Conducted emissions are the electromagnetic radiations generated by a device that gets transferred to its subcircuit or another device via cabling, PCB traces, power/ground planes, or parasitic capacitance. The conducted emission appearing on the interface and power cables must be kept low or they can propagate through cables and reach other devices causing problems in their intended operations. Conducted disturbances are also radiated from the conductors as both electric and magnetic waves like an “accidental transmitting antenna.” Similarly, when these wires, cables, connectors, or any other conductors are exposed to EM fields, currents and voltages are induced in them, thus making the conductor to behaves as an “accidental receiving antenna.”

All of the conducted and accidentally received currents and voltages create interference to cause unacceptable variations in its circuits’ electrical and electronic performances. This also may cause malfunction of the software.

What is Conducted Emissions Testing?                         

   Fig. Conducted Emissions test set-up

All devices emit electromagnetic radiation unintentionally or intentionally. These radiations in form of conducted emissions are needed to keep as low as possible to gain the optimum performance of the machines. Hence, many international organizations have set limits on the emissions that must be complied with by the manufacturers prior to launching them into the market for sale. 

EMC Standards for performing Conducted Emissions Testing:

• CISPR 25: This standard contains limits for the measurement of radio disturbances in onboard receivers caused by vehicles, boats, internal combustion engines, trailers, and devices in the frequency range of 150 kHz to 5 925 MHz. The limits are intended to provide protection for onboard receivers installed (per the manufacturer’s guidelines) in a vehicle from the conducted disturbances produced by components/modules in the same vehicle.
• CISPR 11: This standard specifies the limits of emissions from the equipment in the frequency range 9 kHz to 400 GHz by using a conducted measurement technique on the AC mains supply cable. The conducted emissions limits applied by this standard are intended to protect equipment connected to the same AC mains supply, and also are assumed to protect the radio communications spectrum below 30 MHz from the fields radiated by an equipment’s AC supply cables.
• IEC 61000-6-3: This is a generic standard that defines the emission test requirements for apparatus defined in the scope concerning continuous and transient, conducted, and radiated disturbances. The emission requirements have been selected to ensure that disturbances generated by apparatus operating normally in residential, commercial, and light-industrial locations do not exceed a level that could prevent other apparatus from operating as intended.
• IEC 61000-6-4: This generic standard is similar to IEC 61000-6-3. The only difference is that this standard specifies the conducted emission limits of the devices used in industrial environments. 

Test Equipment Required for Conducted Emissions Testing

The following test equipment are used for conducted emissions testing:

• Line impedance stabilization network (LISN): For conducted emissions measurement LISN is used instead of antennas. A LISN is a lowpass filter that is placed between an AC or DC power source and the EUT to generate a known impedance and to provide an RF noise measurement port. It also isolates the unwanted RF signals from the power source.                      

Fig. Circuit diagram of LISN

• Spectrum analyzer or EMI receiver: to measure noise.
• Ground Plane: the EUT, spectrum analyzer, and LISN are placed over and connected to a common ground plane.

Click here for Emission testing labs.