Products which operate on power mains are the ones that need to be verified for conducted emissions. Battery operated devices that do not have a recharge function, like a handheld FM radio do not have a conductive path to couple noise.
The idea behind testing for conducted emissions is two folds:
- to understand that the noise generated by a device does not get coupled to the power supply network and affect the operation of other devices.
- to make sure that noise generated by a device does not couple itself to the power supply network and radiate through another device that is connected to the network.
So to make sure this doesn’t happen, products that work on power mains are tested for conducted emissions and conducted immunity.
Unlike radiated emission, conducted emission is a problem at lower frequencies. This is because the lower frequencies associated with a circuit need very long antennas or radiating structures to generate radiations. Since such structures are not prominent in every day designs, the noise travels through the conducted path instead of radiating.
Conducted emissions has a major impact on power supply design because of the additional filtering circuit that needs to be added to eliminate noise from getting out of the device. Design decisions have a huge impact on conducted emissions. For example, the type of power supply (linear or switching) used has an impact on conducted emissions performance. Though Switching power supplies are highly efficient, they are very noisy for sensitive designs and call for a variety of noise suppression techniques to be followed.
Regardless of the noise associated with the power supply, a Line Impedance Stabilisation Network (LISN) needs to be used in the test set-up between the Power socket and the Device under Test whenever conducted emissions is measured. As the name suggests, the LISN stabilises the impedance for the DUT over the measurement frequency range and also filters any external power supply noise from getting into the DUT during measurements.
Conducted emissions are regulated by two separate standards called FCC and CISPR 22. FCC is for the American market and defines limits for conducted emissions from 450KHz to 30MHz while CISPR22 is mainly for the European market and defines limits from 150KHz to 30MHz.