The inverse relationship between time and frequency
– If the time-domain description of a signal is changed, the frequencydomain
description of the signal is changed in an inverse manner
– If a signal is strictly limited in frequency, the time-domain description
of the signal will trail on indefinitely ( a signal cannot be strictly limited
in both time and frequency); example: rectangular function and sinc
function (Fourier pair)
– Low
-pass signal: its significant spectral content is centered around the
origin
– Band-pass signal: its significant spectral content is centered around ±f
c
where f
c is a nonzero frequency
– Bandwidth: the extent of significant spectral content of the signal for
positive frequencies; null-to-null bandwidth, 3-dB bandwidth
– Time-Bandwidth Product: The product of signal’s duration and its
bandwidth is always a constant, called time-bandwidth product or
bandwidth-duration product
• Dirac Delta Function (unit impulse), δ(t):