Path: ...!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail From: Bill Sloman Newsgroups: sci.electronics.design Subject: Re: Hand-wound coil in Colpitts oscillator Date: Sun, 5 May 2024 13:12:32 +1000 Organization: A noiseless patient Spider Lines: 63 Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Injection-Date: Sun, 05 May 2024 05:12:37 +0200 (CEST) Injection-Info: dont-email.me; posting-host="e2a83a7688ba29cd184238a92f531728"; logging-data="1746934"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX19TIFwmYX/mjWCFvRzMp7blfyfEbZJ9Dig=" User-Agent: Mozilla Thunderbird Cancel-Lock: sha1:WphfNpOAGlqtuTipFjINQRip3Dw= Content-Language: en-US In-Reply-To: Bytes: 3720 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