Lesson 10/10100%
MODULE 10 — CASE STUDY

Case Study, Best Practices & Summary

A real 300 MHz radiated emission failure, traced from symptom to root cause to fix using the exact troubleshooting methodology from Module 09 — with a real before/after spectrum graph, animations of how the failure actually radiated, real lab equipment images, an EUT test setup diagram, a best-practices checklist, and a summary of everything this course has covered.

Case Study: 300 MHz Networking Board Failure

Problem

A networking board failed radiated emission testing at 300 MHz with a margin of 8 dB above the CISPR limit.

Investigation

Fundamental clock: 100 MHz
Third harmonic: 300 MHz
High emissions on the Ethernet cable

Root Cause

Common-mode current caused by an interrupted return path beneath the clock routing, combined with inadequate common-mode suppression at the Ethernet interface.

Solution

  • ✓ Restored continuous reference plane
  • ✓ Added common-mode choke to Ethernet
  • ✓ Optimized decoupling capacitor placement
  • ✓ Improved cable grounding

Result

Before
+8 dB over limit
Failed at 300 MHz (3rd harmonic of 100 MHz clock)
After
-6 dB margin
Redesigned board passed with 6 dB compliance margin

Before/After Spectrum Plot — a representative pre-compliance sweep of this exact case study, showing the fail trace collapsing below the CISPR 32 Class B limit line after the fix:

Representative quasi-peak trace (CISPR 32 Class B limit at 10 m). The 300 MHz peak — the 3rd harmonic of the 100 MHz system clock — is the point that failed before the fix and cleared the limit by 6 dB afterward. Always verify against the actual measured data from your test lab; this chart is for teaching purposes.

How the failure actually radiated

⏲ Interrupted Return Path Beneath the Clock Trace

A slot or split in the reference plane directly under a high-speed clock trace forces the return current to detour around the gap. That detour enlarges the effective current loop area — and radiated field strength scales with loop area × current × frequency² — which is why fixing the return path alone can remove several dB of emission.

🔌 Common-Mode Current on the Ethernet Cable

Once the 300 MHz harmonic coupled onto the board's ground structure, it drove a small common-mode current onto the shield/return of the Ethernet cable. At 300 MHz, a 1–2 m cable is a significant fraction of a wavelength, so it radiated efficiently — acting exactly like the monopole antenna described in Module 05.

Test Environment & Equipment Used

Semi-anechoic chamber used for EMC radiated emission testing, showing RF absorber cones lining the walls and ceiling
Semi-Anechoic Chamber (SAC) Where the board was retested before and after the fix — absorber-lined walls/ceiling and a conductive floor eliminate reflections.
Wikimedia Commons
Modern real-time spectrum analyzer used as an EMI test receiver to capture the before/after spectrum
EMI Test Receiver / Spectrum Analyzer Captured the quasi-peak trace shown in the before/after plot above, sweeping 30–1000 MHz against the CISPR 32 Class B limit.
Wikimedia Commons
Rotatable log-periodic dipole array antenna used to capture the 300 MHz emission
Log-Periodic Dipole Array Antenna Covers 300 MHz–1 GHz — the exact band containing the 3rd harmonic that failed in this case study.
Wikimedia Commons

EUT Test Setup Diagram — the geometry used to retest the board before and after the fix:

The board sat on a non-conductive turntable atop the ground plane; the receive antenna swept 1–4 m in height while the turntable rotated 0–360° to find the worst-case orientation for the 300 MHz emission.

Typical Test Equipment

📡
EMI Receiver / Spectrum Analyzer

CISPR 16-1-1 compliant receiver with peak, quasi-peak, and average detectors.

📶
Log-Periodic Dipole Array Antenna

Captured the 300 MHz harmonic; loop and biconical antennas cover lower bands.

🔌
Low-Noise Preamplifier

Boosted the weak 300 MHz signal above the receiver noise floor for accurate margin measurement.

🔄
Turntable & Antenna Mast

Automated 0–360° rotation and 1–4 m antenna height scan to find worst case.

🏠
Semi-Anechoic Chamber

Absorber-lined chamber with a conductive ground plane used for both retests.

📊
Near-Field Probe Set

Used on the bench during troubleshooting to localize the interrupted return path before the chamber retest.

Best Practices

  • ✓ Consider EMC from the initial design stage
  • ✓ Perform pre-compliance testing early in development
  • ✓ Use proper PCB stack-ups with uninterrupted reference planes
  • ✓ Keep high-speed loops as small as possible
  • ✓ Verify cable shielding and grounding strategies
  • ✓ Review clock routing and return current paths
  • ✓ Validate enclosure shielding effectiveness
  • ✓ Maintain detailed EMC design guidelines across projects

Summary

Radiated Emission (RE) testing verifies that electronic products do not emit excessive electromagnetic energy that could interfere with other equipment. Compliance with standards such as FCC, CISPR, IEC, MIL-STD, and DO-160 is essential for regulatory approval and reliable operation. Successful EMC performance begins with good design practices, including proper PCB layout, grounding, filtering, shielding, and cable management. Understanding emission sources, selecting the correct test methods, and conducting pre-compliance evaluations significantly reduce certification risks, development costs, and time to market. By integrating EMC considerations early in the product lifecycle, engineers can design products that are both compliant and robust in real-world electromagnetic environments.

Key Takeaways

  • ✓ Radiated emissions are electromagnetic waves unintentionally emitted into free space.
  • ✓ Compliance requirements vary by product category and target market.
  • ✓ Test frequency ranges extend from 9 kHz to 40 GHz, depending on the applicable standard.
  • ✓ Semi-anechoic chambers are the most common environment for compliance testing.
  • ✓ Proper PCB layout, grounding, shielding, and filtering are the most effective methods to control radiated emissions.
  • ✓ Early EMC planning and pre-compliance testing reduce redesign effort and certification delays.
  • ✓ A failing spectrum trace and a passing one often differ by only a few dB at a single harmonic — small layout fixes can be the difference between fail and pass.
FINAL KNOWLEDGE CHECK

Comprehensive Final Quiz

Score: 0 / 14

This final quiz draws from every module in the course. Select the best answer for each question.

Question 1 of 14

Course Complete!

You've finished all 10 modules of the Radiated Emissions (RE) Testing Academy — from EM fundamentals and emission sources through standards, test methods, measurement procedure, troubleshooting, and a real-world case study. You now have the foundation to plan, run, and interpret radiated emission testing on real products.

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