Küpfmüller's uncertainty principle

Küpfmüller's uncertainty principle by Karl Küpfmüller in the year 1924 states that the relation of the rise time of a bandlimited signal to its bandwidth is a constant.^[1]

${\displaystyle \Delta f\Delta t\geq k}$

with ${\displaystyle k}$ either ${\displaystyle 1}$ or ${\displaystyle {\frac {1}{2}}}$

Proof

A bandlimited signal ${\displaystyle u(t)}$  with fourier transform ${\displaystyle {\hat {u}}(f)}$  is given by the multiplication of any signal ${\displaystyle {\underline {\hat {u}}}(f)}$  with a rectangular function of width ${\displaystyle \Delta f}$  in frequency domain:

${\displaystyle {\hat {g}}(f)=\operatorname {rect} \left({\frac {f}{\Delta f}}\right)=\chi _{[-\Delta f/2,\Delta f/2]}(f):={\begin{cases}1&|f|\leq \Delta f/2\\0&{\text{else}}\end{cases}}.}$ 

This multiplication with a rectangular function acts as a Bandlimiting filter and results in ${\displaystyle {\hat {u}}(f)={\hat {g}}(f){\underline {\hat {u}}}(f)=:{{\underline {\hat {u}}}(f)}{{\Big |}_{\Delta f}}.}$ 

Applying the convolution theorem, we also know

${\displaystyle {\hat {g}}(f)\cdot {\hat {u}}(f)={\mathcal {F}}((g*u)(t))}$ 

Since the fourier transform of a rectangular function is a sinc function ${\displaystyle \operatorname {si} }$  and vice versa, it follows directly by definition that

${\displaystyle g(t)={\mathcal {F}}^{-1}({\hat {g}})(t)={\frac {1}{\sqrt {2\pi }}}\int \limits _{-{\frac {\Delta f}{2}}}^{\frac {\Delta f}{2}}1\cdot e^{j2\pi ft}df={\frac {1}{\sqrt {2\pi }}}\cdot \Delta f\cdot \operatorname {si} \left({\frac {2\pi t\cdot \Delta f}{2}}\right)}$ 

Now the first root ${\displaystyle g(\Delta t)=0}$  is at ${\displaystyle \Delta t=\pm {\frac {1}{\Delta f}}}$ . This is the rise time ${\displaystyle \Delta t}$  of the pulse ${\displaystyle g(t)}$ . Since the rise time influences how fast g(t) can go from 0 to its maximum, it affects how fast the bandwidth limited signal transitions from 0 to its maximal value.

We have the important finding, that the rise time is inversely related to the frequency bandwidth:

${\displaystyle \Delta t={\frac {1}{\Delta f}},}$ 

the lower the rise time, the wider the frequency bandwidth needs to be.

Equality is given as long as ${\displaystyle \Delta t}$  is finite.

Regarding that a real signal has both positive and negative frequencies of the same frequency band, ${\displaystyle \Delta f}$  becomes ${\displaystyle 2\cdot \Delta f}$ , which leads to ${\displaystyle k={\frac {1}{2}}}$  instead of ${\displaystyle k=1}$ 

See also

• Heisenberg's uncertainty principle
• Nyquist theorem

References

1. Rohling, Hermann [in German] (2007). (PDF). Nachrichtenübertragung I (in German). Institut für Nachrichtentechnik, Technische Universität Hamburg-Harburg. Archived from the original (PDF) on 2007-07-12. Retrieved 2007-07-12.

Further reading

• Küpfmüller, Karl; Kohn, Gerhard (2000). Theoretische Elektrotechnik und Elektronik (in German). Berlin, Heidelberg: Springer-Verlag. ISBN 978-3-540-56500-0.
• Hoffmann, Rüdiger (2005). Grundlagen der Frequenzanalyse - Eine Einführung für Ingenieure und Informatiker (in German) (2 ed.). Renningen, Germany: Expert Verlag. ISBN 3-8169-2447-6.
• Girod, Bernd; Rabenstein, Rudolf; Stenger, Alexander (2007). Einführung in die Systemtheorie (in German) (4 ed.). Wiesbaden, Germany: Teubner Verlag. ISBN 978-3-83510176-0.

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