Conducted emission testing measures the voltage or current that interference carries out along supply and signal lines. For automotive electronics the main references are CISPR 25 and GB 18655, with testing performed on a LISN (line impedance stabilisation network) inside a shielded room or anechoic chamber.
How the test works
The product is connected to a LISN, which inserts a known impedance into the supply loop, converts the disturbance voltage the product produces on the supply line into a signal the receiver can measure, while supplying stable power and isolating external mains noise.
Compared with radiated emission, conducted emission covers lower frequencies (commonly 150 kHz to 30 MHz or 50 MHz), because low-frequency disturbance travels more readily along conductors than through space.
Test items
Supply-line conducted emission: the LISN is connected at the product's power input and the disturbance voltage leaving along the supply line is measured.
Signal-line conducted emission: some standards require conducted emission measurement on specified signal lines.
Current probe method: a current probe is inserted into the supply loop to measure disturbance current directly, which suits high-power products.
Common causes of an exceedance
The switching frequency of a DC-DC supply and its harmonics exceed the limits.
Insufficient bulk capacitance at the input, or ESR too high, leaving ripple above the limit.
Supply management IC switching frequency drifting in standby.
Switching frequencies of multiple rails interfering with each other and producing additive noise.
Poor grounding design, with return paths crossing digital and analogue areas.
Corrective directions
Add a pi filter at the input, and check that the filter is impedance-matched at rated load.
Choose a supply solution with better switching frequency and harmonic behaviour, adding spread-spectrum techniques where necessary.
Adjust the supply topology and operating frequency so that multiple rails do not overlap.
Re-plan the return paths so that high-current and sensitive signals return in separate areas, with proper partitioning where a ground plane is shared.
Standby power issues can often be improved with a dummy load or by adjusting the power management strategy.
Frequently asked questions
Is conducted emission above the limit the same problem as radiated emission above the limit?
They are different paths. A conducted exceedance usually means insufficient supply or signal filtering; a radiated exceedance usually means a layout or shielding problem. Their root causes are often related, though — a supply module may both generate conducted noise and, through layout coupling, cause a radiated exceedance. It is worth analysing both directions when correcting.
Why is it still above the limit after adding filter capacitors?
There are three common reasons: the capacitor self-resonant frequency is below the failing band; the capacitor impedance has already risen at the actual operating frequency; or the filter sits far from the noise source, so radiation occurs on the trace before filtering. Filters should be placed close to the interference source, and the radiation from the trace itself has to be considered.
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.
