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Hi
when an ic switchs on it draws current from supply line suddenly this sudden change in current causes harmonics and transients we eleminate these effects by bypass capacitor also we call it decoupling capacitor.
There are many viewpoints about having capacitors on motherboard:
If it is large electrolytic capacitors, then the capacitors are used for energy storage as well as providing a smooth shut down to devices and functional systems in the motherboard with respect to the discharging curve of a capacitor.
It is the same concept used in smoothing the rectified power supply.
If it is small ceramic capacitors between 220 to 100nF or 0.1uF, then the capacitors are used for filtering high frequency noises between 1kHz to about 40MHz. It is also used to decouple voltage dips due to SSN (simultaneous switching noise) when multiple CMOS circuits turn on and draws substantial amount of current from supply rail to charge the load.
If you hold a PhD or good MS in VLSI from top university, perhaps you will know these:
If it is PETF capacitors, then it is used for providing the nearest return path for non-ideal paths. These are usually placed evenly all over the motherboard. This reduced the loop path, hence minimises differential-mode radiation. If this is placed close to clock signals and high-speed switching signals, it is used to minimise common-mode radiation.
If you have embedded passive capacitors on motherboard placed closest to processor, then it is used to push the poles due to impedance discontinuity to higher frequencies.
Other than the above, it could be used in a RC resonator or part of a RLC filter somewhere.
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