Early FM pioneers thought frequency modulation (FM) could be potentially be performed with zero bandwidth. However, in theory it could require an infinite amount of bandwidth. This video shows the mathematics behind estimating the bandwidth of an angle modulated signal using power expansion. Ultimately, we can arrive at an expression for a "narrow bandwidth" approximation which is similar to the expression for amplitude modulation (AM).
The information in this video will help you understand concepts found in "Modern Digital and Analog Communication" by B.P. Lathi and Zhi Ding.
You can purchase this textbook here:
https://www.amazon.com/Digital-Commun...