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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: Hand-wound coil in Colpitts oscillator
Date: Sun, 5 May 2024 13:12:32 +1000
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On 5/05/2024 7:16 am, RodionGork wrote:
> Friends and Colleagues, Hi again!
> 
> Here is a Colpitts oscillator scheme I'm experimenting with:
> 
> https://tinyurl.com/2yq9754k (simulation, NPN with common-base I suppose)
> 
> I wound a coil from some length of wire I had at hand - and it has the
> following parameters:
> 
> diameter - about 32mm,
> about 75 turns
> wire - 0.3mm
> 
> It is wound "heap-style" - so it has only about 6-7 mm "length" and perhaps
> about 2mm "thickness".
> 
> I measured frequency with oscilloscope (actually, with two different ones)
> and it is about 350 kHz, which means that inductance is about 400 uH.
> 
> However, calculating by winding parameters over and over by certain
> formulas in books and internet I get result twice lower. For example this
> calculator
> https://www.66pacific.com/calculators/coil-inductance-calculator.aspx
> (formula matches with some old book I have at hand) - gives 250 uH.
> 
> Question 1: What may I be missing? perhaps, capacitance between turns? how
> can I add this in simulation - should it be capacitor in parallel with the
> coil?

There's going to be capacitance between the turns, but it will be 
picofarads rather than nanofarads. You can measure it by measuring 
resonant frequency of the coil in isolation - exciting it from a 
variable frequency generator through a judiciously chosen resistor to 
find the frequency where you get the highest voltage swing across the 
coil. Too small a resistor and you'll  get a rather broad peak, too big 
and you won't have a detectable voltage swing across the coil


> Question 2: If I move ferrite core into the coil, frequency reduces perhaps
> 1.5 times (I tried few different pieces of ferrite found on my desk) -
> which at first amused me as I thought inductance is increased hundreds
> times. But most probably this doesn't work this way because coil has almost
> unidirectional current and the core is saturated by magnetic field?

The inductance might increase a thousand times if you found an ungapped 
pot core pair that could clamp around the core.

Just dropping in a chunk of ferrite will just slightly shorten the 
magnetic path length around the coil. You could clamp a U-core pair 
together around the  coil which would do almost as well as pot core pair.

The optimal solution is to wind your core around a toroid of magnetic 
material, but that get tedious (though there are machines that can do 
the job for you.

Yours is a dumb newbie post - sci.electronics.design is normally 
populated by people who know a bit more about what they are doing.

-- 
Bill Sloman, Sydney