Automotive Emissions
Understanding Automotive Emissions
Every electronic circuit that switches current generates electromagnetic emissions. A switching regulator's MOSFET transitions from fully on (milliohms) to fully off (megaohms) in 5 to 20 nanoseconds. This rapid dV/dt creates current spikes that ring through parasitic inductances in the PCB layout, generating broadband spectral content from the fundamental switching frequency all the way up to hundreds of MHz. In a vehicle, this noise has nowhere to hide: the wiring harness carries it throughout the body, and the vehicle's metal structure acts as both a ground plane and a radiating surface.
Emission Sources by Frequency
| Source | Fundamental Frequency | Harmonic Content Extends To | Victim Receiver |
|---|---|---|---|
| EV Traction Inverter | 10 to 20 kHz | 30 MHz | AM Radio (530 kHz to 1.7 MHz) |
| DC-DC Buck Converter | 500 kHz to 2 MHz | 500 MHz | AM, FM, DAB |
| CAN Bus | 500 kbps to 5 Mbps | 200 MHz | FM Radio (88 to 108 MHz) |
| Ethernet 100BASE-T1 | 66.67 MHz (PAM3) | 500 MHz | FM, DAB, GPS |
| Camera Serializer (GMSL) | 3 to 6 Gbps | 3 GHz | GPS (1.575 GHz), Cellular, V2X |
CISPR 25 Test Setup
CISPR 25 testing uses an Absorber-Lined Shielded Enclosure (ALSE) to isolate the measurement from external ambient signals. The DUT is mounted on a conductive ground plane and connected to a power supply through a Line Impedance Stabilization Network (LISN) that provides a standardized 50-ohm impedance from 150 kHz to 30 MHz. A 1.5-meter wiring harness runs from the DUT to a termination load, and measurement antennas are positioned 1 meter from the harness center.
Amplitude of n-th harmonic ≈ (2 × Vpk) / (n × π) × sin(nπD) × sin(nπfτ) / (nπfτ)
Where:
Vpk = Peak voltage of the switching waveform
D = Duty cycle
f = Fundamental switching frequency
τ = Rise/fall time
n = Harmonic number
The envelope of harmonics drops at -20 dB/decade up to 1/(πτ),
then at -40 dB/decade above 1/(πτ).
Faster edges = more high-frequency content.
The most effective mitigation is slowing down switching edges (increasing τ) to push the -40 dB/decade rolloff below the victim receiver's frequency. A 10 ns rise time produces harmonics to ~30 MHz. Slowing to 50 ns pushes meaningful content below 6 MHz, clearing the FM band entirely.
Frequently Asked Questions
What is the biggest source of automotive emissions?
Switching power converters are the dominant source. A DC-DC buck converter at 2 MHz generates harmonics extending to 500 MHz. In EVs, the traction inverter switching at 10 to 20 kHz with 800V bus voltage generates emissions from 150 kHz to 30 MHz that can saturate the AM radio band.
How is CISPR 25 testing performed?
The DUT is mounted on a ground plane inside an ALSE. A 1.5-meter harness connects it to power through a LISN. Radiated emissions are measured with rod, biconical, and log-periodic antennas at 1 meter distance. Conducted emissions use a current probe clamped around the harness.
What is the difference between Class 3 and Class 5?
The difference is approximately 10 to 20 dB across most frequency bands. Class 5 is the strictest and is required by virtually all OEMs for modules near the vehicle's antenna systems. Most Tier 1 suppliers default to Class 5 to avoid part-number proliferation.