Classical modulation procedures - Signals, Processes, and Systems - page 234

NF-Signal Tiefpass Zeitbereic h FFT F requenzber.

Zeitberei ch Spectrum Freq. domai n

M u ltip lik a t.

Black-Box00

LP fil ter Multipl. LP fil ter

M u ltip li k a t. Tiefpass Signals Multipl. FFT B andpass

Träger

0,250
0,225
0,200
0,175
0,150
0,125
0,100
0,075
0,050
0,025
0,000
0,125
0,100
0,075
0,050
0,025
0,000
0,06
0,05
0,04
0,03
0,02
0,01
0,00
0,030
0,025
0,020
0,015
0,010
0,005
0,000
0,030
0,025
0,020
0,015
0,010
0,005
0,000

1,00
0,50
0,00
-0,50
-1,00
-1,50

2,0
1,5
1,0
0,5
0,0
-0,5
-1,0
-1,5
-2,0
0,4
0,3
0,2
0,1
0,0
-0,1
-0,2
-0,3
-0,4
0,3

0,1

-0,1
-0,3

0,15

0,05

-0,05
-0,15

050 150 250 350 450 550

0 100 250 400 550 700 850

Hz

ms

Illustration 161: Single sideband in lower sideband position

Unlike Illustration 159 here the lower sideband position is filtered out of the double sideband AM signal.
The process which follows is completely identical and, although we have processed a completely different
signal than in Illustration 159, we obtain the source signal in this way.

The demodulation begins with the multiplication of this signal by a sinusoidal carrier
(256 Hz) with which the AM signal was originally generated.

The spectrum in the fourth series results from the sum and difference formation of this
carrier frequency and the single sideband frequency band. The single sideband sum band
is twice as high as was originally the case, the difference band is the frequency band of
the source signal.

This procedure is carried out again in Illustration 161 but here the lower sideband, the
reverse position, is filtered out. Strangely, as our final result we obtain the source signal
as a difference signal by proceeding in the same way - multiplication by a carrier of 256
Hz although the difference really ought to lie in the negative frequency domain.