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BASIC ELECTRONICS
MODULATION
&
DEMODULATION
Introduction
• In radio transmission, it is necessary to send the
audio signal (20 Hz to 20 KHz) such as music, speech
etc. from a broadcasting station over great distances
to a receiver. This communication of audio signal
does not employ any wire and is sometimes called
wireless. The audio signal cannot be sent directly
over the air to appreciable distance because
radiation of electrical energy is practicable only at
high frequencies e.g. above 30 KHz. This difficulty is
overcome by superimposing the electrical audio
signal on a very high frequency wave called carrier
wave
• . The resultant waves are called modulated
waves or radio waves and the process is called
“modulation”. The modulated or radio waves
are sent out from the broadcasting station and
carry the audio signal to larger distances. At
the radio receiver, the audio signal is extracted
from the modulated wave by the process of
“demodulation”. The audio signal is then
amplified and fed to the speaker for sound
reproduction.
Modulation
The process of changing some characteristics (e.g. Amplitude,
frequency or phase) of a carrier wave in accordance with the
intensity of signal is known as ‘modulation’. Modulation means to
“change”. In modulation, some characteristic of carrier is changed in
accordance with the intensity of the signal. The resultant wave is
called the modulated or radio wave and contains the audio signal.
Therefore, modulation permits the transmission to occur at high
frequency while it simultaneously allows carrying of the audio
signal. Depending upon which characteristic of carrier is changed,
the modulation can be of three types:
(i) Amplitude modulation
(ii) Frequency modulation
(iii) Phase modulation
In amplitude modulation, the amplitude of the carrier is
changed in accordance with the intensity of signal while
the frequency of carrier remains unchanged. The audio
signal is contained in the amplitude variation of the
resultant AM wave. In frequency of the carrier is changed
in accordance with the intensity of the signal. The audio
signal is contained in the frequency variations of the FM
wave. In india, amplitude modulation is used in radio
broadcasting. However, in TV transmission, frequency
modulation is used for sound and amplitude modulation
for picture signal.
Modulation Factor
•
An important consideration in amplitude modulation is to describe the extent
to which the amplitude of the carrier wave is changed by the signal. This is
described by a factor called modulation factor m i.e.
Amplitude change of carrier
Modulation factor, m = ––––––––––––––––––––––
Normal carrier amplitude
•
The value of modulation factor depends upon the amplitude of the carrier and
the signal. For example, if both carrier and signal have the same amplitude,
say A, then modulation factor is A/A = 1 or 100%. The modulation factor
indicates the strength and quality of the transmitted signal. The greater the
modulation factor, the stronger and clearer will be the signal. However,
modulation should not be more than 100% otherwise distortion will occur.
Demodulation
• The process of recovering signal from the modulated wave is
known as “demodulation”. It is essentially the reverse of
modulation and is, therefore, rightly called demodulation or
“detection”. Demodulation is done in the radio receiver by a
circuit called “demodulator”. One of such circuits is a diode
demodulator. Here the modulated wave is first rectified
i.e.negative half of the modulated wave is eliminated. The
rectified modulated wave contains the audio signal and the
carrier. The carrier is then removed by a filter circuit and the
recovered audio signal is amplified and fed to the speaker for
sound reproduction.
Superhetrodyne receiver
• It was first designed by Major Edwin H. Armstrong during the First World
War. At present, all modern receivers utilise the superhetrodyne circuit. In
this type of radio receiver, the selected radio frequency is converted to a
fixed lower frequency, called intermediate frequency (IF). This is achieved
by a special electronic circuit called the mixer circuit.
i.
There is a local oscillator in the radio receiver itself. This oscillator
produces high frequency waves. The selected radio wave is mixed with
the high frequency wave in the mixer circuit. In this process, beats are
produced and the mixer produces a frequency equal to the difference
between local oscillator and radio wave frequency. The circuit is so
designed that oscillator always produces a frequency 455 KHz above the
selected radio frequency. Therefore, the mixer will always produce an
intermediate frequency of 455 KHz regardless of the station to which
the receiver is tuned.
ii.
The obtaining of fixed IF (i.e. 455 KHz) is known as superhetrodyne
principle. As an example, if 600 KHz station is tuned, then local
oscillator will produce a frequency of 1055 KHz. Consequently, the
output from the mixer will have a frequency of 455 KHz.
• It is worthwhile to give a passing reference to the utility of getting fixed
intermediate frequency in a superhetrodyne circuit. At this fixed
intermediate frequency, the amplifier circuits operate with maximum
stability, selectivity and sensitivity. As the conversion of incoming radio
frequency to the intermediate frequency is achieved by hetrodyning or
beating the local oscillator against radio frequency, therefore, this circuit is
called superhetrodyne circuit.
Characteristics of a radio receiver
• The ultimate goal of a radio receiver is to extract the signal from the
desired radio wave and build up this signal to such a level so that it can
operate the load (e.g. Speaker, earphones). In checking the specifications
of a radio receiver, four characteristics are of general importance viz
sensitivity, selectivity, fidelity and noise figure.
(i) Sensitivity. It is a measure of the level of the input signal to produce “the
standard output”. Clearly, sensitivity is a function of the amount of
amplification or number of stages used in the receiver. At first thought it
might seem that a receiver could be designed for any degree of sensitivity
merely by increasing the number of stages. Although the gain would be
increased, it may not be usable. When the noise level generated within the
receiver is stronger than the signal to be received, increased sensitivity is
wasted. The receiver output will contain more noise than signal.
(ii) Selectivity. It is measure of the ability of a radio receiver to select the
desired radio wave while rejecting all others. This function is performed by
the tuned circuits ahead of the detector stage. The selectivity of a radio
receiver depends upon the number of tuned circuits and the Q of these
circuits.
(iii) Fidelity. It is a measure of how faithfully the receiver reproduces the
original signal. A receiver is said have high fidelity if it amplifies all the
frequencies equally well.
(iv) Noise figure. Although a receiver can be designed for any gain, the usable
gain is limited by the noise output from the receiver. In a radio receiver is
tuned between stations. An ideal receiver-one that generates no noise—
would have a noise figure of unity or 0 db.
BASIC ELECTRONICS
REGULATED D.C POWER SUPPLY
INTRODUCTION
In general, electronic circuits using tubes or
transistor require a source of d.c power.
Batteries are rarely used for the purpose as
they are costly and require frequent
replacement. In practice, d.c power for
electronic circuits is conveniently obtained from
commercial a.c lines by using rectifier-filter
system, called d.c power supply. The rectifierfilter combination constitutes an ordinary d.c
power
supply.
An ordinary d.c power supply has two
drawbacks. First, the d.c output voltage
changes directly with input a.c voltage.
Second, d.c output voltage decreases tsa
the load current increases because of
increases voltage drop in transformer
windings, rectifier and filter circuit. These
variations in d.c output voltage may cause
inaccurate or erratic operation or even
malfunctioning
of many electronic
circuits.
A regulated d.c power is that in which the d.c
output voltage remains constant irrespective of the
a.c mains fluctuations or changes in load. A
regulated power supply consist of an ordinary power
supply and voltage regulating devices(e.g zener
diode, zener in conjunction with transistor ). The
output of an ordinary power supply is fed to the
voltage regulator which produces the final output.
The output voltage remains constant whether the
load current changes or there are fluctuations in the
input a.c voltage.
Voltage
Regulations.
Voltage regulation is an important consideration
in the selection of a power supply. The
variations of d.c output voltage w.r.i the
amount of load current drawn from the supply is
known
as
“voltage
regulation”.
%
Voltage
regulation
=Vnl-Vfl
VFL
where V NL =d.c output voltage at no load
V FL= d.c output voltage at full load
ZENER
VOLTAGE
REGULATOR.
When a zener diode is operated in the breakdown or zener
region, the voltage acrossit remains substantially constant for
a large change through it. This characteristic permits it to be
used as a voltage regulator. The zener diode of zener voltage
Vz is reverse connected across the load across which constant
output desired. A series resistance is connected in the circuit
which absorb the output voltage fluctuation so as to maintain
constant voltage across the load. The following points may be
noted regarding the zener diode as a voltage regulator:
(i) The zener will maintain a constant voltage (=Vz) across
the load so long as the input voltage does not fall below Vz.
(ii) The current through the series resistance is always
constant. If the load current increases, the current
through the zener must decrease to such a value so as
to keep the same current through the series resistance.
(iii) The zener will cease to operate sa a regulator if
current
through
it
is
zero.
(iv) There is a safe value of current which a zener can
carry. If this value is exceeded, the zener may be
destroyed
due
to
overheating.
(v) A zener should always be reverse connected
because it utilises the reverse characteristic for
voltage regulation.
(vi) When the desired regulated output voltage is
higher than the rated voltage of a zener diode. Two or
more zeners are connected in series. However, in such
circuit care must be taken to select the zener diodes
that have the same current rating .
(vii) For zener diodes with breakdown voltage lower
than 5 V, zener mechanism predominates; between 5V
and 8V both zener and avalanche mechanism are
involved whereas above 8V the avalanche mechanism
alone takes over.
(viii) The zener voltage of a zener diodes
depends upon temperature. In zener
breakdown, the value of breakdown voltage
decreases as the junction temperature
increases. On the other hand, in a avalanche
breakdown, the value of breakdown voltage
increases as the junction temperature increases.
(ix) When zener is in the breakdown region,
current is limited only by the external series
resistance.