T 7.2.1.3 Amplitude Modulation

TPS 7.2.1.3
Solutions
3 Review of amplitude modulation
Answers
3.1 Modulation means the frequency conversion
of an information signal from the AF position
of the baseband into the RF band of the
carrier. Here, the modulating signal
influences an appropriate parameter of the
carrier oscillation, e.g. the amplitude or frequency.
While f C >> f M always holds true in
modulation, mixing entails frequency
conversion being generated between signals
with comparable frequencies.
3.2 Modulation offers the following advantages:
– Matching to the features of the transmission
channel, i.e. improved efficiency
during transmission of information signals.
– Multiple utilization of transmission channels,
e.g. in frequency multiplexing
methods.
– Improved signal-to-noise ratios (modulation
gain)
3.3 The DSB is something "new" produced by
combining the carrier oscillation and the
modulating signal. While the dynamic characteristic
of the AM signal can be observed
as a whole on the oscilloscope the spectrum
analyzer shows the AM broken down into its
components. With constant modulation
signal these components have amplitudes
contstant with respect to time.
3.4 In the dynamic characteristic of the beat a
phase shift of 180° arises in the envelope
curve. The frequency of the envelope curve
is approx. half the differential frequency of
the oscillation components involved. The
beat frequency corresponds to the arithmetic
mean value.
3.5 The amplitude deviation ∆A C indicates the
maximum change permissible for the carrier
amplitude A C . It is dependent on the
modulator constant α and the amplitude A M
of the modulating signal. The modulation
index m is the quotient formed out of the
amplitude deviation and the carrier amplitude.
The modulation index m can assume
values between 0 and 1. Overmodulation
occurs for m > 1.
3.6 In addition to envelope demodulation, synchronous
or coherent demodulation are
common demodulation methods particularly
in commercial communications systems. In
contrast to envelope demodulation, it requires
an auxiliary carrier which is stable in
terms of frequency, phase and amplitude.
3.7 The reduction in carrier power means an
improvement in the transmission efficiency.
The amount of power and amplifier circuitry
in the transmitter can be reduced.
Bandwidth is saved by limiting modulation to
one sideband. However, coherent demodulation
becomes problematic when the carrier
is completely suppressed. For that reason a
residual carrier is transmitted with which the
receiver is synchronized.
3.8 The following applies for the efficiency η:
useful power
η =
total power
Expressed by the modulation index m the
following holds true:
η =
m 2
2+ m 2
For the maximum modulation index
m = 100% you obtain the best efficiency of
the DSB when η = 33%! Regarding the
power needed in DSB at least 2/3 is “squandered”
in the carrier. Since the carrier contains
no information it can be suppressed to
increase the efficiency.
3.9 Methods of carrier suppression
– Suppress the carrier using a bandpass filter
with very sharp cutoffs.
– Addition of a carrier in phase opposition
and of equal amplitude to the modulated
signal.
– Use of a modulation method which does
not permit the carrier to reach the modulation
product, e.g. balanced or ring
modulator.
3.10 Here only coherent demodulation still constitutes
a viable alternative. For this an auxiliary
oscillation is needed in the receiver,
which is in agreement with the original carrier
in terms of frequency and phase and
whose amplitude remains constant.
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