EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: Ben321 on July 04, 2022, 09:19:52 am
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I know, that the inter-winding capacitance of an inductor, combined with its inductance, gives every coil its own resonant frequency. What is the best way to measure this? Can measurement for this be easily made with the L and C settings of an LCR meter, and then manually calculate the resonant frequency? Or could it be made even easier with a device that's designed to measure resonance of an inductor? Does such a device exist? If so, what is it called?
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At resonance the inductor appears open, so a sine wave frequency sweep of any kind plus an AC voltage measurement can find the self resonant frequency quickly. This is essentially what a scalar or vector network analyzer would do and that might be required for a small inductance where the self resonant frequency is high. For the inductors I typically deal with, I just use my function generator and oscilloscope which works to 1s of MHz if not 10s of MHz. A measurement of phase will provide higher accuracy but seems silly since self resonate frequency is not well controlled in typical inductors.
An LCR meter would need variable frequency operation. Measurements of the impedance at three frequencies should allow calculation of the self resonate frequency for a simple inductor, but the math is not trivial.
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Note that there will be more than one resonance within a typical solenoid. At work I do a lot of wideband RF design and it can be challenging to design something like an ultra wideband high pass filter for example.
A typical solenoid has transmission line properties so it can also act as a fairly good short circuit at much higher frequencies. This is usually what limits the upper frequency range of a high pass filter for example. At high frequencies the passband will develop steep notches in the response. The length of the winding in each shunt inductor in the high pass filter gives a good clue as to when it will begin to show a reflection coefficient of close to 1 at 180degrees where it mimics a shorted halfwave transmission line and it crowbars the filter at this series resonance frequency. This causes the unwanted notches in the passband. At even higher frequencies this transmission line behaviour produces even more resonances.
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I use two transistors (TUN or TUP) (Peltz-oscillator) and a counter...
https://wiki.analog.com/university/courses/electronics/comms-lab-peltz-osc (https://wiki.analog.com/university/courses/electronics/comms-lab-peltz-osc)
not very precise due to the miller cap, but ok for me
any negative resistance circuit that matches the load ...
http://www.zen22142.zen.co.uk/Theory/neg_resistance/negres.htm (http://www.zen22142.zen.co.uk/Theory/neg_resistance/negres.htm)
another circuit I also tried for this :
https://www.mikrocontroller.net/topic/436039 (https://www.mikrocontroller.net/topic/436039)
easy readout of the needed negative resistance
just don't add the additional capacitor ...
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A nanovna would usually be the best tool for stuff like this in terms of price vs performance. They can be purchased for about $40 (USD) online.
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I agree ... the nano VNA works great for this.
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If you have a DSO that supports Bode Plots, this feature can be employed to investigate the parallel resonance of inductors.
Here are a few examples (SDS2104X+) we collected showing the parallel resonance. These resonance values agree with our LCR meter (IM3536) readings.
The simple quick setup was using Scope probes at 1X CH1 and CH2 set to 50 ohms, AWG input with clip leads and DUT inductor was connected between CH1 and CH2 probes and AWG connected to CH1 probe.
Note that when the inductor becomes parallel resonate the impedance is high and the plot will show a null, the peak at ~ 77MHz in the plots is the resonance of the setup and cable effects and not due to the DUT.
Images are:
10uH Leaded Inductor (#57)
470uH Leaded Inductor (#60)
100uH Leaded Inductor (#61)
100uH SMD Inductor (#62)
470uH SMD Inductor (#63)
Not a substitute for a VNA but acceptable for use with moderate size inductors like shown.
Best,
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Note that there will be more than one resonance within a typical solenoid. At work I do a lot of wideband RF design and it can be challenging to design something like an ultra wideband high pass filter for example.
A typical solenoid has transmission line properties so it can also act as a fairly good short circuit at much higher frequencies. This is usually what limits the upper frequency range of a high pass filter for example. At high frequencies the passband will develop steep notches in the response. The length of the winding in each shunt inductor in the high pass filter gives a good clue as to when it will begin to show a reflection coefficient of close to 1 at 180degrees where it mimics a shorted halfwave transmission line and it crowbars the filter at this series resonance frequency. This causes the unwanted notches in the passband. At even higher frequencies this transmission line behaviour produces even more resonances.
I'm well aware of the multiple self resonant frequencies in a solenoid, but the one I'd be interested in is the first resonant frequency, as it's the largest. It produces the largest signal amplification at resonance, and thus it's the one you really care about if building a Tesla coil.
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.
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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.
The self-resonance of a coil is a prallel one, hence the current will not peak but dip to a minimum.
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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.
The self-resonance of a coil is a prallel one, hence the current will not peak but dip to a minimum.
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.
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.
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)
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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.
https://www.youtube.com/watch?v=i7hhkUPgfFQ (https://www.youtube.com/watch?v=i7hhkUPgfFQ)
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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.
The self-resonance of a coil is a prallel one, hence the current will not peak but dip to a minimum.
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.
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.
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.
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.
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)
No again. We look at a peak in impedance, which is equivalent to a dip in current.
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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)
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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.
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.
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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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I would use a series through measurement on a network analyzer looking for the dip in S21.
Another method would to be to measure the coil at two different frequencies with a Q meter and calculate the self resonance.
A third method would to find the resonance by loosely coupling it to the coil on a grid dip meter and tuning to find the resonance.
A fourth method would to be to connect the coil to two different small capacitors and hit it with a current pulse (through a large resistor connected to a pulse generator) and monitor the frequency of the ringing with a scope. Don't forget to account for the capacitance of the scope probe. You can then calculate it from the two measurements.
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I would use a series through measurement on a network analyzer looking for the dip in S21.
For many coil types I think this can give a different result compared to a shunt S11 measurement. A lot depends on the way the coil is wound.
I'd expect a Tesla coil to be a large solenoid with lots of turns. Generally speaking, a solenoid is best modelled as a distributed structure, a bit like a complicated transmission line. This means that the classic model of an inductor in parallel with a capacitor will only be valid up to (say) a third of the parallel resonance frequency of the solenoid coil when measured using an S11 measurement with a VNA. Beyond this the simple/classic lumped LC model will show errors.
The parallel resonance frequency using a shunt S11 measurement using a VNA often doesn't coincide with any null seen in a series S21 measurement of the same solenoid. Often the null seen in S21 will be very rounded and the centre of the null will be slightly higher in frequency than the parallel resonance frequency. Depending on the way the coil is wound the difference can become fairly significant. This is because the solenoid is way more complicated than it first appears. It can often be modelled fairly well using a transmission line based model but the model has to be quite elaborate in order to be fairly accurate. A simple transmission line model won't be good enough.