I understand the first 2 graphs, the time domain representation and the frequency domain representation (amplitude spectrum) but I cannot wrap my head around the phase spectrum. Why does it contain a value at f = 100? Is it linked with the bandwidth of the system?
The phase plot is just showing that the phase of the associated sine wave at that frequency is -pi/2. This results from the Fourier transform of a sine wave bing two components, one at positive and one at negative frequency, one with a phase to +90 degrees (pi/2) and the other with a phase of -90 degrees (-pi/2). See The Fourier Transform of the Sine and Cosine Functions for more details
