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| What kind of device can measure a coil's self resonance frequency? |
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| G0HZU:
If you want to measure something huge like a Tesla coil then it's probably best to do it when the coil is mounted in its natural environment. However, I've never tinkered with Tesla coils so I'm really just guessing. One possible method would be to use a nanovna and an H field coil on the source port to lightly excite the base of the coil and then use an E field coil at the receiver port of the nanovna to sniff for resonance near the top of the coil. See below for an old youtube video showing the locations of E field nulls along a helical resonator. You can see the first resonance frequency is quite stubborn. The other resonances show dips in the expected places. |
| Performa01:
--- Quote from: Berni on July 06, 2022, 09:26:18 am --- --- Quote from: Performa01 on July 06, 2022, 07:06:17 am --- --- Quote from: Ben321 on July 06, 2022, 05:01:41 am ---I think all I need is a battery powered device with 2 external connectors. One connector for one end of the coil, the other for the other end of the coil. Internally it would have a sinewave frequency swept generator, as well as a current peak detector. It would sweep the frequency of the sinewave past the resonant frequency of the coil, and the point of resonance would detect the sudden peak in current draw, and the numerical LCD display would display the frequency of the sinewave generator at the moment that the current peak was detected. The displayed frequency would therefore be the self resonant frequency of the coil. That's the kind of device I'd use, if it existed, to find the self resonant frequency of a coil. --- End quote --- The self-resonance of a coil is a prallel one, hence the current will not peak but dip to a minimum. --- End quote --- You can find both a series and parallel resonances if you look at a wide enough spectrum. Some might be more pronounced than others depending on the physical design of the coil. --- End quote --- No. There cannot be a series resonance in a properly connected inductor, because there is no series capacitance. You're probably referring to transmission line effects at very high frequencies, but this is a different mode of operation and we're talking about a Tesla coil here. Wehenver we mention the self-resonance of a coil we certainly mean the lumped element and not the transmission line that only comes into effect far beyond the regular operating frequency range of the component. --- Quote from: Berni on July 06, 2022, 09:26:18 am ---But yes in general what you think of as a coils self resonance point is when the parasitic interwinding capacitance cancels out the inductance (in parallel) and so the inductor starts looking like a low value resistor. --- End quote --- No. A parallel resonance circuit does NOT look like a low value resistor, but rather like an open circuit, like David Hess has already stated a while back in this thread. --- Quote from: Berni on July 06, 2022, 09:26:18 am ---You could calculate it from inductance and interwinding capacitance, but you can't directly measure the capacitance. So it is the easiest to just sweep the inductor across frequency and look for a dip in impedance.(Using a signal gen + scope or a VNA) --- End quote --- No again. We look at a peak in impedance, which is equivalent to a dip in current. |
| Berni:
Sorry i did get it backwards in terms of an inductor. I was focusing too much on a Tesla coil where what you are driving is actually the loosely coupled primary coil. You only get a significant amount of energy transfer between the coils once the main coil reaches resonance, so you get what looks like a dip at resonance and not much elsewhere. (But the dip comes due to the actual secondary oscillating at a high amplitude in such a phase to rob energy from the primary) |
| David Hess:
--- Quote from: G0HZU on July 06, 2022, 09:31:29 am ---One possible method would be to use a nanovna and an H field coil on the source port to lightly excite the base of the coil and then use an E field coil at the receiver port of the nanovna to sniff for resonance near the top of the coil. --- End quote --- Oh, that is another way which I had completely forgotten. In some cases, a dip meter can be used to measure the self resonant frequency of an inductor. |
| G0HZU:
I think in the case of a Tesla coil the E field probe would need to be some distance away in order to minimise any pulling of the resonant frequency of the coil. The H field probe could be loosely coupled into the primary winding when it it connected up to its driver stage. This would minimise any pulling by the primary stage. However, I've not got any experience of Tesla coils apart from watching demos when I was at school! Note that when VNA testing much smaller and easier to manage coils using S11 and S21 measurements, any minimum dip seen on a series S21 measurement will often be a bit higher in frequency than the parallel resonance measured using an S11 measurement. This is because of the distributed (transmission line) behaviour of a typical coil. A lot depends on the way the coil is wound. For use in something like an RF filter, I generally measure a coil with a VNA to get a full two port s2p file of S11, S21, S12 and S22. This is a really powerful method and it produces a decent small signal model that can be used in a simulator. However, this isn't going to be practical with something as huge as a Tesla coil. |
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