Resonance circuit with two coils - Page 1
Forum section: electronics / projects

author: Halide
author_rank: Contributor
timestamp: October 31, 2020, 04:36:53 pm
content: Hi everyone. I built LC resonance circuit with two coils, here is schematic:But as you know the every coil have tolerance. For this reason in TP4 and TP6 I have other voltages:The purple one is +SIN(TP4), the blue one is -SIN(TP6), both of them are divided in capacitance divider. The capacitance divaider is compensetet - the signal amplitude difference is independent ofcapacitors tolerance.Can some one prompt what I can do with this? I want to make coil on coil and feed second one with DC. I think it will saturate the ferrite core. But my knowledge about coils is to poor and I don't know how to make correct caluculations. I try make second coil without this but the cirucuit doesn't work properly - the DC current don't chanege q coil factor.If this is important I'm using TDK B82111EC23 coil.Best regards,Bartosz

author: pwlps
author_rank: Frequent Contributor
timestamp: October 31, 2020, 06:43:07 pm
content: Quote from: Halide on October 31, 2020, 04:36:53 pmA good question: what is the purpose of this circuit ? Your explanations are not very clear.

author: Halide
author_rank: Contributor
timestamp: November 01, 2020, 04:44:12 pm
content: Hi,this is power supply for quadrupole mass analyzer. There is very important to have same amplitude of sin+ and sin- signal.I want to compensate diferences between coils inductance to make sin+ and sin- signal identical.My idea is wind second coil on coil which I buy. I want to feed this coil with DC. The DC should saturate ferrite core and change q factor. When I change q factor the AC signal amplitude should change too.Summarizing, I need sin+ and sin- identical. Maybe you have other idea for this?

author: pwlps
author_rank: Frequent Contributor
timestamp: November 01, 2020, 09:47:43 pm
content: If you partially saturate the ferrite with dc bias it won't be a sin anymore.To control the ac current you can add precision pots in series with the coils, between the opamp output and the dc blocking capacitor.(or, if the currents are too high for this method then just add voltage dividers to modulate the sin amplitude at the LMH buffer inputs).

author: doktor pyta
author_rank: Frequent Contributor
timestamp: November 01, 2020, 11:04:42 pm
content: My points may be completely missed but worth considering:1. I see You are using Rigol oscilloscope. The probes provided with the scope are kinda budget ones.Have You precisely compensated both probes before measurement?2. Requirement of two 180 deg shifted signals at MHZ frequency can be done with success by using transformer eg. wound on a carefully chosen ferrite core.It would be easier task if there was a resonant tank with one high quality L and C tuned by one variable capacitor.

author: pwlps
author_rank: Frequent Contributor
timestamp: November 02, 2020, 03:26:37 pm
content: Quote from: doktor pyta on November 01, 2020, 11:04:42 pmI would add that having a test point in the middle of an L-C series resonant circuit is delicate. At resonance anything you connect there is likely to modify the quality factor etc.If you want to probe the current in the coil without degrading Q you may add a second much smaller coil in series right after the DC blocking capacitor.Edit: I don't have enough information about the exact problem to be able to help you more. You say it is for quadrupole mass analyzer so I first thought the coils are part of the quadrupole magnet, but you also said the coils are discrete on-board components (TDK B82111EC23) so I still don't understand what is the exact function of this circuit.

author: Halide
author_rank: Contributor
timestamp: November 02, 2020, 05:50:13 pm
content: Hi,doktor pyta:1. I don't using oscilloscope probe in this set. I connected BNC cable directly to my measurement circuit. Here is schematic of this circuit - I have there a opamp to buffer capacitor divider:The J3 and J6 are connections for quadrupol.2. Do you mean to use two resonance circuit? I mean one coil and one cap for sin+ and other one coil and cap for sin-? Then pluged quadrupol parallel to this circuit:I think about transformer but that I said my knowlage about ferrite cores and coils is poor - I'm still student. Can you suggest me some articles/books about this issue?pwlps:I know about this and this is reason why I build measurement circuit. I analyse the impedance of whole system - the LC resonance circuit, RF blocking coils and capacitance divider. Here is measurement result from the impedance analyzer:As you can see I have resonance on directly 8MHz.Do I understood you right, all of this measurement components can affect on LC resonant circuit q factor?I marked the capacitor on LC circuit as quadrupol because i use them as capacitor.I try to explain it more clearly. I need increase the amplitude of sine wave to +/-200V at 8MHz. So I built the LC series circuit with two coils and one cap - I have resonance on sine+ and sine- signal. Instead of a capacitor I using quadrupol capacitance. This is student project and I'm using miniature silicon quadrupol - at this moment I don't use any coil to trap the ions. There is four silicon rods. Opposite ones are conected:If something is still not enough clear for you, pleas let me know. I try to explain it more accurate.Edit: On schematic you can notice LM358 opamp but I use AD8092ARZ one.

author: pwlps
author_rank: Frequent Contributor
timestamp: November 02, 2020, 06:29:57 pm
content: Ok, I now understand it's an electric quadrupole, not a magnetic one.Quote from: Halide on November 02, 2020, 05:50:13 pmYou can measure the voltage applied at L1 or L2 but measuring the voltage on Cquadrupol will certainly alter the circuit: this capacitance (4.7pF on your schematic) is similar or smaller than the capacitance of any probe.This quadrupol capacitance is so small that you will also need to minimize any stray capacitances: L1 and L2 should be connected directly to it with very short leads, same for rf choke coils (I mean don't put them on the board unless it really sticks to the quadrupole capacitance).At resonance the impedance of the L1-Cquadrupol-L2 circuit is very small, to minimize the effect of the lead inductance you can use a twisted pair for the connection with the power board.

author: Halide
author_rank: Contributor
timestamp: November 02, 2020, 09:59:27 pm
content: Quote from: pwlps on November 02, 2020, 06:29:57 pmMaybe I don't describe the measurement method enough.I don't connect the scope directly to the LC resonance circuit - the test points TP4 and TP6 only symbolize points where I want to measure voltage at resonance. To measure it i used the capacitance divider with opamp to compensate differences between capacitors(C9÷C13, C14÷C18). Without quadruple and coils I pluged to the TP4 and TP6 sine wave generator(@8Mhz) and set the gain of U1 opamp to get that same voltage on both outputs - after soldering divider caps of course. The divider ratio is something around 700.Then I solder the coils, quadrupole and measure impedance to make sure that I have resonance at 8MHz. After that I pluged the LC resonance circuit with measurement circuit to the amplifier. The scope screenshot in first message show waveform on measurement circuit output - SENSE-_OSC and SENSE+_OSC.Edit:Do you think that I can use other value of coils, use bigger cap and connect the quarupole parallel to new LC series circuit?Maybe you have other idea to trim my LC circuit? I think about pots but I I'm worried about power loss on them.Maybe good idea will be insert there air coils and change space between windings? Just like it used to be on radios to set proper heterodyne oscillator.Quote from: pwlps on November 02, 2020, 06:29:57 pmI make connection between coils and quadrupole the shortest as I can. I connect the coil directly to the quadrupole.Quote from: pwlps on November 02, 2020, 06:29:57 pmDo I understood it right, I should twisted coaxial cables? I using HELUKABEL RG316U coxial cables. Can I twisted it using cordless screwdriver or I can damaged them?

author: pwlps
author_rank: Frequent Contributor
timestamp: November 03, 2020, 12:23:06 am
content: Quote from: Halide on November 02, 2020, 09:59:27 pmOk, I see.QuoteWhat about adding small adjustable inductors in series ? If you find models with adequate current handling capability, I didn't look at it.QuoteNo, don't twist coaxial cablesCoax should be fine unless it is really long (it should be << \$ \lambda/4\$ )If you always work at resonance you can also add an rf transformer between the coax and the LC circuit to better match to the low impedance of the LC circuit, increasing the current will also increase the available voltage range on your quadrupole capacitor for free. (Edit: this is only valid if your LMH drivers work at their max current already, so I can't tell)

author: Halide
author_rank: Contributor
timestamp: November 03, 2020, 09:51:37 pm
content: Quote from: pwlps on November 03, 2020, 12:23:06 amI find something like this: Murata A1313AN-0002 GRG=P3. This is adjustable inductor with range which will be useful for my purpose. I can't find information about maximum current but i want to buy two of them and test.Quote from: pwlps on November 03, 2020, 12:23:06 amMy coax cables have only something around 50cm so they are shorter than λ/4. But one of them is little longer than the other, should I make them equal?

author: pwlps
author_rank: Frequent Contributor
timestamp: November 03, 2020, 10:24:53 pm
content: Quote from: Halide on November 03, 2020, 09:51:37 pmFor 50cm at 8MHz it shouldn't matter. For lengths <<λ/4 your coax just behaves like a capacitance to the ground but it won't change the signal amplitude.