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When using spectrum analyzer to measure the response of a coil (a RLC circuit), I see that there is a peak at one frequency (resonance frequency). This is logical because we can model a coil as a RLC circuit. If I change the frequency of the sinusoidal source, the peak reduces its value.

My question is: why is there a higher peak at resonance frequency? If what I measure is the power dissipated (by the resistor, of course), why is there a dependence with frequency?

If complex power is written as [itex]P = P_{loss}+2j\omega\left(W_m-W_e\right)[/itex], where [itex]P_{loss} = \frac{1}{2}\left|I\right|^2R[/itex] is the dissipated power, [itex]W_e[/itex] is the electric energy stored in the capacitor and [itex]W_m[/itex] is the magnetic energy stored in the inductor, why the analyzer doesn't show a constant peak value (given by [itex]P_{loss}[/itex]) whatever the frequency of the sinusoidal source?

Thank you.