Digital Signal Processing Reference
In-Depth Information
Illustration 45:
Restriction in time involves expansion of the frequency band
As follows from the sequence of pictures from top to bottom, it is not possible to speak of a"time-limited
sinusoidal signal" with one frequency. An oscillation pulse of this kind called a "burst" has a frequency
band which gets wider and wider as the length of the burst gets shorter. The frequency of the sinusoidal
signal in the upper picture is 40 Hz, the time segment of the sinusoidal signal in the upper sequence estab-
lished by measuring was 1 s (only visible in extracts here). For this reason the spectrum in the upper
sequence does not consist of a clear line.
Strangely, with increasing band width the spectrum apparently becomes more and more asymetrical (see
bottom). In addition, the maximum point moves more and more to the left. We shall discuss the reasons
later. Conclusion: there is every reason to speak of uncertainty.
