Hi:

how dose bypass capacitor filter out the noise on the signal?

say i have a 74HC74 chip... with a bypass capacitor connecting the Vcc and

Gnd, i get a really nice wave form. but w/o one, i got noice all over the

place! can't even get a waveform on the scope...

it's like magic!

i know how a cap can filter out high freq. signal. but just can't figure out

how a bypass cap work....

thank you for any answers...

Wires have inductance. Circuit board traces have inductance. It

takes time for current to make its way from a power supply to a chip

that has a sudden drop in resistance across its supply lines (and

modern chips can do this extremely quickly, compared to the time it

takes light to make its way to the supply and back). Inductance takes

time for current to change by the differential equation:

V=L*(di/dt). A capacitor is defined by the differential equation:

I=C*(dv/dt). This means that the current through a capacitor is

proportional to the time rate of change of the voltage across it.

(Whew!)

So taking all that into account at the same time (imagination is

faster than the speed of light) when a chip passes a sudden pulse of

current, instead of that pulse having to make its way through the

circuit board trace inductance and supply wiring inductance (including

the time it takes for the current to change through those inductance

and not even worrying about the speed of light over that distance) and

having to put up with all the voltage sag it takes to drive that

current pulse through all that inductance, if you put a capacitor very

close to the chip, and if the capacitor is big enough, a small sag in

the capacitor voltage allows it to supply a very quick pulse of

current to the chip.

Then the inductive supply distribution system charges the cap back up

before the next pulse.

Isn't that easy?

;-)