signal analysis - Why do we get ripples in the frequency spectrum of a chirp, and how do I remove it?

The ripples in the amplitude spectrum of a linear spectrum is undesirable. I desire to obtain a flat spectrum response. Hence, how can I remove these ripples? Also, which factors (like time bandwidth product, chirp rate) are affecting the ripples, and how?

I designed a chirp with a frequency from 10 to 90 Hz in MATLAB. However, I obtained Fresnel ripples (which normally occurs) in the passband, along with overshoots at the band edges. I need to study the application of chirps on a material and use it for defect analysis. For this, I need to optimize the whole energy applied to the chirp. Hence, I want to get rid of these ripples. So, what parameters like chirp rate and time of sweep will affect the system?