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Source resistance affect to Resonant frequency

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kudjung

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Guys,

Can anyone come up with a simple explanation on why source resistance affect resonant frequency of the following circuit?
Attached is the schematic and simulation result.

TIA,
 

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Hi TIA,
This circuit is not exactly a 2nd-order lowpass (it has a 3rd pole, and a zero at s=0 too), but this is the idea:
The peak of the frequency response of a transfer function containing a pair of poles does not coincide with w0. The location, height and sharpeness of the peak depends of the damping factor too, that in turn depends of the resistance.
Regards

Z
 

Thanks Zorro,

I think I start to better understand this. Here is my understanding. This circuit could be viewed as serie resonance circuit serie with paralle resonant circuit.
For serie resonant circuit, damping factor inverse with resistor. So when increasing resistant, circuit went from overdamp to underdamp.
For parallel resonant circuit, damping factor varied with resistor. So when increasing resistant, circuit went from underdamp to over damp.

When serie this together, we see first peak from the parallel resonant circuit which has higher inductance and second peak from serie resonant which has lower inductancevalue .

Rgds,
 

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