T 7.2.1.3 Amplitude Modulation

TPS 7.2.1.3
Introduction
Harmonic Frequency Amplitude
1
2
3
4
n
f R = 1/T R SR(1)= 4 A
π
3 f R
5 f R
7 f R
(2 n – 1) f R
S R (1)
3
S R (1)
5
S R (1)
7
SR (1)
2n
−1
Amplitude spectrum of a symmetrical squarewave
signal n = 1, 2, 3, 4, ...
ion. Either harmonic oscillations or pulse trains
are used as carrier signals. If, for example, a harmonic
carrier is used with the form:
s C
(t) = A C
cos (2 f C
t + φ ), (1.3)
then the message signal s M (t) can have an effect
either on the amplitude A C , the carrier frequency
f C or the zero phase angle φ. These effects result in
the analog modulation methods:
– Amplitude modulations (AM)
– Frequency modulation (FM)
– Phase modulation (PM).
In the case of analog modulation methods, the
modulation process means a continuous conversion
of the modulating signal s M (t) into a higher
frequency band (frequency conversion). The mod-
R
ulating signal is shifted from the baseband (AF
range, original frequency band), into an RF frequency
band. It no longer appears in the spectrum
of the modulated oscillation. A modulation always
requires that the carrier and the modulation
signal interact. Both of these signals are fed into a
modulator. The original signal s M (t) is recovered
from the modulated signal through demodulation.
Consequently, modulation and demodulation are
mutually related, inverse processes. The complexities
involved in modulation and demodulation are
considerable. The following reasons explain why
modulation is worthwhile:
1. Modulation enables the matching of the
modulating signal to the characteristics of the
transmission channel. (radio links e.g. are
only possible above a certain frequency.)
2. Existing transmission channels can be multiply
exploited using modulation, (frequency
or time division multiplex systems).
3. Improved signal-to-noise ratios can be obtained
using modulation.
The communications system according to Shannon
Electrical communications engineering is divided
into three classical subfunctions:
1. Transmission of the message
2. Processing of the message
3. Relaying the message (telephone technology)
If only a single transmission channel is examined,
(i.e. no telephone technology), then we can concentrate
on the remaining functions illustrated by
the scheme in Fig. 1-3.
Fig. 1-3: The telecommunications system
(A) The telecommunications system
(B) Message transmission
(C) Message processing
1 Message source
(human being, measurment sensor etc.)
2 Converter (microphone,
television camera,
strain gauges, thermo sensor etc.)
3 Transmitter
4 Transmission channel (radio link,
transmission cable, data storage system)
5 Receiver
6 Converter
7 Message recipient
8 Interference source
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