CASE / STRUCTURED ENGINEERING RECORD

150 MHzharness radiated-emissions exceedance L4 Case

A narrowband 150 MHz exceedance is traced through harmonic source correlation, connector mode conversion, harness electrical length and measured common-mode current, then closed with controlled A/B experiments and regression verification.

Open diagnostic workflow ↗Test method ↗

ROOT-CAUSE PHYSICS CHAIN

SourceClock / switching harmonics, Unbalanced drive
PathConnector mode conversion, Reference discontinuity, Parasitic capacitance
ModeCommon mode, Mode conversion
AntennaHarness near λ/2 resonance
VictimCISPR 25 receiver
EvidenceCommon-mode current, Near field, Before/after spectrum

STRUCTURED EVIDENCE RECORD

A case is engineering data—not a conventional article.

The record preserves product and test metadata, failure signature, observations, candidate hypotheses, discriminating experiments, measured evidence, root cause, corrective action, side effects, regression results and transferable principles.

Metadata

Product: Automotive ECU and harness subsystem
Phase: Pre-certification engineering test
Test: CISPR 25 radiated emissions
Frequency: Narrowband exceedance near 150 MHz
Operating mode: ECU in normal operation
Environment: Absorber-lined shielded enclosure

Failure signature

CISPR 25 radiated emissions exceed the applicable limit by approximately 6–10 dB near 150 MHz. The narrowband spacing correlates with a clock or switching fundamental.

Observations

The 150 MHz peak changes with ECU operating mode.
A current probe shows a distinct harness common-mode current peak at 150 MHz.
Near-field scanning identifies strong coupling around the connector region.
Disconnecting the harness substantially reduces the peak.

Candidate hypotheses

2 candidate hypotheses are linked with supporting evidence, counter-evidence and current status.

hypothesis/cable-antenna-resonancehypothesis/connector-mode-conversion

Discriminating experiments

Harness A/B: original 1.0 m length versus 0.6 m · Variable: Harness length

Before: At 1.0 m, the 150 MHz emission is approximately 8 dB over the limit.

After: At 0.6 m, the 150 MHz peak falls and the resonant feature shifts upward.

Conclusion: Harness electrical length is a key factor. Shortening is effective but constrained by vehicle packaging.

Add a common-mode choke at the connector entry · Variable: Common-mode impedance

Before: A pronounced 150 MHz common-mode current peak is present without the choke.

After: The choke reduces common-mode current by approximately 12 dB.

Conclusion: The characterized choke is effective and brings the emission below the limit.

Evidence

Before/after common-mode current comparison
Before/after CISPR 25 spectrum
Near-field scan record

Root cause

Source (clock or switching harmonics) → path (connector-region mode conversion and reference discontinuity) → mode (differential-to-common-mode conversion) → antenna (harness near a λ/2 resonance at 150 MHz) → receiver (CISPR 25 measurement system).

Corrective action

Add a characterized common-mode choke at the connector entry and improve connector-region reference continuity; use harness shortening only as a secondary measure.

Side effects

Verify differential-mode loss across the functional band.
Common-mode impedance may fall under DC bias.
Harness shortening is constrained by vehicle packaging.

Regression verification

Repeat the full CISPR 25 band under the original configuration, confirm removal of the 150 MHz exceedance and absence of new high-frequency failures, then repeat functional testing.

Transferable principle

For a narrowband failure, correlate frequency spacing and harmonics first. Measure harness common-mode current before treating cable radiation, and inspect connectors as likely differential-to-common-mode conversion points.