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-   -   Modifying an inductor? (https://www.vintage-radio.net/forum/showthread.php?t=181003)

Radio Wrangler 13th Jun 2021 6:45 pm

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.

David

mole42uk 13th Jun 2021 8:07 pm

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.....

mole42uk 13th Jun 2021 10:03 pm

Re: Modifying an inductor?
 
Of course, I need a negative capacitor. Now, where’s my RS catalogue.....

Radio Wrangler 13th Jun 2021 11:52 pm

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

David

mole42uk 17th Jun 2021 5:20 pm

Re: Modifying an inductor?
 
1 Attachment(s)
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 15µH 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?

G0HZU_JMR 17th Jun 2021 11:58 pm

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.

G0HZU_JMR 18th Jun 2021 12:31 am

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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