Digital Signal Processing Reference
In-Depth Information
NF-Signal
Tiefpass
Zeitbereic
h
FFT
F
requenzber.
Time domai n
Spectrum
Freq. domai n
Mul ti pl i kat.
Black-Box00
LP filter
Multipl.
LP fil ter
S ignals
Multipl.
FFT fi lter
M
ul ti pl i kat.
Tiefpass
Träger
1,25
1,00
0,75
0,50
0,25
0,00
-0,25
-0,50
-0,75
-1,00
1,5
1,0
0,5
0,0
-0,5
-1,0
-1,5
0,3
0,2
0,1
0,0
-0,1
-0,2
-0,3
0,30
0,20
0,10
0,00
-0,10
-0,20
0,150
0,100
0,050
0,000
-0,050
-0,100
0,200
0,175
0,150
0,125
0,100
0,075
0,050
0,025
0,000
0,10
0,09
0,08
0,07
0,06
0,05
0,04
0,03
0,02
0,01
0,00
0,050
0,045
0,040
0,035
0,030
0,025
0,020
0,015
0,010
0,005
0,000
0,0250
0,0225
0,0200
0,0175
0,0150
0,0125
0,0100
0,0075
0,0050
0,0025
0,0000
0,0250
0,0225
0,0200
0,0175
0,0150
0,0125
0,0100
0,0075
0,0050
0,0025
0,0000
0 100 250 400 550 700 850
050 150 250 350 450 550
Hz
ms
Illustration 160:
Modulation and demodulation in the case of single sideband AM
First the source signal (1st series) is amplitude-modulated by simple multiplication by the carrier. As the
source signal has no offset a double sideband AM signal without a carrier results. By means of a high
precision band pass signal in the “black box” (for the circuit see Illustration 95) the upper sideband - the
regular position - is filtered out (3rd series). We thus obtain a one sideband AM signal.
This is multiplied by a carrier of 256 Hz for demodulation. As before we obtain the sum of two signals (see
frequency domain 4th series), the source signal and a one sideband AM signal at the double carrier
frequency or mid-frequency. Note that both bands have the same amplitude level. If the upper band is
filtered with a lowpass filter, we obtain the retrieved source signal in the lower series.
The single sideband modulation is the only (analog) modulation
process in which the bandwidth is not greater than that of the
source signal. The single sideband modulation is thus the most
economical analog transmission process if the level of distortion
on the transmission path is successfully kept to a minimum.
