Digital Signal Processing Reference
In-Depth Information
Nadelimpuls
kompl. FT
Ausschnitt
kompl. IFT
Zeitbereich
FF T
Fr equenzber .
Symmetric spectrum
3,0
2,5
2,0
1,5
1,0
0,5
0,0
-0,5
-1,0
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0,0
-0,1
-0,2
7,5
5,0
2,5
0,0
-2,5
-5,0
1,50
1,25
1,00
0,75
0,50
0,25
0,00
-0,25
-0,50
-0,75
-1,00
-1,25
0,01
0,01
0,01
0,00
0,00
0,01
0,01
0,01
0,00
0,00
0,01
0,01
0,01
0,00
0,00
0,01
0,01
0,01
0,00
0,00
0 50
0 50
150
150
250
250
350
350
450
450
550
550
650
650
750
750
850
850
950
950
-1000 -750 -500 -250
0
250
500
750
100
ms
ms
Hz
Illustration 119:
The ideal Si-test generator
This circuit which in principle was already used in Illustration 95 makes use of the fact that the Si-function
and the Si-oscillation pulse result as frequency segments cut out from the
δ
-pulse.
The
-pulse is first transformed in the frequency domain (FT). There the real and imaginary section are
both cut out in the same way. If a lowpass signal is required the frequencies from 0 to the cutoff frequency
f
Si
are cut out. In the case of a bandpass signal only the relevant section etc. The rest of the frequency band
is then transformed back into the time domain (IFT). It is now available as a test signal with a precisely
defined frequency range.
δ
Note that the upper lowpass has double the bandwidth from -f
Si
to +f
Si
. The third signal (bandpass) has
exactly the same bandwidth as the first (lowpass). The equivalent is true of the second and fourth signal.
This can be seen in the time domain. The Si-oscillation pulses shown have precisely the Si-signals
described as envelopes.
Noise
The most “exotic” test signal is without doubt pure stochastic noise. As shown at the end
of Chapter 2 it results from random processes in nature. Such processes can be simulated
as desired by the computer with the result that noise can also be generated by means of
the computer.
