Clyde,
CAPACITANCE AND INDUCTANCE ARE SPECIFIED AT A GIVEN FREQUENCY. See the
example.
Yes, often the datasheet states the measurement frequency. But this is invariably a "low" frequency, low enough to keep parasitic effects negligible. The inductance or capacitance of a component won't vary significantly from that frequency down to DC.
The part value starts changing significantly at frequencies high enough to make parasitic effects significant.
Alan,
I have carefully measured normal HF range inductors with a dip oscillator and SDR at working frequencies comparing with a meter that measures
in the 100Khz range. I noticed no significant difference.
When the inductors are built in such a way that in the HF range the parasitics are low, then this is indeed the normal situation. But if you measure them at VHF, the values will very likely change. And at UHF they surely will. It's a simple matter of going high enough in frequency, to make the values of any part change dramatically.
At HF you can often get away with taking the low-frequency values and assuming they will hold true at your working frequency, But not always. And at VHF that becomes rarer, and at UHF it becomes very rare.
Dave:
Since I enjoy challenging you, I will do it again! ;-)
Now, ask yourself: "is an inductor good at bypassing rail noise
as the capacitor was intended to accomplish?" NO!
Well, it depends! A DC-blocked inductor can be a pretty good bypass element, no worse than a capacitor! It just depends on its impedance at the frequency in question. Such a "nasty" 100nF bypass capacitor is resonant at 1.5MHz when its equivalent series inductance is 112nH. A foil-wound capacitor might indeed be that bad. A ceramic capacitor only if mounted with very long leads. Anyway, assuming it has indeed 112nH and thus is resonant at 1.5MHz, how would it behave at 3MHz? Well, it would have a reactance of less than 2?! That's still a pretty good bypass, despite being inductive. At 100MHz it would be bad.
If you replace that nasty 100nF capacitor by a 10nF one, would it be better? NO, if you keep those long leads! It will be much worse bypassing low frequencies, it will be good at its resonant frequency near 5MHz, but at 100MHz it will be almost as bad as the 100nF one.
And what happens if you follow that old rule of putting the 100nF capacitor in parallel with a 1nF one? Well, at some frequency you get a might parallel resonant circuit, with the 100nF capacitor acting as the inductor, and at that frequency you get infinite impedance, and thus NO bypassing! Of course, only if the capacitors have high Q at that frequency. So the important point with bypass capacitors is: They should have enough capacitance for the low frequencies, low enough ESL for the high frequencies, and they should be bad! I mean, they should have a low Q. A high loss factor. That largely pevents getting unbypassed frequencies due to bypass caps happily parallel-resonating with each other.
There is a long-standing myth about electrolytic caps needing a parallel-connected ceramic cap to provide bypassing over a wide frequency range. Using a parallel ceramic cap is indeed useful if this is a chip capacitor. But placing something like a an old-fashioned disc ceramic cap in parallel with an electrolytic of comparable path length doesn't help much, since both have roughly the same ESL.
In some equipment I often see real collections of 6 or more different capacitors in parallel, placed there by some designer who thinks that each frequency will then take the path it likes best. The only problem is that physics don't work like that. Those nice showcases of six different capacitors in parallel are mainly good for one thing: Getting a good laugh!
I also often laugh about that old rule of "one bypass cap per IC". When using slow ICs, often a single bypass cap is enough for the entire board, and in other cases one cap every so much distance is enough. One can save quite a bit of money in series production by leaving out unnecessary parts. Of course without overdoing that...
