My circuit consists of two inductively coupled inductors. A 40 kHz 600 mV sine voltage is applied to the primary inductor. The induced voltage in the secondary inductor shows some ringing in the area of its own resonant frequency (1 MHz). A capacitor with 180 pF placed in parallel to the secondary inductor gets rid of most of the ringing. The voltage is measured with an active differential probe.
What exactely is causing this ringing? And how does the capacitor help to get rid of it? I see that the capacitor lowers the resonant frequency of the secondary circuit. But I don't exactely understand how it reduces the ringing.
Impedance of the primary inductor:
Impedance of the secondary inductor:
Impedance of the secondary inductor with 180 pF in parallel:
Voltage across the secondary inductor without capacitor:
Voltage across the secondary inductor with 180 pF parallel:
Spectrum of signal source: