The following is a relational expression between risetime of pulse waveform and the highest frequency components contained in spectrum;
Fmax = 0.35/Tr where, Tr = risetime of pulse (s), Fmax = the highest frequency components (Hz).The expression has been mentioned without any explanation in a large number of documents as if it is a matter of course. I do not think its basis is self-explanatory at all.
I think you can see intuitively that waveforms of speedy risetime have high frequency components. However,
1) the reason why Fmax and Tr are in an inverse proportion, and 2) what is the basis of the constant "0.35,"cannot be matters of course by any means. What kind of basis on earth, could this expression be derived from?