Digital Signal Processing Reference
In-Depth Information
1,00
0,75
0,50
0,25
0,00
-0,25
-0,50
-0,75
-1,00
125
10
7
50
2
0
-25
-50
-75
-100
-125
1,25
1,00
0,75
0,50
0,25
0,00
55
45
35
25
15
5
without offset
5
15
25
35
45
55
65
75
85
50
150
250
350
450
550
650
750
850
950
ms
Hz
1,00
0,75
0,50
0,25
0,00
-0,25
-0,50
-0,75
-1,00
3
2
1
0
-1
-2
-3
1,25
1,00
0,75
0,50
0,25
0,00
1,25
1,00
0,75
0,50
0,25
0,00
offset 1
5
15
25
35
45
55
65
75
85
50
150
250
350
450
550
650
750
850
950
ms
Hz
1,00
0,75
0,50
0,25
0,00
-0,25
-0,50
-0,75
-1,00
2,0
1,5
1,0
0,5
0,0
-0,5
-1,0
-1,5
-2,0
1,25
1,00
0,75
0,50
0,25
0,00
1,0
0,8
0,6
0,4
0,2
0,0
offset 2 > offset 1
5
15
25
35
45
55
65
75
85
50
150
250
350
450
550
650
750
850
950
ms
Hz
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
1,25
1,00
0,75
0,50
0,25
0,00
1,0
0,8
0,6
0,4
0,2
0,0
offset 3 > offset 2
5
15
25
35
45
55
65
75
85
50
150
250
350
450
550
650
750
850
950
Hz
ms
Illustration 154:
AM generation: multiplication of a source signal by a sinusoidal carrier
A low frequency sine is selected here as the most straightforward form of source signal. A direct voltage
(offset) which becomes greater and greater is superimposed until the sine lies entirely in the positive
region. When this is the case, the original source signal lies in the envelope of the AM signal after multipli-
cation by the sinusoidal carrier.
In this form the AM signal with the above circuit consisting of diode D, resistance R and capacitor C can
be demodulated, i.e this circuit makes it possible to retrieve the original source signal. There are, however,
disadvantages involved in this type of amplitude modulation.
Note:
In the case of analog or computer- (digital) multiplication the frequently mentioned
degree of modulation m
often makes little sense. In theory the mathematical model
of an AM signal is described by the formula
u
AM
(t) = (1 + m sin (
ω
LF
t))
<
Û
carrier
sin(
ω
carrier
t)
The first bracket represents the sinusoidal source signal voltage with a direct voltage
(“1”) superimposed. The last term describes the sinusoidal carrier voltage.
