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?
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?
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
The inductance might increase a thousand times if you found an ungapped
pot core pair that could clamp around the core.
Yours is a dumb newbie post - sci.electronics.design is normally
populated by people who know a bit more about what they are doing.
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