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My physics teacher said that on a LC based radio receiver, the frequency knob changes the capacitance of the capacitor, thus changing the resonance frequency. How is that done?

Kefas
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    Does this answer your question? [Variable Capacitor?](https://electronics.stackexchange.com/questions/66544/variable-capacitor) – Marcus Müller Oct 09 '20 at 19:52
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    The varicap is the answer https://en.wikipedia.org/wiki/Varicap or this one https://en.wikipedia.org/wiki/Variable_capacitor – G36 Oct 09 '20 at 19:52
  • Variable surface area between plates when rotated https://duckduckgo.com/?q=variable+capacitor&t=ffab&iar=images&iax=images&ia=images – Tony Stewart EE75 Oct 09 '20 at 19:53
  • This is a question trivially self-answered with a little research, and as such tends to present a dilemma for the site. Often I've just voted to close these, and that may really be the right response; but I've also noticed that questions like this often tend to accumulate a low quality answer or two along the way to closure, which leaves the question if it's worth trying to provide a solid answer first. Undecided... – Chris Stratton Oct 09 '20 at 20:05
  • [Here's what I use.](https://i.stack.imgur.com/DNQ0e.jpg) None of that cheap miniature stuff that Marcus illustrated. ;) – jonk Oct 09 '20 at 20:47
  • *How is that done?* Why didn't you ask your physics teacher? Type "variable capacitor" in Google and pictures of such devices are shown. Information is so easy to find these days. – Bimpelrekkie Oct 09 '20 at 20:51
  • @TonyStewartSunnyskyguyEE75 those look like cucumber slicers ... lol – jsotola Oct 09 '20 at 21:18
  • @jsotola you must have nano-sized cukes. (Lol) Kefas , have you ever tried to search on WIKI first? https://en.wikipedia.org/wiki/Variable_capacitor – Tony Stewart EE75 Oct 09 '20 at 21:21
  • @jonk I only used the precision gold plated Johanson types. – Tony Stewart EE75 Oct 09 '20 at 22:14
  • @TonyStewartSunnyskyguyEE75 The one I show I can buy today, new. And they work well. Know where I can buy Johanson? The gold plating would help avoid corrosion over one's lifetime (though the sapphire layer that forms instantly on aluminum isn't too bad.) So that could be a nice touch when building a gift for someone. – jonk Oct 09 '20 at 22:21
  • Checkout their website, they may be all subminiature uW types now AM/FM don’t use these anymore – Tony Stewart EE75 Oct 09 '20 at 22:22
  • @TonyStewartSunnyskyguyEE75 They are all sub-miniature and therefore completely uninteresting, from what I see at Knowles. I like building and making gifts. It's personal and fun and something people cannot get any other way, too. A sub-miniature unit just does NOT cut it. (I like large knobs, too.) – jonk Oct 09 '20 at 22:24
  • @jonk I prowl yard sales for radios containing those. That knob is the quality part! Don't wire that capacitor with rotating-plates to vacuum tube plate. – glen_geek Oct 10 '20 at 13:28
  • @glen_geek I definitely won't make the mistake of wiring that capacitor up to a vacuum tube plate voltage! And hopefully, you won't see any yard sales with the radios I make for friends as presents. I try and build quality and for the human touch. Somethings been left behind these days in electronics design due to the high cost of anything requiring quality mechanical components designed for efficient manual use by humans. – jonk Oct 10 '20 at 17:12
  • @jonk no worries with corrosion on your vbl cap, only dust and humidity – Tony Stewart EE75 Oct 10 '20 at 18:09
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    @jonk A word or two may have changed in a comment. My apologies (really) - someone flagged it and I sat and mulled some minutes and in the circumstances decided it was better altered. – Russell McMahon Oct 13 '20 at 10:35

1 Answers1

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There are two commonly used methods.

Mechanically Variable Capacitors

The most traditional technique is to make a capacitor from a stack of sector-like fixed plates, and moving plates on a rotary shaft, and rotate that for more or less overlap with the fixed plates. This is what you typically found in traditional broadcast receivers - eg the classic 365 pF AM tuning capacitor. Often there would be multiple distinct variable capacitors on a common shaft, for example to tune both a tracking preselector filter and the local oscillator, or for use on bands of extremely different frequency.

A variation of this technique sometimes used for trimmer capacitors is to have a set of plates which are compressed to change spacing by a screw (though some better trimmers are just miniature versions of rotary plate capacitors). There'a also the physics lecture apparatus where very large plates would stand on some sort of slider which could vary their spacing over substantial distance.

Electronically Variable Capacitors

The second major technique is to exploit how semiconductor diode junctions vary in capacitance with applied DC vias voltage. There are specific varactor diodes made to be optimal for this, but some other types have sufficiently useful behavior to be sometimes exploited, too. This technique is very useful for making a voltage controlled oscillator such as might be used in a Phase Locked Loop. It also shows up in some simpler electronically tuned radios (eg dirt-cheap "pound shop" radios) and is sometimes chosen for fairly sophisticated gear because it permits using a potentiometer as a tuning control, which can be more easily remote mounted on a front panel and does not suffer from hand capacitance (or LO radiation) the way unshielded physically variable capacitors do, since the actual capacitive element and surrounding tuned circuit is buried inside and only a DC tuning voltage is anywhere near the user's hand or front panel.

There are likely other methods as well, but these are the most common.

Chris Stratton
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