Digital Signal Processing Reference
In-Depth Information
0,125
0,075
0,025
-0,025
-0,075
0,125
0,075
0,025
-0,025
-0,075
-0,125
0,225
0,175
0,125
0,075
0,025
0,25
0,15
0,05
-0,05
-0,15
-0,25
0,325
0,275
0,225
0,175
0,125
0,4
0,2
0,0
-0,2
-0,4
0,525
0,475
0,425
0,375
0,325
0,75
0,50
0,25
0,00
-0,25
-0,50
-0,75
0
100
250
400
550
700
850
ms
Illustration 155:
AM of a low frequency signal curve
In principle the same is represented here as in Illustration 154, only with a typical low frequency signal
segment as the source signal. A direct voltage, which increases from top to bottom is also superimposed on
the source signal.
In the frequency domain this offset corresponds to the sinusoidal carrier which from top to bottom becomes
more and more dominant. The main part of the energy of the AM signal is accounted for by the carrier and
not the information-bearing part of the AM signal.
In the frequency domain the so-called
upper sideband
and
lower sideband
of the original source signal
can be seen clearly to the right and left of the carrier, that is the information is present twice over. For this
reason this kind of AM is often referred to as a “double sideband AM”.
If two voltages are multiplied by each other the unit (V
<
V), i.e. (V
2
) results. This is
incorrect in a physical sense for at the output of a multiplier a voltage appears with
the unit (V). We have recourse to a little trick - the term in the brackets is defined
as a pure number without a unit. m is the
degree of modulation
and is defined as
m = Û
LF
/Û
carrier
(V) is now simplified and for u
AM
(t) the unit (V) results.
The term (1 + m sin(
LF
t))
<
Û
carrie
r
can now be meaningfully interpreted as “time-
dependent amplitude” which changes in the rhythm of the LF signal. If this time-
dependent amplitude is always positive - the term in brackets must be greater than
0, i.e. positive - the time-dependent amplitude (envelope!) lies exclusively in the
positive region or, inverted in the negative region.
ω
