Bypass and Decoupling Capacitors 

Bypass and decoupling capacitors remove noise from power rails as devices switch, reducing the ringing waveform which creates EMC at the ringing waveforms frequency.  They also cause ground currents to return back to the bypass capacitor of the device instead of to the ground input point of the PCB, stopping large loop areas beign created.  See this excellent video demonstrating why bypass and decoupling capacitors are so important for signal integrity and also EMC compliance – a must watch!:

https://www.eevblog.com/2018/05/19/eevblog-1085-bypass-capacitors-visualised/

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