Mixed time/frequency domain data
this is John BG

1.- You have to work with Signal Power, not just input or output:

Circuit resonance is measured with the quality factor Q and does not only have to do with the signal you send in, or the signal that bounces back, but with those AND the impedance that such signals find.

Literature reference Microwave Engineering (Pozar) 1 defines Circuit Resonance the following way



2.- Measure the impedance:

Without the impedance, there's no way you can assess Q.

You are focusing your effort on the test signal and it's spectrum, but there's not a single reference to the impedance of the circuit you are measuring.

There's no way you can measure Q without at least an additional signal, than can be the impedance that the test input faces, or alternatively the reflected signal, that again you haven't mentioned.

3.- Narrow band or wide band? doesn't really matter

Despite the definition of resonance is done with a central frequency f0, it may be the case that the circuit resonates on each of the frequencies within the Chirp signal you use to poke the system.

4.- Use Chirp with constant amplitude

From the amplitude of the signal you shown in the question, the amplitude decreases with increasing frequency, that complicates calculations.

5.- Wrapping Suggestion:

Why don't you 


start with a constant amplitude input chirp, 
measure the return signal
from there get the impedance
calculate the 'sharpness' of the impedance notch over frequency,
which is actually what defines how resonant is a circuit,
from there you can tell whether the circuit under test is resonant
throughout the chirp signal frequency band or only on certain
frequencies.
Once you have all this working, you may want to start playing with
the amplitude of the chirp input signal.


If you follow the above steps, please let us know the results you obtain updating this QA with further comments, whether it worked for you, or if not what code have you written on such regard.

If you send me by email the MATLAB code you used to generate the question signal, I will have a look and look for a way to get the reflected signal.

John BG

Literature Reference (1):

https://www.amazon.co.uk/Microwave-Engineering-Written-Publisher-Hardcover/dp/B00SLRKBV2/ref=sr_1_1?ie=UTF8&amp;qid=1529281040&amp;sr=8-1&amp;keywords=David+Pozar+Microwave+Engineering

Excerpts of the book can be read here: http://www2.electron.frba.utn.edu.ar/~jcecconi/Bibliografia/Ocultos/Libros/Microwave_Engineering_David_M_Pozar_4ed_Wiley_2012.pdf

There's also a solutions manual available here: https://www.scribd.com/doc/176505749/Microwave-engineering-pozar-4th-Ed-solutions-manual