Radiated emission testing measures the electromagnetic field a product radiates into space, and is used to judge whether it will disturb the car radio, navigation, communication modules and other nearby electronics. For automotive work the main references are CISPR 25 and GB 18655, with testing performed in a semi-anechoic or fully anechoic chamber.
How the test works
While operating, a product radiates electromagnetic energy through enclosure seams, cables and connectors. The product is positioned on the turntable inside the chamber as the standard requires, an antenna receives the signal at the specified measurement distance (commonly 1 m for automotive electronics), and the receiver records field strength band by band.
The chamber walls are lined with absorber that swallows boundary reflections of free space, so what is measured is the product's own radiated emission rather than the result of wall reflections adding up.
Test conditions
Frequency range: CISPR 25 requires coverage from 150 kHz to 1 GHz; the exact limits and test layout follow the standard version in use.
Measurement distance: 1 m for automotive electronic devices in most cases (the 1 m method); some projects also assess the corresponding limits at 3 m.
Operating state: the product runs in its normal operating mode, covering several typical conditions where necessary (ignition, charging, switching loads) so that the worst-case emission peak is captured.
Sample state: assembled as it would be installed in the vehicle, including enclosures, harnesses, shields and any other structure that affects radiation.
Common causes of an exceedance
Clock traces routed without impedance control, so harmonics radiate above the limit.
Insufficient filtering in the switching supply module, leaving differential- and common-mode noise high.
Poor shielding at board gaps and enclosure seams, creating a slot antenna.
Unshielded harnesses, or a shield grounded poorly, effectively turning the harness into an antenna.
Connector shield shells not bonded reliably, leaving a gap.
Corrective directions
PCB layout: shorten clock traces, widen spacing, avoid long parallel runs, and add decoupling capacitors at key nodes.
Filtering: add common-mode chokes and differential-mode filters at the input, and choose low-noise supply solutions.
Mechanical: add conductive gaskets, increase bonding area, and extend the bonded width at screw compression points.
Harness: use shielded cable with 360-degree shield termination, and avoid grounding both ends of a shielded twisted pair, which creates a ground loop.
A targeted re-test after corrective work can confirm the improvement by comparing spectra.
Frequently asked questions
Do radiated and conducted emission have to be tested together?
Yes, both. Interference can leave a product through space or along supply and signal lines, and in real products the two paths usually coexist. Testing only one leaves the other path uncovered, and the product still fails if that path exceeds the limit.
What operating state should the product be in during the test?
It has to cover the typical conditions of real use. Automotive electronics, for example, should be measured separately in ignition, charging and load-switching states, and the worst result is taken as the basis for the verdict.
Service notes
Our laboratory holds both CMA and CNAS accreditation, covers about 10,000 m2 and runs more than 500 sets of equipment. All testing is carried out in our own laboratories — nothing is subcontracted. Plant visits and on-site witnessed testing are welcome.
Scheduling follows the actual project pipeline. You can state your preferred lead time when you submit samples; we will confirm a feasible plan against current capacity and keep you updated at each milestone. Once the report is issued we send the electronic copy; corrective-action advice can be arranged separately.
