-thought-out PCB layouts are essential to mitigate EMI risks.

Noise Reduction Techniques

To reduce noise in DSP systems, address three areas:

  1. Suppress emissions at the source.
  2. Minimize the coupling path.
  3. Increase the receiver’s immunity to interference.

1. Board Construction Techniques

  • Use ground and power planes for low impedance decoupling.
  • Employ multi-layer PCBs to shield traces and minimize loops.
  • Separate analog, digital, transmitter, and receiver grounds/power lines.
  • Avoid cutting PCB traces near sensitive components.
  • Maintain trace widths (4-8 mils) to reduce capacitive coupling and high-frequency damping.
  • Ensure proper grounding with multi-point configurations to handle high-frequency currents effectively.

2. Trace Routing Techniques

  • Use 45° turns instead of 90° to avoid characteristic impedance changes.
  • Keep trace spacing greater than the trace width to reduce crosstalk.
  • Minimize clock loop areas and ensure high-speed signals are short and direct.
  • Avoid routing sensitive traces parallel to high-current switching signals.
  • Isolate noisy signals like clock lines and buses from I/O lines and connectors.

3. Filtering Techniques

  • Filter all power lines and signals entering the PCB.
  • Decouple IC pins with high-frequency, low-inductance ceramic capacitors (e.g., 0.1µF for <14MHz, 0.01µF for >15MHz).
  • Employ multi-stage filters to address multi-band power noise.
  • Bypass all power and reference voltage pins of analog circuits.

4. Advanced Noise Control

  • Embed and ground crystals to reduce oscillation noise.
  • Use series termination resistors to minimize transmission line reflections and resonance.
  • Place decoupled line drivers and receivers near I/O interfaces.
  • Shield critical components and use twisted pairs for interfering leads.

Designing DSP Systems for EMC

Proactive noise reduction is vital for ensuring EMC compliance in DSP systems. Techniques like proper grounding, decoupling, and PCB layout design play pivotal roles in mitigating EMI. Shielding, filtering, and careful component placement further enhance the system’s robustness. By addressing EMC challenges during the design phase, you can optimize performance and meet stringent industry standards.

Key Takeaways

  • Prioritize grounding and decoupling in your designs.
  • Optimize PCB layouts to minimize noise coupling paths.
  • Use advanced filtering and shielding techniques to handle high-frequency noise.

By implementing these noise reduction techniques, you can create efficient and EMC-compliant high-speed DSP systems.

KKPCB conducts research on special processing technologies such as ordinary double-sided boards, thick copper circuit boards, high-frequency circuit boards, HDI circuit boards, rigid-flexible circuit boards, FPC flexible boards, buried blind hole circuit boards, and IC carrier boards. Provides PCB design, PCB layout, PCB prototyping and PCB assembly services.

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