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Some transmitters are made to transmit in one side
band only which is called single side band
transmission or SSB transmission. In
telecasting, the video signal is transmitted in
the SSB mode.
BAND WIDTH
a The total width of a broadcasting channel
including upper and lower side bands is caleed
its bandwidth. If the signal frequency is 10 kHz
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then the band width will be 20 kHz. Therefore,
the T.R.F. amplifiers employed in transmitters
and receivers are designed in such a way that
they can amplify the complete band width of
the carrier frequency. The side bands and the
band width of a 500 kHz carrier along with 10
kHz signal frequency are shown in Fig. 23.4.
F.M. bandwidth is usually higher than that of
AM; it is equal to double of the sum of RF
variations and AF variations
The average frequency of an audio signal is..
MODULATION
a The frequency of audio signals extends upto
20 kHz and even the major portion of a
video signal also extends upto 20 kHz. The
transmission of the waves of such a low
frequency is impracticable. There are two
reasons behind the said fact (i) The average
frequency of an audio signal is 10 kHz and
its wavelength will be 30,000 metres. Since,
the length of a transmitting aerial should be
, hence the direct transmission of audio
signals would require a 15 kilometre long
antenna and the instalation of such a
lengthy antenna would be quite
impracticable. (ii) The transmitter would
require thousands and lakhs of watts output
power for having an effective broadcasting
zone. The construction of transmitting
tumes having such a huge power handling
capacity would also be a difficult task.
a Owing to above mentioned facts the
broadcasting would be impracticable in the
frequency range of 20 Hz to 20 kHz. The
problem was solved by the process called
modulation. Carrier waves are capable to
travel upto infinite distances in the space
but they do not have any informative audio
or video signal. On the other hand, an audio
or video signal has information but their
direct transmission is impracticalbe.
Therefore, the signal wave is superimposed
on carrier wave and then transmitted into
the space. The process of superimposing
(or mixing) signal wave on the carrier wave
is called modulation. For example, if a 12
MHz carrier is employed for broadcasting
then 12 MHz=25 metres carrier would
require only a 12.5 metres long antenna.
a In this way, a transmitter includes an
oscillator for the production of carrier
waves, amplifier for the amplification of
signal waves and a modulator for the
modulation of these two types of waves.
SIGNAL WAVE
a An electrical wave having sound, picture
or any other type of information or effect
is called a signal wave. The frequency of
sound waves lies between 20 Hz to 20 kHz,
whereas, the frequency of video signals lies
between 0 to 7 MHz. Normally kthe
amplitude of a signal wave does not remain
constant but it varies continuously in
accordance to the signal strength.
CARRIER WAVES
a The electromagnetic waves having
constant amplitude and of the frequency
a The F.M. was invented during the search of
Ravi Bolleddu
a process which could eliminate the
Golkonda Institute of
electrical interference. In practice, the A.M.
Police
wave received by a receiver has amplitude
Dilsuknagar, Hyd.
variations on account of atmospheric,
electrical and mechanical disturbances. A
9000422100
radio receiver cannot differentiate between
the signal wave and the noise wave.
accordance to the highest and lowest signal
Therefore, if the frequency of the signal
frequency. In this way, the depth of
wave is modulated then other types of
modulation of a F.M. wave is termed as
amplitude variations cannot affect the
modulation index.
reception.
a The other advantage of F.M. is this that the
Modulation index
band width of the modulated wave is kept
sufficiently wide so that all the useful
a A frequency deviation of 75 kHz in a F.M.
frequencies of the audio band of 20 Hz to
wave is considered equivalent to 100%
20 kHz remains present in the F.M. signal.
modulation of an A.M. wave. Therefore,
Generally, two A.M. transmitting stations
the value of frequency deviation should not
are spaced by 10 to 15 kHz and all
be higher than 75 kHz. If the frequency
frequencies of the signal wave are not
deviation is 75 kHz and the maximum
accommodated in the A.M. transmission
signal frequency is 15 kHz then the
due
to
short
bandwidth.
In
the
F.M.
system,
deviation ratio will be qual to 75 kHz/15
Constable
TYPES OF MODULATION
all teh frequencies between 20 Hz to 15
kHz=5.
(communication)
kHz (practical A.F. range) of a signal wave
a The principal methods of
ExamSpecial remains present in the F.M. wave. The F.M. SIDE BAND
modulation are as follows:
a Though, the aim of A.M. is to produce
is employed in the frequency range of 30 to
1. Amplitude modulation
variations in the carrier amplitude only but
300 MHz and above. The F.M. wave of a
2. Frequency modulation
a few new frequencies are also developed
pure tone audio signal is shown in Fig. (d)
3. Pulse modulation
1. Amplitude modulation. The process of
Comparison between Merits and Demerits of A.M. and F.M
modulation in which the amplitude of
carrier wave is varied in accordance to the
A.M
F.M.
amplitude of signal wave is called
1.
Electrical,
mechanical
and
atmospheric
1.
All
sorts
of
interferences
are minimum.
amplitude modulation. The signal wave
interferences
are
maximum.
may be of audio, video or any other type.
2. The modulated signal does not contain
2. The modulated signal contains all the fundamental
The modulation of a pure tone audio signal
all the fundamental and harmonic
and harmonic frequencies of the signal hence the
is shown in Fig. (c). Amplitude modulation
frequencies
of
the
signal.
modulated wave is of high quality.
is abbreviated as A.M. and it is used in
3. More electrical power is consumed
3. Less electrical power is consumed in the process.
radio telephony and television etc.
in the modulation process.
a The modulation is performed in a transistor
4. A.M. can be employed in all the wave
4. F.M. is practicable only in VHF, and UHF bands.
r.f. amplifier circuit. The circuit is called
bands i.e., medium, short and
modulator circuit or only modulator. A
micro-wave band.
transistor circuit does not obey Ohm’s law,
5. A.M. channel width is small.
5. F.M. channel width is large.
hence the current flowing in it, is not
6. A.M. transmitter and receiver
6. F.M. transmitter and receiver circuits are complex
proportional to the applied voltage.
circuits are simple.
range between 20 kHz to 3 x 106 MHz are
called carrier waves or radio waves or
Hertzian waves or continuous waves.
These waves are generated by a valve or
transistor oscillator circuit. Though, the
waves can neither be heard nor seen, but
they can be used as carrier for audio and
video signals that is why they are named as
carrier waves or continuous waves.
a Continuous waves can be received in the
form of Morse signals with the help of a
special type of reciever. A carrier wave is
shown in Fig. (b).
2. Frequency modulation. The process of
modulation in which the frequency of
carrier waves is varied in accordance to the
amplitude of signal wave is called
frequency modulation. A frequency
modulated wave is shown in Fig. (d). The
frequency modulation is abbreviated as
F.M. and it is used in television and microwave communication.
3. Pulse Modulation. The process of
modulation in which discontinuous waves
(e.g., square waves) are used in place of
continuous waves is called pulse
modulation. The types of this modulation
are as follows:
i) Pulse Amplitude Modulation. In this
method, the amplitude of carrier pulses is
varied in accordance to the signal wave.
ii) Pulse Duration Modulation. In this method,
the time period of carrier pulses represents
the signal wave.
iii) Pulse Code Modulation. In this method the
carrier pulses are radiated in accordance to
binary code.
MODULATION INDEX
a Since, the amplitude of carrier wave is kept
constant in the frequency modulation,
hence the depth of modulation depends on
the frequency deviation. The deviation in
the carrier frequency takes place in
in the process which were absent before the
modulation. These new frequencies are
equal to the sum and the difference of the
carrier and signal frequencies. If the carrier
frequency is 500 kHz and the signal
frequency is 10 kHz then the lower side
frequency will be 500 - 10 = 490 kHz.
Similarly, the upper side frequency will be
500 + 10 = 510 kHz, see Fig. 23.4. In this
way, a modulated wave has two side bands.