Electromagnetic interference, or EMI, is one of the most common problems engineers encounter when developing electronic and industrial control systems.

A PCB may work perfectly on the laboratory bench but begin to experience communication errors, unexpected resets, sensor noise, or intermittent operation after it is installed inside a machine.

Why does this happen?

The problem is often not a single defective component. Instead, EMI can result from the interaction between:

• PCB layout
• Power supply
• Grounding
• High-speed signals
• Switching circuits
• Cables
• Connectors
• Shielding
• External equipment

A useful way to troubleshoot EMI is to follow the complete path:

Noise Source → PCB → Ground/Return Path → Cable → Connector → External System

Good EMI design starts by controlling the source and its current-return path rather than simply adding a shield at the end. Recent Analog Devices guidance also emphasizes placement, grounding architecture, return-current control, power routing, and shielding as parts of an integrated EMI strategy.

1. What Is EMI?

EMI stands for Electromagnetic Interference.

EMI occurs when unwanted electrical or electromagnetic energy interferes with another circuit or system.

For example:

Switching Regulator
       │
       │ High-frequency noise
       ▼
     PCB
       │
       ├── MCU
       ├── Sensor
       └── Communication
              │
              ▼
            Cable
              │
              ▼
       External Equipment

The result could be:

• MCU reset
• Communication error
• RS-485 communication failure
• ADC measurement noise
• Sensor malfunction
• Display errors
• Motor-control problems
• Unexpected system shutdown

EMI can be conducted through electrical connections or radiated through electromagnetic fields.

2. Conducted EMI vs Radiated EMI

Understanding this distinction makes troubleshooting much easier.

Conducted EMI

Noise travels through:

• Power lines
• Ground connections
• Signal wires
• Communication cables
• Supply rails

Example:

DC/DC Converter
      │
      │ Noise
      ▼
    VCC ───────► MCU
      │
      └────────► Sensor

If the switching regulator creates excessive ripple, that noise can propagate through the power distribution network.

Radiated EMI

Noise travels through electromagnetic fields.

For example:

High-speed PCB trace
       ))))))))))
       ))))))))))   Electromagnetic field
       ))))))))))
             │
             ▼
       Sensitive circuit

Long traces, large current loops, poor grounding, and rapidly changing signals can increase the possibility of radiation or susceptibility.

Analog Devices notes that PCB grounding and return-current design are important for controlling EMI/RFI, while ground planes can help provide low-impedance return paths and reduce emissions.

3. The First Question: Where Is the Noise Coming From?

When an EMI problem occurs, don’t immediately replace components.

First ask:

What changed?

For example:

• Did the motor start?
• Did communication begin?
• Did a switching regulator turn on?
• Did the load current increase?
• Did the cable become longer?
• Did the PCB move into a metal enclosure?
• Did a new sensor get connected?
• Did the grounding structure change?

This question is extremely useful in industrial troubleshooting.

4. Identify the Noise Source

Typical PCB EMI sources include:

Switching regulators

DC/DC converters can generate significant high-frequency switching currents.

Look carefully at:

• Switching node
• Inductor
• Diode/MOSFET
• Input capacitor
• Output capacitor
• Power and ground return paths

The high dV/dt and dI/dt portions of the circuit are particularly important.

MCU clocks

MCU clock signals can contain significant high-frequency components even when the fundamental clock frequency appears relatively low.

For example: 

Fast edges contain higher-frequency components.

Therefore, EMI is not determined only by the nominal clock frequency.

Motors and relays

Industrial systems can be especially challenging because motors, solenoids, relays, and contactors can generate substantial transient noise.

Typical events include:

Motor ON
   ↓
Large current change
   ↓
Voltage disturbance
   ↓
Ground/power noise
   ↓
PCB circuit affected

5. Check the Ground First

Ground is one of the most important areas to investigate.

A ground connection is not a perfect zero-impedance reference.

Every conductor has:

• Resistance
• Inductance

Therefore:V=IRV=IR

and at high frequencies:V=LdidtV=L\frac{di}{dt}

Even a small inductance can create a significant transient voltage when current changes rapidly.

Analog Devices specifically discusses how ground-plane impedance, return current, and inductance can affect noise and circuit performance.

6. Follow the Return Current

One of the most important EMI troubleshooting techniques is:

Don’t look only at the signal path. Look at the return path.

Consider:

Signal Source ─────────────► Load
      │                       │
      └──── Return Current ◄──┘

The signal and return path form a loop.

A larger loop can increase electromagnetic coupling.

For high-frequency signals, return current tends to follow a low-impedance path associated with the signal path. A continuous reference plane can therefore be very useful.

7. Avoid Unnecessary Ground-Plane Gaps

A common PCB layout mistake is creating unnecessary gaps or slots in the ground plane.

For example:

Signal
──────────────────────────►

Ground Plane
██████████     ███████████
             GAP

The return current may have to travel around the gap.

That can increase:

• Loop area
• Inductance
• Noise
• Crosstalk
• EMI

Therefore, don’t automatically divide a PCB ground plane simply because the circuit contains analog and digital sections.

Analog Devices notes that blindly splitting ground planes can increase return-path inductance and voltage noise; good circuit partitioning can often avoid the need for a split.

8. PCB Layout: Separate Noisy and Sensitive Circuits

Good physical placement can significantly reduce EMI problems.

A practical arrangement might look like: 

Try to keep high-noise circuits away from sensitive circuits.

Examples of noisy circuits:

• Switching regulators
• Motor drivers
• Relay drivers
• High-current outputs
• Fast clocks

Sensitive circuits may include:

• ADC
• Sensors
• Analog amplifiers
• Crystal/oscillator circuits
• Communication receivers

9. Keep High-Speed Signal Loops Small

Consider two layouts.

Poor layout 

 Large loop area.

Better layout

Source ─────────► Load
       ◄─────────
       Return

Smaller loop area.

The basic objective is:

Keep the signal and its return path close together.

This is particularly important for:

• Clock signals
• Fast digital signals
• Differential interfaces
• High-speed buses
• Switching power circuits

TI’s EMC guidance similarly recommends reducing trace length and loop area and maintaining appropriate ground structures.

10. EMI Problems on RS-485

Industrial RS-485 systems are a good example.

A typical system may look like:

PLC
 │
 │ RS-485
 │
 ▼
Controller
 │
 ▼
Sensor

Potential problems include:

• Incorrect termination
• Poor grounding
• Long cable
• Shielding problems
• Cable routing near motors
• Ground potential differences
• Reflections
• Common-mode noise

For RS-485 troubleshooting, check:

1. Cable type
2. Twisted pair
3. Characteristic impedance
4. Termination
5. Bias resistors
6. Shielding
7. Ground/reference
8. Cable routing
9. Oscilloscope waveform

This connects directly with the previous PowerTechGuide articles on industrial communication and RS-485 termination.

11. Cable Routing Is Extremely Important

A perfectly designed PCB can still experience EMI problems because of its cables.

For example:

Motor Cable
══════════════════════

       ↑ EMI coupling

Signal Cable
───────────────

Avoid routing sensitive signal cables parallel and close to high-current or high-noise cables for long distances when possible.

Typical noise sources include:

• Motor cables
• AC power cables
• Solenoid wiring
• Relay wiring
• Inverter outputs

Separate noisy and sensitive wiring whenever the mechanical design permits.

12. Shielding

Shielding can help prevent electromagnetic energy from coupling into sensitive circuitry.

A simplified structure is: 

But shielding is not a substitute for good PCB layout.

A shield must also have an appropriate connection strategy.

For cable-connected systems, the relationship between circuit ground, chassis/shield ground, and external equipment can become important. Analog Devices discusses shielding and common-mode currents as important considerations when connecting separate ground systems.

13. Don’t Automatically Connect Every Ground Together

This is an important engineering point.

There are potentially different concepts:

• Signal ground
• Power ground
• Chassis ground
• Earth ground
• Shield

They may need to be connected depending on the system architecture, safety requirements, EMC requirements, and isolation design.

Therefore:

Never assume that connecting every ground together is automatically the best solution.

Analyze where the current will flow.

14. Use an Oscilloscope During EMI Troubleshooting

An oscilloscope is one of the most useful tools for investigating PCB noise.

Measure:

VCC

Check:

• Ripple
• Overshoot
• Undershoot
• Switching spikes
• Startup behavior

RESET

Check: 

Normal: 

 RESET 

Problem: 
 RESET   

Communication signals

Check:

• Ringing
• Overshoot
• Undershoot
• Distorted edges
• Common-mode disturbance

15. Be Careful With Oscilloscope Ground Leads

A long oscilloscope ground lead can itself create misleading waveforms.

For example:

Probe
  │
  │
  └──────────── Long Ground Wire
                 ↑
             Antenna effect

At high frequencies, the probe connection can pick up noise.

For better measurements, use a short ground connection or an appropriate spring/low-inductance probing method.

Otherwise, you may end up troubleshooting the measurement setup rather than the PCB.

16. Compare Good Board vs Bad Board

This is one of the most effective practical engineering techniques.

If you have:

• Good PCB
• Bad PCB

measure both under the same conditions.

Compare:

Measurement	Good Board	Bad Board
VCC DC	___ V	___ V
VCC ripple	___ mVpp	___ mVpp
RESET	Stable	Unstable
Clock	Clean	Noisy
RS-485	Clean	Ringing
Ground noise	Low	High
Temperature	___ °C	___ °C

This often reveals the problem much faster than randomly replacing components.

17. Real-World Engineering Experience

In one controller development project, I experienced a startup problem related to an unstable power-reset signal.

The controller did not always start correctly when the incoming power condition was unstable.

The troubleshooting process showed that the problem was not simply the MCU itself.

The power-reset behavior and boot-control signals needed to be improved.

I modified:

• The power reset circuit
• The reset signal behavior
• Boot-control signals
• Pull-up configuration

After the changes, the controller became much more reliable during startup.

This experience reinforced an important engineering lesson:

Power stability, reset stability, signal integrity, and PCB layout are closely connected.

An EMI or noise problem can sometimes appear as a software problem, communication problem, or random startup problem.

18. Industrial Example: PLC + Robot + Vision System

Consider a manufacturing line:

      PLC

Now add:

Motor
  │
  └── High-current switching

Possible EMI paths include:

 

The resulting symptom may be:

Motor ON → communication error

But the real cause may be:

Motor switching → EMI → cable coupling → PCB disturbance → communication error

This is why troubleshooting should identify the complete noise path.

19. Practical EMI Troubleshooting Flow

Use the following sequence.

Step 1 — Define the symptom

Example:

RS-485 communication fails when the motor starts.

Step 2 — Reproduce the problem

Determine exactly when the failure occurs.

Step 3 — Identify the noise source

Check:

• Motor
• Relay
• DC/DC
• Clock
• PWM
• Solenoid

Step 4 — Measure VCC

Use an oscilloscope.

Step 5 — Measure RESET

Check whether the controller is actually resetting.

Step 6 — Check communication waveform

Look for:

• Ringing
• Overshoot
• Noise
• Ground disturbance

Step 7 — Check ground

Follow the return-current path.

Step 8 — Check cable routing

Look for parallel routing with noisy cables.

Step 9 — Check shielding

Check shield termination and chassis connections according to the system design.

Step 10 — Make one change at a time

For example:

Change grounding
       ↓
Test

Change cable routing
       ↓
Test

Add filtering
       ↓
Test

This makes the root cause much easier to identify.

Step 6 — Check communication waveform

Look for:

• Ringing
• Overshoot
• Noise
• Ground disturbance

Step 7 — Check ground

Follow the return-current path.

Step 8 — Check cable routing

Look for parallel routing with noisy cables.

Step 9 — Check shielding

Check shield termination and chassis connections according to the system design.

Step 10 — Make one change at a time

For example:

Change grounding
       ↓
Test

Change cable routing
       ↓
Test

Add filtering
       ↓
Test

This makes the root cause much easier to identify.

20. Common EMI Troubleshooting Mistakes

Mistake 1: Adding a capacitor everywhere

More capacitors do not automatically solve EMI.

First identify:

• Noise frequency
• Noise path
• Source
• Return path

Mistake 2: Splitting the ground plane automatically

A ground split can sometimes increase return-path inductance.

Use it only when the circuit architecture actually requires it.

Mistake 3: Looking only at the signal

Always ask:

Where does the current return?

Mistake 4: Ignoring cables

The PCB may be fine while the cable acts as an antenna or coupling path.

Mistake 5: Measuring only with a multimeter

A multimeter may show:

3.30 V

while the oscilloscope could reveal:

3.30 V with 300 mV high-frequency spikes.

The DC number alone may therefore not explain an intermittent failure.

21. EMI Troubleshooting Checklist

Before changing the PCB, check:

Power

• VCC voltage
• VCC ripple
• Startup voltage
• Load transient
• DC/DC switching waveform

Ground

• Continuous ground plane
• Return-current path
• Ground-plane gaps
• Ground vias
• Power-ground current path

Signal

• High-speed traces
• Clock routing
• Differential pairs
• Communication waveform
• Signal loop area

Cable

• Cable length
• Cable type
• Shield
• Connector
• Routing
• Motor-cable separation

Mechanical

• Metal enclosure
• Chassis connection
• Shielding
• Connector location

Test

• Oscilloscope
• Short probe ground
• Good/bad board comparison
• Maximum-load test
• Motor ON/OFF test
• Communication test

22. The Most Important EMI Engineering Rule

When troubleshooting EMI, don’t ask only:

“Where is the noise?”

Also ask:

“Where does the noise current flow?”

A successful EMI solution usually requires understanding three things:

Source → Coupling Path → Victim

For example:

     SOURCE
        │
        ▼
   Motor Switching
        │
        │ EMI coupling
        ▼
    Cable / Ground
        │
        ▼
       PCB
        │
        ▼
      VICTIM
     RS-485

Once this path is understood, the solution becomes much more systematic.

PowerTechGuide Takeaway

PCB EMI troubleshooting is not simply about adding filters, capacitors, or shields.

A reliable approach is to understand the complete electrical system:

Power → Ground → Return Current → PCB Layout → Signal → Cable → Shield → External Equipment

Good EMI performance starts during PCB design, not after the first EMC test failure.

For practical engineering, remember:

Control the noise source.
Control the return path.
Minimize loop area.
Separate noisy and sensitive circuits.
Measure before modifying.

These principles are especially important in industrial systems involving PLC, Robot, Vision Systems, RS-485, sensors, motors, and embedded controllers.