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Lecture 5

The document discusses the power efficiency of Amplitude Modulation (AM) signals, highlighting that only 33% efficiency is achieved with m=1 modulation. It explains Double Side Band Suppressed Carrier (DSB-SC) modulation, which removes the carrier for higher efficiency but requires complex receivers for synchronization. Additionally, it covers the generation, detection, and comparison of various AM methods including Full AM, DSB-SC, Single Sideband (SSB), and Vestigial Sideband (VSB) modulation.

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32 views25 pages

Lecture 5

The document discusses the power efficiency of Amplitude Modulation (AM) signals, highlighting that only 33% efficiency is achieved with m=1 modulation. It explains Double Side Band Suppressed Carrier (DSB-SC) modulation, which removes the carrier for higher efficiency but requires complex receivers for synchronization. Additionally, it covers the generation, detection, and comparison of various AM methods including Full AM, DSB-SC, Single Sideband (SSB), and Vestigial Sideband (VSB) modulation.

Uploaded by

sadoopatola
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PDF, TXT or read online on Scribd
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.

Calculate the power efficiency of AM signals

• The ratio of useful power, power efficiency :

sidebands power m2 / 2 m2
 
total power 1  m / 2 2  m2
2

• In terms of power efficiency, for m=1 modulation, only


33% power efficiency is achieved which tells us that only
one-third of the transmitted power carries the useful
information.
Double Side Band Suppressed Carrier
(DSB-SC) Modulation

• The carrier component in full AM or DSB-LC does not convey any


information. Hence it may be removed or suppressed during the
modulation process to attain higher power efficiency.

• The trade off of achieving a higher power efficiency using DSB-SC


is at the expense of requiring a complex and expensive receiver due
to the absence of carrier in order to maintain transmitter/receiver
synchronization.
. Derive the Frequency Spectrum for Double Sideband
Suppressed Carrier Modulation (DSB-SC)

1 Consider the carrier


sc (t )  Ac cos(ct ) where c  2f c
2 modulated by a single sinusoidal signal
sm (t )  Am cos  mt where  m  2f m
3 The modulated signal is simply the product of these two
s (t )  Ac cos( c t ) Am cos( m t )
 Ac Am cos( c t ) cos( m t )
1
since cos A cos B  cos( A  B )  cos( A  B ) 
2
Am Ac Am Ac
 cos( c   m )t  cos( c   m )t
2   2  
US B LS B
sc (t )  Ac cos ct

sm (t )  Am cos  mt X s(t )  Ac cos(ct ) Am cos( mt )

Frequency Spectrum of a DSB-SC AM Signal

fc-fm fc fc+fm
• All the transmitted power is contained in the two sidebands
(no carrier present).

• The bandwidth is twice the modulating signal bandwidth.

• USB displays the positive components of sm(t) and LSB


displays the negative components of sm(t).
Generation and Detection of DSB-SC

• The simplest method of generating a DSB-SC signal is


merely to filter out the carrier portion of a full AM (or
DSB-LC) waveform.

• Given carrier reference, modulation and demodulation


(detection) can be implemented using product devices or
balanced modulators.
BALANCED MODULATOR

Sm(t) S1(t)
AM Modulator 1

Sm(t) Accos(ct)
S(t)
Carrier
DSB-SC
Accos(ct)

AM Modulator 2
-Sm(t) S2(t)
• The two modulators are identical except for the sign
reversal of the input to one of them. Thus,

s1 (t )  Ac (1  m cos( mt )) cos(ct )

s2 (t )  Ac (1  m cos( mt )) cos(ct )

s(t )  s1 (t )  s2 (t )
 2mAc cos( mt ) cos( ct )
COHERENT (SYNCHRONOUS) DETECTOR OR
DSB-SC (PRODUCT DETECTOR)

v(t) vo(t)
DSB-SC Signal s(t) X LPF

Cosct

Local Oscillator

• Since the carrier is suppressed the envelope no longer


represents the modulating signal and hence envelope
detector which is of the non-coherent type cannot be used.
v(t )  s (t ) cos( c t )  2mAc cos( mt ) cos( c t )cos( c t )
Am
2 Ac cos( mt ) cos ( c t )
2

Ac
 1  cos 2 c t 
 2 Am cos( mt ) 
 2 
 Am cos( mt )  Am cos( mt ) cos(2 c t )
since sm (t )  Am cos( mt )
 sm(t)  sm(t ) cos ( 2 c t)
 
Unwanted erm(remove
t d by LPF)
• It is necessary to have synchronization in both frequency
and phase between the transmitter (modulator) & receiver
(demodulator), when DSB-SC modulation ,which is of the
coherent type, is used.
Both phase and frequency must be known to demodulate
DSB-SC waveforms.
LACK OF PHASE SYNCHRONISATION

Let the received DSB-SC signal be


sDSBSC (t )  sm (t ) cosct   Ac
if  is unknown,
v(t )  sDSB SC (t ) cos  ct
 Ac sm (t ) cos  c t    cos  c t
Ac
 sm (t )cos   cos 2 c t   
2
Output of LPF
Ac
vo (t )  sm (t ) cos 
2
But we want just
Ac
vo (t )  sm (t )
2
Due to lack of phase synchronization, we will see that the
wanted signal at the output of LPF will be attenuated by an
amount of cos.
In other words, phase error causes an attenuation of the
output signal proportional to the cosine of the phase error.
The worst scenario is when =/2, which will give rise to
zero or no output at the output of the LPF.
LACK OF FREQUENCY SYNCHRONISATION

Suppose that the local oscillator is not stable at fc but at


fc+D f, then
v(t )  sDSB SC (t ) cos  c  D t
 Ac sm (t ) cos  ct cos  c  D t
Ac
 sm (t )cos Dt  cos 2 c t  D 
2
Output of LPF
Ac
vo (t )  sm (t ) cos Dt
2
Thus, the recovered baseband information signal will vary
sinusoidal according to cos D t
This problem can be overcome by adding an extra
synchronization circuitry which is required to detect  and
D t and by providing the carrier signal to the receiver.

A synchronizer is introduced to curb the synchronization


problem exhibited in a coherent system.

Let the baseband signal be


sm (t )  Am cos  mt
Received DSB-SC signal
s(t )  Ac sm (t ) cos ct
SYNCHRONISER

( )2 PLL BPF 2

Mathematical analysis of the synchronizer is shown below:


s 2 (t )  Ac2 Am2 cos 2  mt cos 2  c t
Ac2 Am2
 1  cos 2 mt 1  cos 2 ct 
4
Ac2 Am2
 1  cos 2 mt  cos 2 ct  cos 2 mt cos 2 ct 
4
Ac2 Am2  1 1 
 1  cos 2 t  cos 2 t  cos 2   t  cos 2   t
4 
m c c m c m 
2 2 

Output of BPF Ac2 Am2


cos 2 ct
4
Output of frequency divider
k cos  ct
where k is a constant of proportionality.

DISADVANTAGE OF USING COHERENT SYSTEMS

• The frequency and phase of the local oscillator signal must


be very precise which is very difficult to achieve.

It requires additional circuitry such as synchronizer circuit


and hence the cost is higher.
Single-Sideband Modulation
Single Side Band Modulation (SSB)

How to generate SSB signal?


• Generate DSB-SC signal
• Band-pass filter to pass only one of the sideband
and suppress the other.
For the generation of an SSB modulated signal
to be possible, the message spectrum must have
an energy gap centered at the origin.
• Example of signal with -300 Hz ~ 300 Hz energy gap
Voice : A band of 300 to 3100 Hz gives good
articulation
• Also required for SSB modulation is a highly selective filter
• Vestigial
VestigialSideband Modulation
Side Band Modulation (VSB)
Instead of transmitting only one sideband as SSB, VSB
modulation transmits a partially suppressed sideband and a
vestige of the other sideband.
Comparison of Amplitude Modulation methods
Comparison of Amplitude Modulation methods

Full AM (or DSB-LC)


- Sidebands are transmitted in full with the carrier.
- Simple to demodulate / detect
- Poor power efficiency
- Wide bandwidth ( twice the bandwidth of the information
signal)
- Used in commercial AM radio broadcasting, one
transmitter and many receivers.
Comparison of Amplitude Modulation methods

DSB-SC
- Less transmitted power than full AM and all the transmitted
power is useful.
- Requires a coherent carrier at the receiver; This results in
increased complexity in the detector(i.e. synchroniser)
- Suited for point to point communication involving one
transmitter and one receiver which would justify the use of
increased receiver complexity.
Comparison of Amplitude Modulation methods

SSB
- Good bandwidth utilization (message signal bandwidth =
modulated signal bandwidth)
- Good power efficiency
- Demodulation is harder as compares to full AM; Exact
filter design and coherent demodulation are required
- Preferred in long distance transmission of voice signals
Comparison of Amplitude Modulation methods

VSB
- Offers a compromise between SSB and DSB-SC
- VSB is standard for transmission of TV and similar signals
- Bandwidth saving can be significant if modulating signals
are of large bandwidth as in TV and wide band data
signals.
• For example with TV the bandwidth of the modulating
signal can extend up to 5.5MHz; with full AM the
bandwidth required is 11MHz

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