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LC oscillator with very low capacitance?

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alf

Wed Jul 13 2011, 11:31AM

Registered Member #3925 Joined: Fri Jun 03 2011, 10:50AM
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Posts: 121

hey,

it would be cool if i could make a colpitts oscillator for example, which will oscillate at audiable frequencys, and use a capacitor which consists of 2 metal plates which could be moved apart to increase or decrease frequency, but im not sure if this possible, is it?

thanks. Alf.

Ash Small

Wed Jul 13 2011, 11:38AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Why not use a conventional variable capacitor?

Adam Munich

Wed Jul 13 2011, 11:45AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

That reduces the coolness!

alf

Wed Jul 13 2011, 12:28PM

Registered Member #3925 Joined: Fri Jun 03 2011, 10:50AM
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Posts: 121

Grenadier wrote ...

That reduces the coolness!

it does indeed! :D

Proud Mary

Wed Jul 13 2011, 12:46PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Oscillators using inductance (L) and capacitance (C) tuned circuits to set their frequency are absolutely not for use at audio frequencies, and here's why:

The variable capacitor you describe might well have a maximum capacitance of 100 pF.

Using a 100 pF cap to make a tuned circuit resonate at 1 kHz (a note of medium pitch) would require an inductance of more than 250 H - a giant value as inductors go, and miles and miles of wire.

At its resonant point this turned circuit would have an impedance (written as 'Z' - a special type of resistance found in AC circuits) of 1.6 Megohm - a very great deal of impedance, which you can think of as an obstruction in the circuit, preventing all but a miserable trickle of electrons from going backwards and forwards at the resonant frequency.

But these things, impractical though they may be, are not the reason such a circuit could not be made to work. A real world inductor or coil of 250 H would have a self-capacitance (the capacitance between all those thousands of turns) of far more than 100 pF - the value I have guessed for your variable capacitor - and would completely swamp any attempt to vary the frequency with a small variable capacitor.

Lastly, all those miles of wire would have to be very fine in order to get them all onto a former, so your 250 H inductor would have a lot of ohmic resistance (R) and would not be able to hold a very constant frequency - imagine a bell chiming a very clear note, and then imagine the same bell with a great slab of sticky toffee stuck to it. No more clear chime on a single frequency! This is the effect such a high resistance of the wire will have on the tuned circuit. The resistance degrades the 'goodness' of the tuned circuit - its ability to ring - which we denote by the symbol Q. A high Q tuned circuit will chime a single pure note, but as the resistance is increased, the chime will turn into a dull clunk - a sound spread over a wide range of frequencies, so no good for a tuned circuit.

If you want to make a variable frequency audio oscillator, you should choose a design where the frequency is set by combinations of capacitance (C) and resistance (R). You'll find any number of such circuits online if you search Google Images for 'audio oscillator schematic'.

PS Edit: There is a type of oscillator suitable for use at very low frequencies which behaves likes an LC oscillator, but uses some clever techniques to simulate the behaviour of a large inductance. Such an inductance simulator is called a gyrator.

Electra

Wed Jul 13 2011, 11:11PM

Registered Member #816 Joined: Sun Jun 03 2007, 07:29PM
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Posts: 156

Perhaps you could make the inductor variable instead, Wound on a ferrite core that you can vary the spacing, or has a tuning slug that can be slid in or out of the centre.

C can be made a fixed capacitor of a few hundred nF to suit the inductance used.

The negative resistance oscillator, is one LC oscillator that should work down in the few KHz range.

Ps.
I think the coolest of Low frequency oscillators is the electronically maintained Tuning fork and you can even hear them without a speaker.

Patrick

Thu Jul 14 2011, 12:33AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

You can get to 40-250H by adapting coils like HEI igition coil secondaries, but as Proud Mary has said the resistance and capacitence will be wild.

radiotech

Thu Jul 14 2011, 06:06AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

"it would be cool if i could make a colpitts oscillator for example, which will oscillate at audiable frequencys, and use a capacitor which consists of 2 metal plates which could be moved apart to increase or decrease frequency, but im not sure if this possible, is it?
"

Here is a suggestion, make two oscillators operate at radio frequncies,
and then by moving a single plate back and forward, you change the
frequency of one of them and keep the other constant. Mix the two
oscillators, and you get an extremely pure tone capaple of being
varied over 17octaves ( the entire range of human hearing) just by
moving the two plates

Now dispense with one plate and wave your hand over the other, and
you get the same effect. How cool would that be?.

Ð›ÐµÌÐ² Ð¡ÐµÑ€Ð³ÐµÌÐµÐ²Ð¸Ñ‡ Ð¢ÐµÑ€Ð¼ÐµÌÐ½ thought is was.

Proud Mary

Thu Jul 14 2011, 09:40AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Electra wrote ...

I think the coolest of Low frequency oscillators is the electronically maintained Tuning fork and you can even hear them without a speaker.

Great fun, and probably more stable than you'd guess once thermal equilibrium has been reached.

I have some audio frequency oscillator crystals - slabs with plated sides mounted in a vacuum - that were used as tone generators in ancient telecomm systems. I am not sure whether they be quartz or not - a number of different piezoelectric crystals were used in the 1940s and 50s - but they are very fussy about the conditions of oscillation.

Dr. Dark Current

Thu Jul 14 2011, 09:54AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

You can make a RC (not LC) oscillator with some ICs with very low leakage current / FET input stage (maybe a relaxation oscillator with the 74HC14 hex inverter would work?)

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