AMPLITUDE%20MODULATION

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AMPLITUDE MODULATION
Communication Systems Prof. Ravi Warrier
Main Topics Double sideband modulation,
demodulation Amplitude Modulation
- Suppressed carrier Single
sideband Vestigial Sideband
Superhetrodyne AM Receiver
AMPLITUDE MODULATION DOUBLE SIDEBAND SUPPRESSED
CARRIER(DSB-SC) m(t) message signal , M(?)
its spectrum , cos(?ct) carrier signal, ?c
carrier frequency. ?(t) m(t)cos(?ct)
modulated signal, EXAMPLE
0
0
Lower sideband (LSB)
Upper sideband (USB)
0
Notice that the carrier part is missing in the
spectrum of ?(t) (impulse functions disappeared).
Therefore, this type of modulation is called
AM-Double Side band Suppressed Carrier (AM-DSBSC)
modulation.
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Communication Systems Prof. Ravi Warrier
DEMODULATION e(t) ?(t) cos(?ct)
m(t)cos2(?ct)
Lowpass filter e(t) to get m(t).
0
e(t)
LPF
Demodulation requires a local oscillator at the
receiver and the frequency and phase of the local
sine wave should be synchronized with the
transmitter. This requires expensive hardware.
Read Tone modulation Modulators -
Nonlinear modulator, switching modulators and
ring modulators Frequency
conversion. Important result
Multiplying two sinusoids results in two
frequencies which are the sum and difference of
the frequencies of the sinusoids multiplied. To
change the carrier frequency ?c of a modulated
signal to an intermediate frequency ?I we use an
oscillator to generate a sinusoid of frequency
?MIX such that
BPF_at_ ?I
EXAMPLE Let m(t) be as shown.
m(t)
?(t)
e1(t)
SPECTRA
0
0
0
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Communication Systems Prof. Ravi Warrier
EXERCISE Let m(t)cos(10t) and ?c 200
rad/s. a) Sketch ?(t) m(t)cos(?ct) . b) Sketch
the spectra of m(t) and ?(t). c) What is are
upper sideband and lower sideband components of
g(t) ? Express them in frequency domain and time
domain.
AMPLITUDE MODULATION (AM) A dc offset is added to
m(t) before modulation.
The last impulse functions indicate that the
carrier is not suppressed in this case. For some
M() shown, the modulated signal spectrum is as
shown.
0
0
With this type of AM the demodulation can be
performed without a local oscillator
synchronized with the transmitter.
EXAMPLE m(t) has a minimum value of about
-0.4. Adding a dc offset of A1 results in Am(t)
being always positive. Therefore the positive
envelope of is just Am(t). An envelope detector
can be used to retrieve this.
A1
m(t)
Am(t)
0.7
1.
0.
-0.4
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Communication Systems Prof. Ravi Warrier
The choice of dc offset should be such that
Am(t) should always be positive. Otherwise
envelope detector cannot be used. For example,
the minimum value of m(t) -0.4 . Therefore A gt
min(m(t)) for successful envelope detection.
What if Alt m(t). In the previous example let
A0.3.
A0.3
Am(t)
m(t)
0.7
0
0.
-0.4
Envelope detection cannot be employed in this
case . The condition for envelope detection is
that Am(t) ? 0 for all t. Let mp be the
absolute negative peak of m(t).
EXAMPLE Single-tone modulation. Let
m(t)2sin(20t) and carrier frequency be 200
rad/sec. What is the dc offset required for i)
?0.5 , ii)?1. Sketch the AM signal if the dc
offset used is 1.
m(t)
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EXERCISE Let m(t)2cos(20t)cos(40t) and
carrier frequency be 200 rad/sec. What is the dc
offset required for i) ?0.5 , ii)?1. mp
2.25 for this m(t). Why ?)
Communication Systems Prof. Ravi Warrier
Sideband powers
EXERCISE For AM wave with m(t)2cos(20t)cos(40
t) calculate the efficiency if i) ?0.5 ,
ii)?1.
Generation of AM signals Read in TEXT
-
m(t)
AM output
-
c cos(?ct)
DEMODULATION OF AM WAVES 1. Rectifier detector
READ in TEXT
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Communication Systems Prof. Ravi Warrier
2.DEMODULATION USING ENVELOPE DETECTION
vc(t) -
AM signal
R
C
EXAMPLE Let E(t)ABcos(?mt). Design RC
for envelope detection.
AM signal BANDWIDTH AM signal bandwidth is
twice the bandwidth of the modulating signal.
(Why?) A 5kHz signal requires 10kHz bandwidth
for AM transmission. If the carrier frequency is
1000 kHz, the AM signal spectrum is in the
frequency range of 995kHz to 1005
kHz. QUADRARTURE AMPLITUDE MODULATION is a scheme
that allows two signals to be transmitted over
the same frequency range. READ TEXT.
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Communication Systems Prof. Ravi Warrier
Single Sideband (SSB) AM Purpose to reduce the
bandwidth requirement of AM by one-half. This is
achieved by transmitting only the upper sideband
or the lower sidebband of the DSB AM signal.
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0
SSB (Upper sideband)
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Math description
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Communication Systems Prof. Ravi Warrier
Next we show that the SSB signal can be expressed
in terms of m(t) and its Hilbert transform as
follows.
How to generate mh(t) ?
H(?)
M(?)
Mh(?)
Transfer function of a Hilbert transformer
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Communication Systems Prof. Ravi Warrier
SSB Signal Generation 1. Selective Filtering
using filters with sharp cutoff characteristics.
Sharp cutoff filters are difficult to design. The
audio signal spectrum has no dc component,
therefore , the spectrum of the modulated audio
signal has a null around the carrier
frequency.This means a less than perfect filter
can do a reasonably good job of filtering the DSB
to produce SSB signals.
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2. Phase shift method using Hilbert transformer.
x

m(t)


Hilbert Transformer
X
DEMODULATION OF SSB AM SIGNAL 1. Synchronous
demodulation
SSB signal can be transmitted with carrier as
well by adding a dc off set.
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Communication Systems Prof. Ravi Warrier
EXAMPLE Tone modulation m(t) cos(?mt). What
is the Hilbert transform of this m(t)
? VESTIGIAL SIDEBAND (VSB) AM VSB is a
compromise between DSB and SSB. To produce SSB
signal from DSB signal ideal filters should be
used to split the spectrum in the middle so that
the bandwidth of bandpass signal is reduced by
one half. In VSB system one sideband and a
vestige of other sideband are transmitted
together. The resulting signal has a bandwidth gt
the bandwidth of the modulating (baseband) signal
but lt the DSB signal bandwidth.
DSB
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SSB (Upper sideband)
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VSB Spectrum
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m(t)
e(t)
Hi(?)
LPF Ho(?)
Transmitter
Receiver