I want to simulate a transformer using coupled inducors
in Spice (LTspice). I found some unexpected behaviour at
very high frequencies. My observation can be boild down to the
circuit given below: An AC source delivers a voltage. A resistor
converts the voltage to a current through a transformer (perfect coupling K=1).
The transformer is secondary shorted (by a 0-Volt AC souce, yes that works) and
I measure the secondary side current. ( My original application is understanding
and simulation of current transformes). At frequencies above 100MHz the current
drops, as if the coupling would not be perfect or as if an additional series
inductance of approx 2nH would be involved (see Mag/Phase Plot).
Has someone an explanation for that. Is there someone a good explanation what
spice does here. I am noramlly very very satisfied with spice results especially
because I can do simulations of very (unrealistically) broad frequancy ranges
and I can get an eye on every single cause/effect.