Help on Behavior of Capacitor - Transformer combination and resonance

[gauthamtechie]

The screen shot of LTspice simulation:


The response seen for series current is a band pass one. So a Transformer is behaving as the L here and the LC resonant frequency is about 175.5 MHz as seen in the screenshot. The transformer is 1:1, so I took equivalent inductance as the same value as either one of them.

Applying f=1/(2∏√(LC)) for C = 0.4n; L=1H, frequency is 7.957 KHz, which is not the value seen in the LTspice simulation.

How do I arrive at this value?

View attachment CapTr.asc

 

[MikeMl]

Make the coupling coefficient something less than 1.
Put some realistic resistance between the source and the input winding.

 

[ericgibbs]

hi,

These two Sims should give you an idea what has happened to the 'effective' inductance/impedance of the L2 winding with the almost virtual short across L1. [1R]

Especially with the 100% coupling L2:L1 in your circuit.

E.

 

[MrAl]

Hi Guys,


The theoretical transfer of the transformer with ratio 1:1 and coupling factor of 1 would make that resistor look like it was in parallel with the mutual inductance of the transformer i think. That would mean it would couple perfectly to the resistor as if it was an inductor in parallel with the resistor. For this we would see zero resistor current at DC and as the frequency increased the current would stay low until a certain frequency and then rise up and up until it hit 1 amp (with a 1v drive source). It would then remain at 1 amp for any frequency above that.
Interesting, the inductor current rises up and up and reaches a peak at around 1Hz, then continues until it reaches around 40MHz where it starts decreasing, then reaches -3db at around 400MHz. So the inductor current looks like a sort of very broadband bandpass response while the resistor current has a high pass response.

The fact that the spice simulation does not do this exactly but rather has a peak point tells us that the model used for this particular simulation is not an ideal transformer. So to get the response the same way this spice shows it you would have to look into what kind of model they are using and then analyze the circuit using that model rather than an ideal (perfect) 1:1 transformer.