Digital Signal Processing Reference
In-Depth Information
P
Carrier
Sideband
f
Figure 6.5
Each modulation of a sinusoidal signal — the carrier — generates so-called (mod-
ulation) sidebands
To find the duty factor
m
we calculate the arithmetic mean of the keyed and unkeyed
amplitude of the carrier signal:
u
0
+
u
1
2
u
m
=
(
6
.
1
)
The duty factor is now calculated from the ratio of amplitude change
u
0
−ˆ
ˆ
u
m
to
the mean value
u
m
:
u
m
u
m
u
0
−
u
m
u
m
u
0
−
u
1
u
0
+
u
1
m
=
=
=
(
6
.
2
)
In 100% ASK the amplitude of the carrier oscillation is switched between the carrier
amplitude values 2
u
m
and 0 (
On-Off keying
; Figure 6.6). In amplitude modulation
using an analogue signal (sinusoidal oscillation) this would also correspond with a
modulation factor of
m
=
1 (or 100%) (Mausl, 1985).
The procedure described for calculating the duty factor is thus the same as that
for the calculation of the modulation factor for amplitude modulation using analogue
∆
û
m
û
0
û
m
û
1
t
m = 0.5; (ASK 50%)
Figure 6.6
In ASK modulation the amplitude of the carrier is switched between two states by
a binary code signal
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