Re: [funwithtransistors] twice the voltage expected

Oct 16, 2015



#2009 Oct 16, 2015

I made a 5mc crystal oscillator that need to have 2 outputs. I used a coil I had hanging around for the buffer amp. The coil has 30 turns on it with a slug core so I can use a fixed 47pf capacitor to tune to 5mc. The coil is wound such that it has 20 turns on the first layer and then starts back down with the other 10 turns like a 2 layer coil. I had wrapped 5 turns of wire on the top and bottom parts of the coil to get my 2 outputs. I expected the 2 outputs to be the same but when I measured them I had about .5 volts on the bottom coil and 1 volt on the top coil where it is 2 layered. The only thing I can think to explain this is that the top coil has twice the magnetic field density as the bottom coil due to the 2 layers. I will put a photo of the coil in the files section.


#2013 Oct 17, 2015

The difference could also be because of capacitive coupling between windings.. The winding near whichever end of the coil is not at AC ground, may have more common-mode signal due to that coupling, and it might be being converted into differential mode.

But yes, depending on how the coil is wound and how much & where the core is and where the windings are, the mutual inductance isn't that much, and there is lots of leakage inductance that doesn't couple between every turn and every other turn.. Then the magnetic field density would depend on position along the main coil, as you say.

Possibly both effects may be happening.



#2014 Oct 23, 2015

Thanks for the reply. It could be some capacitive coupling going on. I would like to try some expiriments with different coils and see if I can get the same result.

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