The measurement of conducted emissions of switched mode power supplies (SMPSs) is discussed for the 2-1000-kHz frequency range, analyzing variability caused by setup elements and test conditions. The tested SMPSs have 12-VDC output and about 20-W nominal power. They are fed by a controlled ac power supply, instead of mains, through a line impedance stabilization network and various loading levels are applied. Identified causes of variability (supply voltage and frequency variations, thermal stabilization influencing internal mechanisms of emissions) indicate a relevant impact on the spread of measured conducted emissions, ranging from a fraction of decibels to about 10 dB (sample standard deviation). Such phenomena represent a cause first of all of systematic error, but also of increased uncertainty. No EMC standards are known to take them into account, nor the scientific literature reports extensive quantification, as in the present case. The shift of the line spectrum of emission consequential to a mains frequency change revealed to be exploitable also for better identification of internal sources of emission.
