Same head unit, same hardware revision, same operating mode: laboratory A reads 32 dBµV/m and laboratory B reads 38 dBµV/m. The equipment may not differ, but the set-up certainly does — a large part of a radiated emission reading is decided by how the harness runs, how the ground plane is bonded and where the antenna stands.
1. What CISPR 25 radiated emission measures
It measures the disturbance field strength a component radiates through space. The frequency range covers 150 kHz to 2.5 GHz, received by an antenna in an absorber-lined shielded enclosure (ALSE) at a measuring distance usually in the 1 m band, with the antenna type changed by frequency range. Limits are graded from Class 1 to Class 5, the class corresponding to how much interference the mounting position can tolerate.
The domestic counterpart is GB 18655; the correspondence in limits and set-up is compared clause by clause in CISPR 25 versus GB 18655. On a real project, which version and which class applies is whatever the technical agreement states.
2. The set-up decides the reading: four places where it goes wrong
| Item | Standard requirement | Common deviation |
|---|---|---|
| Ground plane and bonding | Material, dimensions and the method of bonding to the chamber ground are all specified | Oxidised bonding points or too few of them, so high-band readings come out high |
| Harness length and routing | Length is banded, routing follows the standard diagram | Harness coiled, pressed against the plane or left hanging, which changes the radiation path |
| Antenna position and distance | Distance and height specified by frequency band, polarisation chosen to match the field | Antenna rotated or distance shortened, so near-field coupling is exaggerated |
| Sample operating state | Run at the worst-case condition, supply voltage per the agreement | Measured in standby or on a single channel, missing the true maximum emission state |
Of the four, harness routing is the most under-estimated. The harness is an antenna in the vehicle and it is an antenna in the chamber: ten-odd centimetres of length or a few centimetres closer to the plane changes the radiation efficiency of the common-mode current. When two sets of data diverge, ask for the set-up photos — more useful than reading the conclusion.
3. Class levels: the same part can pass and fail
Class 1 to Class 5 grade interference tolerance by mounting position: parts near sensitive receivers (modules close to an antenna, for example) are the strictest, parts in a well-shielded area can be relaxed.
The choice of class belongs neither to the laboratory nor to the supplier — it sits in the OEM's mounting position definition. The same controller may pass at one class in the boot and hit the limit when moved near the dashboard. So the question "will this part pass" cannot be answered without the class.
4. After a failure: prove the set-up first, then discuss corrective action
The handling order for a failing report should be fixed:
- Re-check the set-up: ground plane bonding, harness length and routing, antenna distance and polarisation, sample operating mode — all four against the standard diagram;
- Re-check the ambient: with a high chamber noise floor, the margin judgement in weak-signal bands is distorted;
- Locate the frequencies: map the failing frequencies to switching frequencies and clock harmonics and decide whether the emission is broadband or narrowband;
- Only then correct: start with the harness and grounding (terminating the shield, raising common-mode impedance), then move to board-level filtering and layout.
Skipping the first two steps and reaching for ferrites is the most common waste during development — the problem may not be in the sample at all. For how radiated and conducted work relate in a corrective plan, what automotive EMC testing actually measures sets out the fuller path.
5. A few honest words
Radiated emission is the item most likely to look like black magic, yet it is the most evidence-driven: a compliant set-up, a clean ambient and a well-defined operating mode make the data stable and reproducible. Conversely, a report with a vague set-up will not stand up in an OEM review even if the conclusion is a pass.
When TMC LAB issues this kind of report, the set-up photos and operating mode records are archived with it, so conditions can be matched directly during customer review or third-party comparison.
For the corresponding immunity methods see the ISO 11452 standard page; for emission limits and class definitions see the CISPR 25 standard page. If the end product needs export approval, start with the UN ECE R10 standard page for the type approval route.
Turnaround time is arranged according to the actual project. You are welcome to state your target date when submitting samples; we will check it against the current schedule, confirm a feasible plan with you, and update you at each milestone.
