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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 13th Jun 2021, 5:45 pm   #21
Radio Wrangler
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Default Re: Modifying an inductor?

Remember, your inductor is not a pure inductance, there is distributed stray capacitance amongst its turns. This distorts your inductance readings and apparent resonant frequencies, especially as you approach the self-resonance frequency. Modelling errors can get out of hand and rather counter-intuitive things appear to happen. 2-layer coils really make the capacitive effects worse. expecially if you laid the turns out there-and-back-again-over-the-top.

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Old 13th Jun 2021, 7:07 pm   #22
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Default Re: Modifying an inductor?

Yes, I sort of know about those unwanted capacitances. I just don't have any experience to know what to expect or how to design them into the filter. So I just work it out using what I do know, look up how some other people have done it, verify my idea with a design tool, check with someone on here who knows a lot more than I, then breadboard it and see what happens. So far I've been into several dead-ends, or learning experiences as I prefer, and have learned something about how LC filters function. What I haven't done yet is to achieve the design objective.....
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Old 13th Jun 2021, 9:03 pm   #23
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Default Re: Modifying an inductor?

Of course, I need a negative capacitor. Now, where’s my RS catalogue.....
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Old 13th Jun 2021, 10:52 pm   #24
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Default Re: Modifying an inductor?

Your inductor IS a negative capacitor!

Snag is its value changes with frequency. And you get free, stray, positive capacitors with it

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Old 17th Jun 2021, 4:20 pm   #25
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Default Re: Modifying an inductor?

A successful bit of breadboarding confirms that the values now are near enough correct. Using the circuit below and a TTL-level square-wave input, I get a peak sine wave output a few cycles under 1MHz - I have ten 15ÁH chokes to try, so I'm sure one of them will be closer to my preferred frequency. I have a choice of poly 3n3 or ceramic 3n3 - both work, but will it matter which I use?
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Old 17th Jun 2021, 10:58 pm   #26
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Default Re: Modifying an inductor?

It probably doesn't matter much but if the 3n3 ceramic caps are class 2 types they won't be that stable over temperature and the Q won't be that great either. I'd fit the poly ones assuming they aren't ancient and/or enormous.

A good choice would be a modern Wima 3.3nF cap as these are small, accurate and stable.
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Old 17th Jun 2021, 11:31 pm   #27
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Default Re: Modifying an inductor?

Note also that if the ceramic caps are class 2 they will also have a degree of non-linearity with voltage. A drive level of 4Vpkpk might be enough to generate some unwelcome distortion in the capacitors but I think it depends on the dielectric type and also the voltage rating of the cap.
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