Separated role of on-chip and on-PCB decoupling capacitors for reduction of radiated emission on printed circuit board

The power/ground fluctuation is known as a significant source of radiated emission. We discuss the separated functions of on-PCB and on-chip decoupling capacitors on the suppression of electromagnetic radiated emission. Due to the different ranges of parasitic inductance and the different locations of the on-chip current drivers, on-PCB and on-chip decoupling capacitors exhibit separated frequency characteristics in terms of suppression efficiency of radiation. The roles of on-PCB and on-chip decoupling capacitors are estimated by circuit simulation and a simple antenna model, and are confirmed by experiments. It is found that the on-chip decoupling capacitors are mainly effective for the suppression of radiated emission over 100 MHz frequency. Increase of the on-chip decoupling capacitance and decrease of the parasitic inductance of the package produce an improved suppression ratio at high frequency range. Combined placement and sizing of the decoupling capacitors have achieved more than 10 dB suppression of the electromagnetic radiated emission over a wide spectrum range