It's said that electrolytic capacitors behave as inductors at high frequencies, which is why we put small ceramic caps in parallel with them:
Electrolytic, paper, or plastic film capacitors are a poor choice for decoupling at high frequencies; they basically consist of two sheets of metal foil separated by sheets of plastic or paper dielectric and formed into a roll. This kind of structure has considerable self inductance and acts more like an inductor than a capacitor at frequencies exceeding just a few MHz.
Capacitor impedance vs. frequency.
Yet I also see a few things like this:
The "inductance problem" associated with electros is another idiotic myth - they have no more inductance than a length of wire the same as the length of the cap.
or
A popular myth is that electros have considerable inductance because of the way the foil is wound inside the can. This is nonsense - the foils are usually joined at the ends in the much the same way as with film caps. High frequency performance usually extends to several MHz, even with standard off-the-shelf electros and bipolar (non-polarised electrolytic) caps.
What is the exact nature of this effect and in what applications and frequencies do we need to worry about it? What are the practical implications?
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