In PCB design, electromagnetic interference (EMI) control is one of the core challenges for ensuring products pass radiated and conducted emissions testing. Component placement is not just about putting parts on the board — it directly affects the intensity of electromagnetic radiation and immunity to external interference.

This guide explores how to effectively reduce EMI and improve signal integrity through proper circuit partitioning and component placement.

1. PCB Partitioning: The Foundation of EMI Control

PCB partitioning (also called board partitioning) is a method used to organize different circuit sections on a printed circuit board and keep them isolated from each other. This technique improves overall board performance, especially in EMI control. It not only helps reduce electromagnetic interference but also enhances signal integrity in PCB designs.

The core principles behind PCB partitioning include:

• Confining high-energy components of high-frequency digital signals

• Avoiding common-impedance coupling between different circuit types on the board

• Reducing current loop area to lower radiation and improve immunity to external interference

2. Core Concepts: Understanding High-Speed Signals and Return Paths

2.1 High-Speed vs. Low-Speed Signals and Their Harmonics

The first core concept involves controlling the high-energy harmonic components generated by fast-switching signals and their rate of current change over time (di/dt). The higher the current change rate, the stronger the harmonic energy in the signal, and the greater the radiation risk.

The second concept is that a signal's return current varies with signal frequency. This is because the impedance encountered during signal propagation includes not only conductor resistance but also capacitance, especially loop inductance. As signal frequency increases, the inductive component of impedance (frequency-dependent) increases significantly.

2.2 Differences in Return Paths

Since current always seeks the lowest-impedance path, it's important to understand:

• High-frequency signals: Return current closely follows the signal current to minimize the inductive loop

• Low-frequency signals: Inductance effects diminish, and the resistive component of impedance dominates. Return current spreads across the conductor surface to find the path of least resistance

For PCB designers, the key takeaway: the path of return current back to the source depends on signal frequency.

3. Partition Design: How to Correctly Divide Circuit Areas

As PCB designers, our task is to minimize interference between return currents and avoid common-impedance coupling that can cause electromagnetic radiation.

To achieve this, specific areas or partitions can be defined on the PCB, with each partition dedicated to a specific circuit type. This also reduces current loop area and lowers differential-mode radiation.

For mixed-signal boards, typical partitions include:

4. Key Principle: Don't Split the Return Reference Plane

Some designers may try to split the return reference plane (RRP) to further isolate return current paths between different circuits. But this practice does not align with EMC best practices — it creates a voltage difference between two metal regions, forming an antenna-like structure that causes electromagnetic radiation.

The correct solution:

Use a complete, low-impedance return reference plane so return currents can find the preferred path back to the source. The plane should have no cuts, splits, or large gaps that could become common-mode noise sources.

Component placement should be divided into independent areas based on circuit type and function — but the reference plane should remain whole and continuous.

5. Cable-Formed Antenna Structures: Critical Design for I/O Areas

The input/output (I/O) area is critical:

• On one hand: Noise injection into cables must be limited

• On the other: Filtering and shielding are needed to reduce external interference or suppress board noise radiation

Layout must consider cables and surrounding equipment or structures.

The digital section should be kept away from the I/O area — ideally located at the center of the board rather than the edge, to prevent high-energy harmonics from coupling into cables or radiating from edges.

Typical problem: Clock signals couple into networks connected to other parts of the board (such as power and analog sections), which may connect to cables. The cables then act as antennas, intensifying radiation.

Design Recommendations:

• Concentrate all input/output cables on one side of the board, not multiple edges

• Avoid voltage differences between cables

• Prevent antenna-like structures that can intensify digital signal or noise radiation

6. Summary

Through proper circuit partitioning and component placement, radiated emissions can be effectively reduced, and external interference coupling into the design can be limited. Core principles:

1. Partition by function: Digital, analog, power, I/O, and filter sections

2. Keep the reference plane intact: Do not split the return reference plane

3. Keep digital section away from I/O: Avoid harmonic coupling into cables

4. Concentrate I/O cables on one side: Avoid antenna-like structures

5. Understand return current paths: High frequency follows the signal; low frequency spreads across the surface

Following these guidelines effectively reduces radiated emissions and limits external interference coupling into the design.

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