"Jim Starret"

The following circuit is a MOSFET driver.

When I feed a 20KHz squarewave in, I get 20KHz out ... but as a
1.25MHz ring. This is as unloaded.

http://users.tpg.com.au/hburns/index.html

I assume this has to do with the air core coupling, but would like to
know more about how this happens. IOW what factors control the
presence and frequency of the ringing.

Can anyone please explain?


** That secondary winding has almost no effect unless it is loaded.

Your primary winding is therefore a simple " tank circuit " - ie a
parallel tuned circuit, tuned by the stray capacitance between individual
turns and that mosfet driving it.

The 8 to 10 cycles of visible ringing indicates the " Q factor " of your
100t coil is around 8 to 10 also - a neat rule of thumb to remember.

Add a bit of ferrite inside the coil - see the ringing frequency take a
nose dive and the number of cycles go through the roof !!!

Real electronic devices, in a hand on situation, plus real time scope view
beats ANY simulation, anyday.




........ Phil