# chirp signal - Frequency spectrum characteristics

I am confused with my own intuition of the frequency response of a chirp signal.

In this case, the chirp signal is a frequency modulated signal with initial frequency starting at 100 Hz and ending at 300 Hz for 2 seconds.

1. Since the signal contains equal amounts of frequencies from 100Hz to 300 Hz, the magnitudes of the frequencies should be equal? However, in the plot below, the frequencies at 100Hz and 300 Hz have higher magnitude than the middle frequencies e.g. at 200Hz? Should the plot be just a straight line?

2. Also, why does it appear that the middle frequencies are denser? Is it correct to assume that since there are more frequencies in the middle e.g. 100ish to 200ish than the end frequencies of 100Hz and 300Hz, then this should explain the increased density?

What is wrong with my assumptions?

• Intuition is not helping here. Do not neglect the impact of the 2 Hz "amplitude modulation". – Moti Apr 15 '18 at 4:36