||This article relies largely or entirely upon a single source. (September 2013)
Küpfmüller's uncertainty principle by Karl Küpfmüller states that the relation of the rise time of a bandlimited signal to its bandwidth is a constant.
with either or
A bandlimited signal with fourier transform in frequency space is given by the multiplication of any signal with with a rectangular function of width
as (applying the convolution theorem)
Since the fourier transform of a rectangular function is a sinc function and vice versa, follows
Now the first root of is at , which is the rise time of the pulse , now follows
Equality is given as long as is finite.
Regarding that a real signal has both positive and negative frequencies of the same frequency band, becomes , which leads to instead of