Series LCR experiment
Most signal generators have an output impedance set to usually 50 ohms. Some can be set to 600 ohms and some can be set to zero ohms although those set to zero ohms will not be able to supply amps of signal rather they will limit in some haphazard way.

At resonance, the impedance of L+C+R will be R because Xc and Xl are identical |impedances| but of opposite polarity. Net impedance is R: -




R then loads the output of the signal generator and you get full signal at low and high frequencies and some fraction of the signal at resonance.