UNIT-2

DSB & SSB MODULATION

Dr.P.Sunitha,M.Tech,Ph.D
Associate Professor
Dept.of ECE
Contents
Double side band suppressed carrier modulators
Time domain and frequency domain description.
 Generation of DSBSC Waves.
 Balanced Modulators, Ring Modulator,
Coherent detection of DSB-SC Modulated waves, COSTAS
Loop.
Frequency discrimination method for generation of AM SSB
Modulated Wave,
Time domain description
Phase discrimination method for generating AM SSB
Modulated waves.
Demodulation of SSB Waves
Double Sideband Full Carrier (DSBFC)
• In the process of Amplitude Modulation, the modulated wave consists of the carrier

wave and two sidebands. The modulated wave has the information only in the
sidebands. Sideband is nothing but a band of frequencies, containing power, which are
the lower and higher frequencies of the carrier frequency.

• The transmission of a signal, which contains a carrier along with two sidebands can be

termed as Double Sideband Full Carrier system or simply DSBFC. It is plotted as

shown in the following figure.
Double Sideband Suppressed Carrier
• The modulated signal m(t).cos (ct) has components of

frequencies (c ± m) but does not have a component of the

carrier frequency.

• For this reason this scheme is referred to as double-sideband

suppressed-carrier (DSB-SC) modulation.

• However, such a transmission is inefficient. Because, two-thirds of the power

is being wasted in the carrier, which carries no information.

• If this carrier is suppressed and the saved power is distributed to the two

sidebands, then such a process is called as Double Sideband Suppressed

Carrier system or simply DSBSC. It is plotted as shown in the following
figure.
Mathematical Expressions

• Let us consider the same mathematical expressions for modulating

and carrier signals as we have considered in the earlier chapters.
• i.e., Modulating signal

m(t)=Am cos(2πfmt)
• Carrier signal

c(t)=Ac cos(2πfct)
• Mathematically, we can represent the equation of DSBSC wave as
the product of modulating and carrier signals.
• s(t)=m(t)c(t)

⇒s(t)=AmAccos(2πfmt)cos(2πfct)
Bandwidth of DSBSC Wave
Power Calculations of DSBSC Wave
Generation of DSBSC
• Product Modulator

Now if m(t) = cos (mt), then the modulated signal

The component of frequency (c + m) is the upper side band, and that of
frequency (c - m) is the lower sideband corresponding to the modulating
signal of frequency m.
Nonlinear modulators (DSB-SC modulator)
• A semiconductor diode or a transistor is a nonlinear component.
Nonlinear characteristics can be approximated by a power series as:
i  a.v  bv 2
We shall consider that the nonlinear element is in series with the resistor R as a
composite element whose terminal voltage v and the current i are related by the
power series as mentioned above. The voltage v1 and v2 in the following figure are
given by:
v1  cos ct  m(t) and v 2  cos c t  m(t)
(b) Nonlinear element
i1

+
(a) m(t) R

k.m(t).cos (ct)
i cos (ct) v1 Band-
v0 pass
+ + filter
v m(t) v2  c
R

i2
Nonlinear element
Balanced Modulator
Hence, the currents i1 and i2 are given by: i1  a.v1  bv1 2
i1  a.cos c t  x(t) bcos  tc  x(t)
2

i2  a.cos c t  x(t) b cos c t  x(t)

2

v0  i1 R  i2 R  2R2bx(t) cos c t  ax(t)

The element a.x(t) in this equation can be filtered out by using a band-pass tuned to
ωc at the output terminals. Implementation of this scheme is shown in the following
figure.

R
cos (ct)

k.x (t).cos (ct)

x(t) +
R
As we can see that the baseband input signal is applied at the input of
2 diodes that are 180⁰ phase reversed with each other through a centre
tapped transformer.
Hence the input at D ,1

v = cos ω t + x(t)
1 c

and input at D ,
2

v = cos ω t – x(t)
2 c

At the output side tuned bandpass filter is obtained by parallel

connection of RLC circuit.
So, the current through D is given as
1
Similarly,

• The output voltage is given by

vo = i1 R – i2 R

• On substituting the above-given value of i1 and i2 in the output equation, we

will have,

vo = R [2 a x(t)] (+ 4b x (t) cos ωct)]

• Therefore, the output is,

vo = 2aR x(t) + 4bRx (t) cos ωct

• 2aR x(t) = modulating signal

• 4bR x(t) cos ωct = DSB-SC signal

• Thus, from the above expression, it is clear that output voltage is a

combination of modulating signal along with the DSB-SC signal.

• After the elimination of the modulating signal, the DSB-SC signal is

then passed to the LC bandpass and is received at the output.

• Thus we will have,

• 4bR x(t) cos ωct = K x(t) cos ωct at the output.

Non-Linear DSB-SC Modulator
• The following two modulators generate DSBSC wave.
• Balanced modulator
• Ring modulator
• Balanced Modulator
• Balanced modulator consists of two identical AM modulators. These

two modulators are arranged in a balanced configuration in order to

suppress the carrier signal. Hence, it is called as Balanced modulator.

• The same carrier signal c(t)=Ac cos(2πfct) is applied as one of the

inputs to these two AM modulators.

• The modulating signal m(t) is applied as another input to the upper

AM modulator.

• Whereas, the modulating signal m(t) with opposite polarity,

i.e., −m(t) is applied as another input to the lower AM modulator.
• We get the DSBSC wave s(t) by subtracting s2(t) from s1(t).

• The summer block is used to perform this operation.

• s1(t) with positive sign and s2(t) with negative sign are applied as

inputs to summer block.

• Thus, the summer block produces an output s(t) which is the

difference of s1(t) and s2(t).

Ring modulator
D1
a c
Another switching
+ modulator is the ring

k.m(t).cos (ct)
Band-pass filter
D4
modulator as shown here.
m(t)

D2
-
b D3 d
 -
A. Cos (ωct) (a)

k’(t)

t→
m(t) (b)
v1=m(t).k’(t)

t→
t→
(c)
(d)
2
4
Ring modulator
• In this diagram, the four diodes D1,D2,D3 and D4 are connected in

the ring structure. Hence, this modulator is called as the ring

modulator.

• Two center tapped transformers are used in this diagram. The

message signal m(t) is applied to the input transformer. Whereas, the

carrier signals c(t) is applied between the two center tapped
transformers.

• For positive half cycle of the carrier signal, the diodes D1 and D3 are

switched ON and the other two diodes D2 and D4 are switched OFF.
In this case, the message signal is multiplied by +1.
Ring modulator
• For negative half cycle of the carrier signal, the diodes D2 and D4 are

switched ON and the other two diodes D1 and D3 are switched OFF.
In this case, the message signal is multiplied by -1.

• This results in 1800 phase shift in the resulting DSBSC wave.

• From the above analysis, we can say that the four

diodes D1, D2, D3 and D4 are controlled by the carrier signal. If the
carrier is a square wave, then the Fourier series representation
of c(t) is represented as
Demodulation of DSB-SC signals
Demodulation of a DSB-SC signal is identical to modulation as it is seen from the following
figure. 1
m(t) cos  c t m(t) cos 2  c t Low-pass 2
m (t )
filter
cos  c t

1
m(t)(cos  c t)(cos  c t)  m(t)  m(t) cos 2 ct)
2
(m(t) cos c t)(cos  c t)  M ()  M (  2 c )  M (  2 c )
1 1
2 4
• At the receiver, we multiply the incoming signal by a local carrier of frequency
and phase in synchronism with the carrier used at the modulator.
• Pass the product through a low-pass filter.
• For demodulation, receiver must generate a carrier in phase and frequency
synchronous with the incoming carrier.
• These demodulators are called synchronous, or coherent (also homodyne)
demodulators.
Demodulation of DSBSC
• The process of extracting an original message signal from DSBSC

wave is known as detection or demodulation of DSBSC. The following

demodulators (detectors) are used for demodulating DSBSC wave.

• Coherent Detector

• Costas Loop
Coherent Detector

• Here, the same carrier signal (which is used for generating DSBSC

signal) is used to detect the message signal. Hence, this process of

detection is called as coherent or synchronous detection.

Coherent Detector(Cont.)

• In this process, the message signal can be extracted from DSBSC wave
by multiplying it with a carrier, having the same frequency and the phase
of the carrier used in DSBSC modulation.
• The resulting signal is then passed through a Low Pass Filter. Output of
this filter is the desired message signal.
• Let the DSBSC wave be

s(t)=Ac cos(2πfct)m(t)
• The output of the local oscillator is

c(t)=Accos(2πfct+ϕ)
• Where, ϕ is the phase difference between the local oscillator signal and
the carrier signal, which is used for DSBSC modulation.
Coherent Detector(Cont.)
Costas Loop
• Costas loop is used to make both the carrier signal (used for DSBSC modulation) and

the locally generated signal in phase. Following is the block diagram of Costas loop.
• Costas loop consists of two product modulators with common

input s(t) which is DSBSC wave.

• The other input for both product modulators is taken from Voltage

Controlled Oscillator (VCO) with −90 degrees phase shift to one

of the product modulator as shown in figure.

• We know that the equation of DSBSC wave is

s(t)=Accos(2πfct)m(t)

• Let the output of VCO be

c1(t)=cos(2πfct+ϕ)
• The output of this Low pass filter has −90degrees phase difference

with the output of the upper low pass filter.

• The outputs of these two low pass filters are applied as inputs of the

phase discriminator.

• Based on the phase difference between these two signals, the phase

discriminator produces a DC control signal.

• This signal is applied as an input of VCO to correct the phase error in

VCO output.

• Therefore, the carrier signal (used for DSBSC modulation) and the

locally generated signal (VCO output) are in phase.

Advantages of DSB-SC modulation

It provides 100% modulation efficiency.

Due to suppression of carrier, it consumes less power.

It provides a larger bandwidth.

Disadvantages of DSB-SC modulation

It involves a complex detection process.

Using this technique it is sometimes difficult to recover the

signal at the receiver.

It is an expensive technique when it comes to

demodulation of the signal.
Applications of DSB-SC modulation

During the transmission of binary data, DSB-SC system is

used in phase shift keying methods.

In order to transmit 2 channel stereo signals, DSB signals

are used in Television and FM broadcasting.

DSB-SC technique allows us to have a transmission that

reduces overall power consumption rate, thereby ensuring

a stronger signal at the output.
Single Sideband (SSB) Modulation

• Definition: Single sideband modulation (SSB) is an amplitude

modulation scheme in which only a single sideband is transmitted
through the channel.

• It is also known as SSB-SC which is an acronym for Single Sideband

Suppressed Carrier as it allows suppression of one sideband and

carrier completely.

• As we know, DSB-SC modulation technique generates an output wave

having twice the bandwidth as that of the original modulating signal. So,
in order to avoid doubling factor of bandwidth in such modulation
system, the SSB-SC modulation technique was introduced.
Why is the suppression of a sideband allowed in SSB modulation?

• Single sideband modulation technique was adopted, as by

suppression of one of the two sidebands along with the carrier causes

no loss of information.
• This is so because the two sidebands of the modulated

signal are particularly related to each other. In other words,

we can say the two sidebands carry similar information.

• Thus, for the transmission of information, we need only one

sideband. So, by suppressing one sideband along with the

carrier, no any information is lost.

• Hence, the bandwidth requirement also gets reduced to half

and there are chances for an accommodation of twice

number of channels using the SSB modulation technique.
• Mathematical Expression
• Let the modulating signal be,
m(t) = Am cos (2πfmt)
• and carrier signal
c(t) = Ac cos (2πfct)
• As we have discussed the similar expression in DSB-SC
modulation. So, here we can write,

• It is a combination of 2 sidebands,
Bandwidth of SSBSC Wave

• We know that the DSBSC modulated wave contains two sidebands

and its bandwidth is 2fm.

• Since the SSBSC modulated wave contains only one sideband, its

bandwidth is half of the bandwidth of DSBSC modulated wave.

• i.e., Bandwidth of SSBSC modulated wave =2fm/2=fm

• Therefore, the bandwidth of SSBSC modulated wave is fm and it is

equal to the frequency of the modulating signal.

Advantages

Less Bandwidth or spectrum space is requires compare

to that of AM and DSBSC waves.

Allowing transmission of more number of signals.

Less Power consumption.

Capacity to transmit high power signal.

Reduced interference of noise.

Signal fading is less likely to occur.

Disadvantages

The generation and detection of SSBSC wave is quite a

complex process.

The signal quality gets affected if the SSB transmitter and

receiver does not possess an excellent frequency stability.

Applications

Applied for power saving requirements and low bandwidth

requirements.

In land, air, and maritime mobile communications.

In point-to-point communications.

In radio communications.

In television, telemetry, and radar communications.

In military communications, such as amateur radio, etc.

Generation of SSBSC
We can generate SSBSC wave using the following two
methods.
Frequency discrimination method
Phase discrimination method
Frequency Discrimination Method
• In this method, first we will generate DSBSC wave with the help of the

product modulator. Then, apply this DSBSC wave as an input of band

pass filter.

• This band pass filter produces an output, which is SSBSC wave.

• Select the frequency range of band pass filter as the spectrum of the

desired SSBSC wave.

• This means the band pass filter can be tuned to either upper

sideband or lower sideband frequencies to get the respective SSBSC

wave having upper sideband or lower sideband.
Phase Discrimination Method
• This block diagram consists of two product modulators, two −90degrees
phase shifters, one local oscillator and one summer block. The product
modulator produces an output, which is the product of two inputs.
The −90degrees phase shifter produces an output, which has a phase
lag of −90degrees with respect to the input.
• The local oscillator is used to generate the carrier signal. Summer block
produces an output, which is either the sum of two inputs or the
difference of two inputs based on the polarity of inputs.
• The modulating signal Amcos(2πfmt) and the carrier
signal Accos(2πfct) are directly applied as inputs to the upper product
modulator.
• So, the upper product modulator produces an output, which is the
product of these two inputs.
• The output of upper product modulator is
Demodulation of SSB Waves

• The process of extracting an original message signal from

SSBSC wave is known as detection or demodulation of

SSBSC.
• Coherent Detector
• Here, the same carrier signal (which is used for generating SSBSC

wave) is used to detect the message signal. Hence, this process of

detection is called as coherent or synchronous detection.
Block diagram of coherent detector.
• In this process, the message signal can be extracted from SSBSC

wave by multiplying it with a carrier, having the same frequency and

the phase of the carrier used in SSBSC modulation.

• The resulting signal is then passed through a Low Pass Filter. The

output of this filter is the desired message signal.