So if I know the noise on my power supply is say 20kHz how would I go about choosing a bypass capacitor value?

Hi

Without knowing a *lot* more than the frequency, picking a cap (or a bank of caps) is a bit premature.

1) Is the noise a sine wave tone or a square wave ripple? Different approach for each of those.

2) Is the noise broadband with a peak peak at 20 KHz? again a special case

3) Is the noise from the load rather than the supply? Again a different set of things to look at.

4) Is this some sort of massive supply current wise? (I have a 100A adjustable sitting over there .. some day I'm *sure* it will move it's self. I certainly am not picking it up ..).

5) Is this some sort of very high voltage supply? (another bunch of odd caps to pick between)

That's just the here and there questions. You will eventually get to:

1) What is the output impedance of the supply at 20 KHz?

2) How much noise do you have?

3) How much noise do you want?

If the supply has a 0.01 ohm output impedance and 100 whatever units of noise. You want 1 whatever unit of noise. Your cap needs to be 0.0001 ohms at 20 KHz. If you start with caps that have a 0.01 ohm ESR, you will need at least 100 of them in parallel. Since we have a hundred of them, they each will need to have a reactance of <0.01 ohms at 20 KHz. A thousand microfarads looks like a good bet.

Yes those are bogus numbers. No they don't apply directly to *your* supply. You need the numbers for your device. It does illustrate that tossing caps at the problem *can* have it's downside. Even if you go to 0.1 ohms Zo, you still have a hundred caps with a pretty low ESR.

Depending on your answers to all the goofy questions in the first part, your supply may not *have* a well defined output impedance or it's noise may come from a stability issue. Your giant cap bank *might* make things worse.

Bob