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  Generation and demodulation of AM wave are easy.
 AM systems are cost effective and easy to build.

Disadvantages:
 AM contains unwanted carrier component, hence it requires more
transmission power.
 The transmission bandwidth is equal to twice the message
bandwidth.

To overcome these limitations, the conventional AM system is modified at the cost of

increased system complexity. Therefore, three types of modified AM systems are discussed.

DSBSC (Double Side Band Suppressed Carrier) modulation:

In DSBC modulation, the modulated wave consists of only the upper and lower side bands.
Transmitted power is saved through the suppression of the carrier wave, but the channel
bandwidth requirement is the same as before.

SSBSC (Single Side Band Suppressed Carrier) modulation: The SSBSC

modulated wave consists of only the upper side band or lower side band. SSBSC is suited for
transmission of voice signals. It is an optimum form of modulation in that it requires the
minimum transmission power and minimum channel band width. Disadvantage is increased
cost and complexity.

VSB (Vestigial Side Band) modulation: In VSB, one side band is completely passed
and just a trace or vestige of the other side band is retained. The required channel bandwidth
is therefore in excess of the message bandwidth by an amount equal to the width of the
vestigial side band. This method is suitable for the transmission of wide band signals.
 DSB-SC MODULATION

DSB-SC Time domain and Frequency domain Description:

DSBSC modulators make use of the multiplying action in which the modulating
signal multiplies the carrier wave. In this system, the carrier component is eliminated and
both upper and lower side bands are transmitted. As the carrier component is suppressed,
the power required for transmission is less than that of AM.

Consequently, the modulated signal s(t) under goes a phase reversal , whenever the
message signal m(t) crosses zero as shown below.

Fig.1. (a) DSB-SC waveform (b) DSB-SC Frequency Spectrum

The envelope of a DSBSC modulated signal is therefore different from the

message signal and the Fourier transform of s(t) is given by
Generation of DSBSC Waves:

Balanced Modulator (Product Modulator)

A balanced modulator consists of two standard amplitude modulators arranged in a

balanced configuration so as to suppress the carrier wave as shown in the following block
diagram. It is assumed that the AM modulators are identical, except for the sign reversal of
the modulating wave applied to the input of one of them. Thus, the output of the two
modulators may be expressed as,
Hence, except for the scaling factor 2ka, the balanced modulator output is equal to the
product of the modulating wave and the carrier.

Ring Modulator

Ring modulator is the most widely used product modulator for generating DSBSC wave and
is shown below.

fig 4
The four diodes form a ring in which they all point in the same direction. The diodes are
controlled by square wave carrier c(t) of frequency fc, which is applied longitudinally by
means of two center-tapped transformers. Assuming the diodes are ideal, when the carrier is
positive, the outer diodes D1 and D2 are forward biased where as the inner diodes D3 and D4
are reverse biased, so that the modulator multiplies the base band signal m(t) by c(t). When
the carrier is negative, the diodes D1 and D2 are reverse biased and D3 and D4 are forward,
and the modulator multiplies the base band signal –m(t) by c(t).

Thus the ring modulator in its ideal form is a product modulator for square wave
carrier and the base band signal m(t). The square wave carrier can be expanded using Fourier
series as

From the above equation it is clear that output from the modulator consists entirely of
modulation products. If the message signal m(t) is band limited to the frequency band − w < f
< w, the output spectrum consists of side bands centred at fc.
Power calculation of DSB-SC wave
Consider the following equation of DSBSC modulated wave.

Power of DSBSC wave is equal to the sum of powers of upper sideband and lower sideband
frequency components.

We know the standard formula for power of cos signal is

First, let us find the powers of upper sideband and lower sideband one by one.
Upper sideband power

Similarly, we will get the lower sideband power same as that of upper sideband power.

Now, let us add these two sideband powers in order to get the power of DSBSC wave.

Bandwidth
The DSBSC modulated wave has only two frequencies. So, the maximum and minimum
frequencies are fc+fm and fc−fm respectively.

Substitute, fmax and fmin values in the bandwidth formula.

Introduction of SSB-SC

Standard AM and DSBSC require transmission bandwidth equal to twice the message
bandwidth. In both the cases spectrum contains two side bands of width W Hz, each. But the
upper and lower sides are uniquely related to each other by the virtue of their symmetry about
the carrier frequency. That is, given the amplitude and phase spectra of either side band,
the other can be uniquely determined. Thus if only one side band is transmitted, and if both
the carrier and the other side band are suppressed at the transmitter, no information is lost.
This kind of modulation is called SSBSC and spectral comparison between DSBSC and
SSBSC is shown in the figures 1 and 2.

Frequency Domain Description

side band is transmitted; the resulting SSB modulated wave has the spectrum shown in figure
6. Similarly, the lower side band is represented in duplicate by the frequencies below fc and
those above -fc and when only the lower side band is transmitted, the spectrum of the
corresponding SSB modulated wave shown in figure 5.Thus the essential function of the SSB
modulation is to translate the spectrum of the modulating wave, either with or without
inversion, to a new location in the frequency domain.
The advantage of SSB modulation is reduced bandwidth and the elimination of high power
carrier wave. The main disadvantage is the cost and complexity of its implementation.

Generation of SSB wave:

Frequency discrimination method

Consider the generation of SSB modulated signal containing the upper side band only.
From a practical point of view, the most severe requirement of SSB generation arises from
the unwanted sideband, the nearest component of which is separated from the desired
side band by twice the lowest frequency component of the message signal. It implies that,
for the generation of an SSB wave to be possible, the message spectrum must have an energy
gap centered at the origin as shown in figure 7. This requirement is naturally satisfied by
voice signals, whose energy gap is about 600Hz wide.
 The frequency discrimination or filter method of SSB generation consists of a product
modulator, which produces DSBSC signal and a band-pass filter to extract the desired side
band and reject the other and is shown in the figure 8.

Application of this method requires that the message signal satisfies two conditions:
1. The message signal m(t) has no low-frequency content. Example: speech, audio, music.
2. The highest frequency component W of the message signal m(t) is much less than the
carrier frequency fc.

Then, under these conditions, the desired side band will appear in a non-overlapping
interval in the spectrum in such a way that it may be selected by an appropriate filter.

In designing the band pass filter, the following requirements should be satisfied:
1.The pass band of the filter occupies the same frequency range as the spectrum of the
desired SSB modulated wave.

2. The width of the guard band of the filter, separating the pass band from the stop
band, where the unwanted sideband of the filter input lies, is twice the lowest frequency
component of the message signal.
Hilbert Transform & its Properties:

The Fourier transform is useful for evaluating the frequency content of an energy signal, or in
a limiting case that of a power signal. It provides mathematical basis for analyzing and
designing the frequency selective filters for the separation of signals on the basis of their
frequency content. Another method of separating the signals is based on phase selectivity,
which uses phase shifts between the appropriate signals (components) to achieve the desired
separation.
o
In case of a sinusoidal signal, the simplest phase shift of 180 is obtained by “Ideal
transformer” (polarity reversal). When the phase angles of all the components of a given
o
signal are shifted by 90 , the resulting function of time is called the “Hilbert transform” of
the signal.
Consider an LTI system with transfer function defined by equation 1
 The device which possesses such a property is called Hilbert transformer. Whenever a
signal is applied to the Hilbert transformer, the amplitudes of all frequency components of the
o
input signal remain unaffected. It produces a phase shift of -90 for all positive frequencies,
o
while a phase shifts of 90 for all negative frequencies of the signal.
If x(t) is an input signal, then its Hilbert transformer is denoted by xˆ(t ) and shown in
the following diagram.
Now consider any input x(t) to the Hilbert transformer, which is an LTI system. Let the
impulse response of the Hilbert transformer is obtained by convolving the input x(t) and
impulse response h(t) of the system.
Properties:

3. Hilbert transform of an odd signal is even and Hilbert transform of an even signal is
odd.
4. Signal and its Hilbert Transform are orthogonal.
5. Energy is same.

Time Domain Description:

The time domain description of an SSB wave s(t) in the canonical form is
given by the equation 1.
 Following the same procedure, we can find the canonical representation for an SSB
Wave s(t) obtained by transmitting only the lower side band is given by

Phase discrimination method for generating SSB wave:

Time domain description of SSB modulation leads to another method of SSB

generation using the equations 9 or 10. The block diagram of phase discriminator is as shown
in figure 15.
 The phase discriminator consists of two product modulators I and Q, supplied with
carrier waves in-phase quadrature to each other. The incoming base band signal m(t) is
applied to product modulator I, producing a DSBSC modulated wave that contains reference
phase sidebands symmetrically spaced about carrier frequency fc.

The Hilbert transform mˆ (t) of m (t) is applied to product modulator Q, producing a

DSBSC modulated that contains side bands having identical amplitude spectra to those of
modulator I, but with phase spectra such that vector addition or subtraction of the two
modulator outputs results in cancellation of one set of side bands and reinforcement of the
other set.

The use of a plus sign at the summing junction yields an SSB wave with only the
lower side band, whereas the use of a minus sign yields an SSB wave with only the upper
side band. This modulator circuit is called Hartley modulator.

(Another method for generation of SSB is Weaver’s Method-Please go through yourself)

Bandwidth of SSB signal=2fm/2=fm

Demodulation of SSB Waves:

Introduction to Vestigial Side Band Modulation

Vestigial sideband is a type of Amplitude modulation in which one side band is

completely passed along with trace or tail or vestige of the other side band. VSB is a
compromise between SSB and DSBSC modulation. In SSB, we send only one side band, the
Bandwidth required to send SSB wave is w. SSB is not appropriate way of modulation when
the message signal contains significant components at extremely low frequencies. To
overcome this VSB is used.

Frequency Domain Description

The following Fig illustrates the spectrum of VSB modulated wave s (t) with respect to
the message m (t) (band limited)
Assume that the Lower side band is modified into the vestigial side band. The vestige of the
lower sideband compensates for the amount removed from the upper sideband. The
bandwidth required to send VSB wave is

The vestige of the Upper sideband compensates for the amount removed from
the Lower sideband. The bandwidth required to send VSB wave is B = w+fv, where fv
is the width of the vestigial side band.

Therefore, VSB has the virtue of conserving bandwidth almost as efficiently as SSB
modulation, while retaining the excellent low-frequency base band characteristics of
DSBSC and it is standard for the transmission of TV signals.

Generation of VSB Modulated Wave

VSB modulated wave is obtained by passing DSBSC through a sideband shaping filter
as shown in fig below.

Fig.17. Block Diagram of VSB Modulator

The exact design of this filter depends on the spectrum of the VSB waves. The
relation between filter transfer function H (f) and the spectrum of VSB waves is given by
 S(f) = Ac /2 [M (f - fc) + M(f + fc)]H(f) ------------------------- (1)
Where M(f) is the spectrum of Message Signal. Now, we have to determine the
specification for the filter transfer function H(f) It can be obtained by passing s(t) to a
coherent detector and determining the necessary condition for undistorted version of the
message signal m(t). Thus, s (t) is multiplied by a locally generated sinusoidal wave cos
(2πfct) which is synchronous with the carrier wave Accos(2πfct) in both frequency and phase,
as in fig below,
The spectrum of Vo (f) is in fig below,
Independent Side Band (ISB)

 Independent Sideband (ISB) is an amplitude modulated technique in which a single carrier

frequency is independently modulated by two different modulating signals.
 In other words, ISB is a form of double sideband transmission system in which the transmitter
comprises of two independent single sideband suppressed carrier modulators. One modulator
generates only the upper sideband, and the other generates only the lower sideband.
 The single sideband output signals from the two modulators are combined to form a double
sideband signal in which the two sidebands are completely independent of each other, except
that they are symmetrical about a common carrier frequency.
 The block diagram of ISB transmitter is shown in Fig9 below.

1. ISB essentially consists of two SSB channels added to form two side bands around the
reduced carrier. Each side band is quite independent of the other. It can simultaneouly
convey totally different transmission.
2. Each 100 Hz - 6 KHz channel is fed to its own balanced modulator each modulator also
receiving the output of the 100 KHz crystal oscillator. Each modulator modulates each
message (100 Hz to 6 KHz) on the frequency 100 KHz.
3. The USB filter and LSB filter suppresses the unwanted side band in such a way that one
filter suppresses the lower side band and the other filter suppresses the upper side band
respectively.USB = 100.1 - 106 KHz LSB = 94 - 99.9 KHz
4. Both outputs are added at the hybrid adder with the 10% reduced carrier.
5. The output is then fed to the balanced mixer where it is mix with the output of the crystal
oscillator, the frequency is then raised to 28.1 MHz +- 6 KHz.
6. The resulting RF ISB signal is then amplified by the linear amplifier, until it reaches the
ultimate level then is fed to the antenna for transmission.

Applications of different AM systems:

  Amplitude Modulation: AM radio, Short wave radio broadcast

  DSB-SC: Data Modems, Color TV’s color signals.
  SSB: Telephone
 VSB: TV picture signals