CHAPTER ONE
CHAPTER ONE1.10 INTRODUCTION
Communication, in its very rudimentary level, could be defined
as the transfer of information from one point to another. It is the
transfer of information from one encoder, through a medium, a
decoder. It is said to be incomplete until the information from the
seat or encoder gets to the destination or decoder.
In the early days, communication started in the form of the use
of natural voice, talking drums, whistling birds, smoke signals and
other forms of communications. After which, to effect long distance
communication, man resorted to the use of mechanical and optical
means like telegraphy, torch light focusing mirrors etc. All these
were in use in the 1840s. However, this had a lot of drawbacks,
that is, some were very slow, unreliable, and unsafe and later
became primitive to use.Radio is one of the longest established
applications of electronics. In fact, prior to the second world
war, radio could probably be considered to be the application of
electronics. Today, however, it is just one of many fields which
are based on the use of electronics. Radio plays a very significant
part in our daily lives and thus an introduction to the subject
makes a fitting part of this project.
The military application of radio. Transmitters were first
exploited during the first world war (1914 to 1918), and during
that period radio was first used in aircrafts. Broadcasting
followed later in the 1920s and 1930s and most homes in the United
Kingdom boasted of a wireless set or wireless radio. Radio is thus
one of the longest established applications of electronics. Radio
communication was made possible through the invention of triode
valve and was greatly simulated by the work done during the World
War II. With the discovery of radio communication, information
could now be propagated through space as an electronic wave. It has
subsequently become even more widespread and refined through the
invention and use of transistors, integrated circuits and other
semiconductor devices. The propagation of wave through space was
then known to depend on the frequency of transmission, such as
audio wave frequency (Af), radio frequency (Rf) and microwave
frequency (Mf) and other characteristics. After this, electrical
communication began with its origin from elementary wire telegraphy
which has finally developed into the invention of telephones,
radio, television, radar, laser, to the modern means of
communication using satellite communication systems.
In these modern days, communication or telecommunications could
be defined as the sending, reception or processing of information
by electrical means. The purpose of communication therefore is to
transmit information bearing signals from a source located at one
point in space, to a user destination, located at another point in
space. As a rule, the message produced by the source is not
electrical in nature. Accordingly, an input transducer is used to
convert the message generated by the source into a time-varying
electrical signal called the message signal. By using another
transducer at the receiver, the original message is recreated at
the user destination.
Fig 1.0 Block Diagram of a Communication System1.11 AIM AND
OBJECTIVE: The aim of this project is to design and construct a
frequency modulated(FM) transmitter with a range of 100metres and
generated signal frequency of about 105MHz.1.12 SCOPE:
The project is limited to the construction of an FM transmitter
from an educational design perspective utilizing discrete
components. CHAPTER TWO2.10 LITERATURE REVIEW
2.11 INTRODUCTION
In radio transmission, it is necessary to send audio signal
(e.g. music, speech etc) from a broadcasting station over a great
distance to a receiver. This communication of audio signal does not
employ any wires(wireless). The audio signal cannot be sent
directly over the air for appreciable distance. Even if the audio
signal is converted into electrical energy, the latter cannot be
sent very far without employing large amount of power. The energy
of a wave is directly proportional to its frequency. At audio
frequencies (20Hz to 20KHz) the signal power is quite small and
radiation is not practicable. The construction of this project was
carried out using discrete electronic components as will be
discussed below.
2.11.1 RADIO WAVESAs with light, radio waves propagate out wards
from a source of energy (the transmitter and transmitting aerial)
and comprises electric (E) and magnetic (H) field at right angle to
each other. These two components, the E field and the H field are
inseparable and the resulting wave travels always from the source
with the E and H lines mutually perpendicular to the direction of
propagation,
Radio waves are said to be polarized in the plane of the
electric (E) field. Thus, if the E- field is vertical, the signal
is said to be vertically polarized. Whereas, if the E-field is
horizontal, the signal is said to be horizontally polarized.
The transmitting aerial is supplied with a high frequency
alternating current. This gives rise to an alternating electric
field between the ends of the aerial and an alternating magnetic
field around and at right angles to it. The direction of the E-
field lines is reversed on each cycle of the signal as the waveform
moves outwards form the source. The receiving aerial intercept the
moving field and voltage alongside current is induced in it. As a
consequence this voltage and current are similar but of smaller
amplitude to that produced by the transmitter. The transmitting
antenna radiates the radio waves in space in all directions, these
radio waves travels with the velocity of light 1.e. 3x 108 m/sec.
The radio waves are electromagnetic waves and posses the same
general properties. These are similar to light and heat waves
except that they have longer wave lengths. It may be emphasized
here that radio waves are sent without employing any wire. It can
be easily shown that at high frequency, electrical energy can be
radiated into space.
2.11.2 RADIO FREQUENCY
Radio frequency signals are generally understood to occupy
frequency range that extends from a few tens of kilohertz (KHZ) to
several hundred Gigahertz (GHz). The lower part of the radio
frequency range that is of practical use below 30KHz is only
suitable for narrow- band communication at the frequency, signals
propagate as ground waves (LF) following the curvature of the
earth, over very long distance.
At the other extreme, the highest frequency range that is of
practical importance extends above 30.GHZ. at these microwave
frequencies considerable bandwidth are available (sufficient to
transmit many television channels using point to point links or
permit very high definition radar system) and signals tend to
propagate straightly along line of sight paths space wave
At other frequencies signals may propagate by various means,
including reflection or more correctly refraction from ionized
layers in the ionosphere. Sky waves, at frequencies between 3MHZ
and 30MHZ monophonic propagation, regularly permit intercontinental
broadcasting and communications.
For convenience, the radio frequency spectrum is divided into a
number of bands each spanning a decade of frequency. The use to
which each frequency range is put depends upon a number factors
paramount amongst which is the propagation characteristics within
the band concerned. Other factors that need to be taken into
consideration include efficiency of practical aerial systems in the
range concerned and the band width.The radiation of electrical
energy is possible only at high frequencies e.g. above 20KHz. The
high frequency signals can be sent thousands of miles even with
comparatively small power. Therefore, if the audio signal is to be
transmitted properly, some means must be devised which permit
transmission to occur at high frequencies while it simultaneously
allows the carrying or transportation of the audio signal. This is
achieved by superimposing the electrical audio signal on a high
frequency carrier. The resultant wave is known as a modulated wave
or radio wave, while the process is called Modulation.
At the radio receiver, the audio signal is extracted by a
process known as Demodulation(the reverse of modulation). The
extracted intelligence signal is then amplified and reproduced into
sound by the loudspeaker.2.12 TRANSMITTERThe transmitter is a
device capable of capturing signals representing sound and light
and converting them by the process of modulation into a form
suitable for transmission as electromagnetic waves in the radio
spectrum. Any radio communication system that transmits
intelligence from one point to another requires a high power
transmitting module that prepares signal at the radio frequency and
drives power to the transmission medium through the antenna. The
transmitter consists of a transducer, an audio amplifier, an
oscillator to generate the signal, sometimes frequency multiplier,
radio frequency amplifier, power amplifier and antenna.Transmitters
are used to propagate intelligence signals between distant
locations. Propagation (transmission) could be over a variety of
media, including transmission lines, optical fibers, waveguide and
free space. The signals to be broadcast may be in the form of voice
i.e. speech or digitally coded data. The output of which is of very
small signal level, usually of the order of milli-volts, hence the
need for amplification before further processing could be
effected.
Every transmitter has three basic fundamental functions;
Transmitter must generate a signal of the correct frequency at
the desired point in the spectrum.
It must provide sufficient power amplification to ensure that
the signal level is high enough in order to cover the desired
distance.
It must provide some form of modulation that causes information
signal to modify the carrier signal.
With various types of transmitters available, this project
concentrates on FM type of a transmitter. The output power ranges
from the milli watt level up to the 100KW for broadcast FM. We will
note that FM is not used at frequency below about 33MHz. This is
due to the phase distortion introduced to the FM signals by the
earths ionosphere to these frequencies.
Transmitting aerial
microphone
Fig 2.0 Generalized block diagram of an FM transmitter2.12.1
BASIC BUILDING BLOCKS OF AN FM TRANSMITTERListed below are the
basic building blocks of an FM transmitter.2.12.1.1 TransducerThe
function of any transducer is to convert energy from one form to
another For this project, an electret microphone is being used.
2.12.1.2 The Audio Amplifier
This is sometimes called a low frequency amplifier. It is
basically designed to amplify electrical signal of about 20Hz
-20KHz. The two principal types of audio amplifiers are the voltage
and power amplifiers. Primarily, a voltage amplifier is designed to
produce large output voltage with respect to the input voltage. A
power amplifier develops, primarily, a large signal current in the
output circuit. Schematically, there is no way to distinguish
between the two types of the audio amplifier except their types of
load.
In this project design, the audio amplifier circuit was employed
since the audio signal from the microphone is quite weak and
requires amplification. The amplified output from the last audio
amplifier is fed to the modulator for rendering the process of
modulation.
2.12.1.3 The OscillatorsAn electric oscillator may be defined as
one of the following;
- A circuit which converts DC energy to AC energy at a very high
frequency.
-An electronic source of alternating current of high voltage
having sine, square or saw tooth or pulse shapes.
-A circuit which generates an output signal without requiring
any externally applied input signal.
-An unstable amplifier.
-A circuit that produces an output which varies its output with
time.
These definitions exclude electromagnetic alternators which
convert mechanical or heat energy into electrical energy. An
oscillator differs from an amplifier in one basic respect, in that
the oscillators do not require an external signal either to start
or maintain energy conversion process as shown in the figure below.
It keeps producing an output so long as the DC power source is
connected.This stage generates the carrier signal on which the
audio signal from the AF amplifier is super imposed for effective
transmission. Radio frequency parallel L-C oscillator was used in
this project to generate about 100MHz oscillator frequency.
Signal output
DC power inputFig 2.2 Block Diagram of an oscillatorMoreover,
the frequency of the output is determined by the passive component
used in the oscillator and can be varied at will. Electronic
oscillators may be broadly divided into two groups namely;
sinusoidal and non- sinusoidal oscillators. Sinusoidal (or
harmonic) oscillators: These are oscillators which can produce an
output having sine waveforms and produce any of the following
oscillations; damped or undamped oscillations.
Non-sinusoidal (relaxation) oscillator These are oscillators
which produce an output which ahs square, rectangular or saw tooth
wave-form.
-Damped oscillations:
Oscillations whose amplitudes keeps decreasing (or decaying)
with time are called damped oscillations. The waveform of such
oscillations is shown in the figure below. These are produced by
those oscillator circuits in which IR losses takes place
continuously during each oscillation without any arrangement for
compensating the same.
Fig 2.3 Damped Oscillation waveformUltimately, the amplitude of
the oscillations decays to zero when there is not enough to supply
circuit losses. However, the frequency or time-period remains
constant because it is determined by the circuit parameters.
Sinusoidal oscillators serve a variety connection in
telecommunications and in electronics. Its most important
application in telecommunication is the use of sine waves as
carrier in both radio and cable transmission. Since wave are also
used in frequency response testing of various types of systems and
equipment including analogue communications channels, amplifier and
filters and closed-loop control system. Undamped
OscillationsOscillations whose amplitude remains constant, that is,
does not change with time are called undamped oscillations. These
are produced by those oscillators circuit which have no losses or
if they have, there is provision for compensating them; the
constant-amplitude and constant frequency sinusoidal waves. Shown
below is an example of a carrier wave used in communication
transmitter for transmitting low-frequency audio information to far
distant places.
Fig: 2.4 Undamped Oscillation waveform
In addition, oscillators can also be described as an electronic
circuit whose function is to produce an alternating electromotive
force (emf) of a particular frequency and wave. Its purpose in the
design is the generation of sinusoidal carrier signal.
The basic types of oscillators are Phase oscillator, Hartley
Oscillator, Colpitts Oscillator etc.
The oscillator in this project is the Colpitts Oscillator.
Capacitor and inductors are the two component found in an RF
oscillator or tank circuit. These two components are used together
to form an L-C circuit which provide selectivity that we need in a
radio receiver. when used together we refer to them as tuned
circuits or resonant circuit.
In practice we have both series and parallel tuned circuits.
this two behave quite differently. In the case of series tuned
circuit and assuming that both of the components are perfect, the
impedance of the circuit will be zero at the resonant frequency
this circuit is thus sometimes referred to as an acceptor circuit,
in other words, it will accept signal at the resonant frequency and
reject signal at other frequencies.
In the case of the parallel L-C circuit and assuming that both
of the components are perfect, the impedance of the circuit will be
infinite at resonant frequency this circuits is thus sometimes
referred to as a rejecter circuit. in other words it will reject
signals at resonant frequency.
In the case of both the series and parallel circuit the
frequency of resonance can be calculated, Determination of Resonant
Frequency:
Where
L = Inductor
C = Capacitor
XL = Inductive reactance
Xc = Capacitive reactance
XL = 2fL 2.1
Xc = 1 .2.2 2fCResonance occurs at XL=Xc 2.3At resonance,
2fL = 1 2.4 2fC
Making the subject of the formula, we obtain
f = 1
....................................................................................2.5
2LC
This is the resonant carrier frequency of a Colpitt
Oscillator
The tank or resonant circuit has three main specifications,
namely
Bandwidth
Quality Factor or Q factor
Insertion LossThese parameters define the pass band, shape and
loss of the tank circuit response.2.12.1.4 Radio Frequency (RF)
Amplifier: RF amplifier is better described as power amplifier. It
is used in radio transmitters to amplify the carrier frequency to
the desired power output level. RF power amplifier is operated
under either class B or class C condition.2.12.1.5 The Modulator:
This is another component of a transmitter whose operation in
transmission is highly expedient. It, as the name implies,
modulates by combining an audio frequency (AF) signal with a radio
frequency (RF) carrier wave. During modulation, some
characteristics of the carrier wave are varied in time with the
modulating signal are accomplished by combining the two. The
resultant wave produced is called the modulated wave.
2.13 Modulation
Modulation is the process of superimposing information contained
in a lower frequency electronic signal into higher frequency
signal. The higher frequency is called the carrier signal while the
lower frequency, the modulation signal. In the process of
modulation, some characteristics are varied in accordance with the
instantaneous value of modulating signal such as sine wave which
may be represented by the following equation.
e = E sin (wt+ ).2.6Where:
e is the instantaneous value of the sine wave, called the
carrier;
E is its maximum amplitude,
w is the angular velocity
is its phase relation with respect to some reference value.
Any of these last three characteristics or parameters (, w, and
) of the carrier may be varied by the modulating signal, giving
rise to amplitude, frequency or phase modulation respectively in
this project, frequency modulation is considered.
`2.13.1 Need for Modulation
Modulation is needed due to the following reason.
For efficient radiation and reception of radio waves, the
transmitting and receiving antennal must have heights in the
multiple of y/4 (y = c/f). where y is the signal wavelength. At low
frequencies, the antenna height will be too long to be
realized.
Signals of low frequencies cannot travel far, hence, it is of
importance to superimpose it on a signal of higher frequencies for
a wider coverage on the other hand, and unmodulated carrier cannot
be use to convey information. By standard, the bandwidth for
commercial quality speech is 30Hz 3400Hz. To allow for
discrimination, each individual signal is modulated by different
carriers through the process called frequency Division Multiplexing
(FDM). By this method, a telephone cable is capable of carrying up
hundreds of channels. 2.14 Types of Modulation TechniquesBasically,
these are two types of modulation namely Amplitude modulation and
Angle modulation. Angle modulation is further divided into
frequency and phase modulation. They are each discussed
below.2.14.1 Amplitude Modulation
A signal is said to be amplitude modulated when the amplitude of
the carrier wave is varied in proportion to the instantaneous
amplitude of the information signal or RF signal.
Obviously, the amplitude (and hence the intensity) of the
carrier waves is changed while the frequency remains constant.
2.14.1.1 LIMITATION OF AMPLITUDE MODULATION
Although theoretically highly effective, amplitude modulation
suffers from the following draw backs;
Noisy Reception: In an FM wave, the signal is in the amplitude
variations of the carrier. Practically all the natural and man made
noises consist of electrical amplitude disturbances. As a radio
receiver cannot distinguish between amplitude variations that
represent noise and those that contain the desire signal, therefore
reception is generally noisy Low efficiency: In amplitude
modulation, useful power is in the side bands as they contain the
signal. Small operating range: Due to low efficiency of the
amplitude modulation, transmitters employing this method have a
small operating range i.e. message cannot be transmitted over large
distances. Lack of audio quality: This is a distinct disadvantage
of amplitude modulation. In order to attain high fidelity reception
all audio frequencies up to 15 KHz must be reproduced. This
necessitates bandwidth of 30 KHz since both sidebands must be
reproduced. But FM broadcasting stations are assigned bandwidth of
only 10 KHz to minimize the interference from adjacent broadcasting
station. This means that the highest modulation frequency can be
5KHz which is hardly sufficient to reproduce the music
properly.2.14.2 Phase Modulation.
Here, the information signal changes the phase of the waves with
the frequency and the amplitude kept constant.
2.14.5 Frequency Modulation
In this case the frequency of the carrier wave is varied in
sympathy with some property of the modulating signal. A better and
more vivid explanation of frequency modulation is given along the
write up.
2.15 DEMODULATION
The process of recovering the audio signal from the modulated
wave is known as demodulation or detection.
At the broadcasting station, modulation is done to transmit the
signal over large distances to receiver when the modulated. Wave is
picked up by the radio receiver. It is necessary to recover the
audio signal from it. This process is accomplished in the radio
receiver and is called demodulation.
2.15.1 NECESSITY OF DEMODULATION
It was noted previously that amplitude modulated wave consists
of carrier and sideband frequency. The audio signal is contained in
the sideband frequencies which are radio frequencies. If the
modulated wave after amplification is directly fed to the speaker
as shown in fig 2.3 a, no sound will be heard. It is because
diaphragm of the speaker is not all able to respond to such high
frequencies. Before the diaphragm is able to more in one direction,
the rapid reversal of current tends to move it in the opposite
direction i.e. diaphragm will not move at all. Consequently, no
sound will be heard. Loud speaker
receiver antennae (No sound)
Fig 2.8 Block Diagram of a Demodulation Process
From the above discussion, it follows that the audio signal must
be separated from the carrier at a suitable stage in receiver. The
recovered audio signal is than amplified and fed to the speaker for
conversion into sound. 2,16 THEORY OF FREQUENCY AND PHASE
MODULATIONFrequency modulation is a system of modulation in which
the amplitude of the modulated carrier is kept constant, while its
frequency and rate of change are varied by the modulating signal.
The first practical system was put forward in 1936 as an
alternative to A.M in an effort to make radio transmissions me
resistant to noise. Phase modulation is a similar system in which
the phase of the carrier is varied instead of the frequency: as in
FM, the amplitude of the carrier remains constant.
Let us assume for the moment that the carrier of the transmitter
is at its resting frequency (no modulation) of 100MHz and we apply
a modulating signal. The amplitude of the modulating signal will
cause the carrier to deviate from this resting frequency by a
certain amount. If we increase the amplitude (loudness) of the
modulating signal we will increase the deviation to a maximum of
75khz as specified by the Federal Communications Council. If we
remove the modulation, the carrier frequency shifts back to its
resting frequency(100MHz).
It can be shown that the deviation of the carrier is
proportional to the amplitude of the modulating voltage. The shift
in the carrier frequency in comparison to the amplitude of the
modulating voltage is called the Deviation Ratio. A deviation ratio
of 5 is the maximum allowed in commercially broadcast FM.
The rate at which the carrier shifts from its resting point to a
non resting point is determined by the frequency of the modulating
signal. Frequency modulation can also be described as the process
of changing a particular property of the carrier wave in sympathy
with the instantaneous voltage or current which is the signal. The
most commonly used method of modulation are amplitude modulation
(AM) and frequency modulation (FM) in the former case, the carrier
amplitude (its peak voltage varies according to the voltage at any
instant of the modulation signal in the latter case, the carrier
frequency is varied in accordance with voltage, at any instant of
the modulating signal.
2.16.1 DESCRIPTION OF SYSTEMS
The general equation of an unmodulated wave, or carrier, may be
written as
X= A sin (wt + ) from eqxn 2.6Where X= instantaneous value (of
voltage or current)
A= (maximum) amplitude
w= angular velocity, radians per second (rads/sec)
= phase angle, rad
Note that wt represents the angle in radians
If any one of these parameters is varied in accordance with
another signal, normally of a lower frequency, then the second
signal is called the modulation, and the first is said to be
modulated by the second.
Amplitude modulation is achieved when the amplitude is varied.
Alteration of the phase angle will yield phase modulation. If the
frequency of the carrier is made to vary, frequency modulation is
achieved.
It is assumed that the modulating signal is sinusoidal. This
signal has two important parameters which must be represented by
the modulation process without distortion, specifically, its
amplitude and frequency. By the definition of frequency modulation,
the amount by which the carrier frequency is varied from its
unmodulated value, called the deviation, is made proportional to
the instantaneous amplitude of the modulating voltage. The rate at
which this frequency variation changes or takes place is equal to
the modulating frequency. All signals having the same amplitude
will deviate the carrier frequency by the same amount.
Consequently, all signals of the same frequency will deviate the
carrier at the same rate no matter what their individual
amplitudes. The amplitude of the frequency modulated wave remains
constant at all times. This is the greatest single advantage of
FM.
The effect of frequency modulation on a sinusoidal carrier is
shown in the figure below(note that the modulating signal is in
this case, also sinusoidal in practice many more cycles of RF
carrier would occur in the time span of one cycle of the modulating
signal.
+V
t
-V
Modulating signal
+V
t
-V
Frequency modulation
The modulating or audio signal is described as:
Va = A sin 2at. 2.8Where A represents the maximum amplitude, a
represents the frequency of the audio signal, t represents time and
Va ,the instantaneous value of the modulating signal voltage.The
carrier frequency. F, will vary around a resting Fc thus:
F = Fc + F2fat.. 2.9The frequency modulated wave will have the
following description:
V = A sin (2Fc + F2fat) }.. 2.10
In this frequency modulated situation, is the maximum change in
frequency the modulated wave undergoes. It is called the frequency
deviation, and the total variation in frequency from the lowest to
the highest is referred to as a carrier swing. Therefore for a
modulated signal which has equal positive and negative peaks, such
as pure sign wave, the carrier swing is equal to two times the
frequency deviation.
F = frequency deviation
Carrier swing = 2 x frequency deviation =2F..2.11It can be shown
that the equation for the frequency modulated wave can be
manipulated into:
V = A sin {2fct + (F/a) cos 2at} .2.12It must be noted that in
this equation, the cosine term is preceded by the F/a. This
quantity is called the modulation index and is indicated as M
Modulation index = M = F/a 2.132.16.2 ADVANTAGES OF FREQUENCY
MODULATION
It gives noiseless reception as discussed before, noise is a
form of amplitude variations and a FM receiver will reject such
signals.
The operating range is quite large. It gives high fidelity
reception. The efficiency of transmission is very high. 2.16.3
APPLICATIONS OF FREQUENCY MODULATION
The five major categories in which FM is used are as
follows;
Non commercial broadcast at 88MHz to 90MHz.
Commercial broadcast with 200 KHz channel bandwidth at 90 to
108MHz.
Television audio signals with 50 KHz channel bandwidth at 54 to
88MHz, 174 to 216MHz and 470 to 806MHz.
Narrow band public service channels from 108 to 174MHz and in
excess of 806MHz. Narrow band amateur radio channels at 29.6MHz, 52
to 53MHz, 144 to 147.99MHz, 440 to 450MHz and in excess of 902MHz.
Digital FSK: Frequency Shift Keying (FSK) is used on HF for low
speed telegraphy or data transmission, eg RTTY at speeds of 45.45
or 50 baud. FSK is also used on VHF for data transmission at 4800
bps using the HAPN Direct Frequency Modulation (DFM) technique, or
G3RUH modulation at 9600bps. Digital AFSK: Audio Frequency Shift
Keying is the use of a frequency shift keyed audio tone to modulate
a FM or SSB transmitter. This is commonly used for speeds of 300bps
on HF and 1200bps on VHF/UHF. On VHF/UHF, the AFSK signal is fed
into the microphone input of the transmitter to pick up
pre-emphasis, and de-emphasized audio is used for the
demodulator.2.17 NOISE AND FREQUENCY MODULATIONNote that there are
several other forms of modulation particularly associated with
digital communication like pulse code modulation, pulse width
modulation etc.
Frequency modulation is much more immune to noise than amplitude
modulation and is significantly more immune than phase modulation.
A signal-noise frequency will affect the output of a receiver only
if it falls within its band pass. The carrier and the noise
voltages will mix, and if the difference is audible, it will
naturally interfere with the reception of wanted signals. Noise
rejection is obtained only when the signal is at least twice the
noise peak amplitude. Other forms of interference found in
receivers include:
Adjacent channel interference
Frequency modulation offers not only an improvement in the S/N
ratio but also better discrimination against other interfering
signals, no matter what their source. Also wideband FM broadcasting
channel occupies 200KHz( of which only 180KHz is used), and the
remaining 20KHz guard band goes a long way toward reducing adjacent
channel interference even further.
Co-channel interferencecapture effect
Fm receivers incorporate the use of amplitude limiters, which
work on the principle of passing the signal and eliminating the
weaker. This was the reason for mentioning earlier that noise
rejection is obtained only when the signal is at least twice the
noise peak amplitude. A relatively weak interfering signal from
another transmitter will also be attenuated in this manner, as much
as any other form of interference. This applies even if the other
transmitter operates at the same frequency as the desired
transmitter2.18 PRE-EMPHASIS AND DE-EMPHASISNoise has a greater
effect on higher modulating frequencies than on the lower ones.
Thus, if the higher frequencies were artificially boosted at the
transmitter and correspondingly cut at the receiver, an improvement
in noise immunity could be expected, thereby increasing the
signal-to-noise ratio. This boosting of the higher modulating
frequencies, in accordance with a pre-arranged curve, is termed
pre-emphasis, and the compensation at the receiver is called
de-emphasis. The standard unit for defining emphasis is
microseconds. A 75-s pre-emphasis in FM gives a noise rejection at
least 24dB better than AM.
CHAPTER THREE3.10 METHODOLOGYThe overall method and steps
involved during the design of this project are briefly explained
here. These can best be explained using the block diagram below,
Input audio to antenna
Fig 3.1 A block diagram of an FM transmitterThe various
components used in the construction of this project include:
resistors, transistors, capacitors and a length of flexible
cord.
The major sections that constitute this design are
The power supply unit
The audio pre amplification unit
RF oscillator unit
Antennae The indicator
3.11 THE POWER SUPPLY UNIT
This unit consists of a 3 volts DC battery made up by a pair of
1.5 volts DC battery. The power supply ensures the circuit
functions effectively. To an extent, it determines the carrier
frequency of the oscillator circuit.
3.12 THE AUDIO PRE-AMPLIFIER UNIT
The function of this stage is to pre-amplify the audio signal
from the microphone which is very weak so that it ca be set for
modulation. This stage consists of NPN transistor, common emitter
configuration, with collector feedback biasing, biasing resistors
and capacitors. The input to this stage is from the base of the
transistor while the output is from the collector. The capacitors
this unit serve as a coupling unit, filter networks and frequency
determination of input signal.
3.13 RF OSCILLATOR UNIT
This unit consists of a parallel resonant circuit which is
responsible for producing the carrier wave upon which the
intelligence signal is to be superimposed for modulation.
3.14 THE ANTENNAE UNITThe antennae is responsible for the
transmission of the modulated signal through space. For this
project, the antennae is a piece of flexible cord. It should be
noted that extending the length of the wire antennae consequently
extends the range of signal transmission as observed during
testing.3.15 THE INDICATOR
This section consists only of a Light Emitting Diode whose
function is to indicate power supply to the rest of the
components.For the design and construction of this project, some
fundamental components were used. An insight into their properties
and their characteristic behavior relevant to the design under
consideration are discussed below.3.16 RESISTORFor a resistor,
according to Ohm`s law, the voltage dropped across it is
proportional to the amount of current flowing through it. i.e. V=
IR ..3.1Where V is the voltage across the resistor,
I = the current flowing through the resistor and R, the
resistance of the resistor.Any current waveform across a resistor
will produce the same voltage waveform across the
resistor.Resistors are essential to the functions of almost every
electronic circuit and provide means of controlling the circuit
and/or voltage present. There are almost as many types as their
application. Resistors are used in amplifiers as loads for active
devices in bias networks and as feedback element. In combination
with capacitors they establish time constant and act as filters,
they are used to set operating currents and signals levels.
Resistors are used in power to measure currents and to discharge
capacitor after moving power source. They are used in precision
circuit to establish currents to provide accurate voltage ratio and
to set precise gain values.
3.17 INDUCTORThe voltage across an inductor leads the current
through it by 90 degrees. This is due to the fact that the voltage
across an inductor depends on the rate of change of current
entering the inductor. The impedance of an inductor is +jwL (w=2f)
which reflects the fact that the voltage leads the current.Given
the dimensions of an inductor coil such as average radius of the
coil(r), number of turns of the coil(N), length of the coil(L), the
inductance in micro Henrys(H) can be computed using this
relationship.
L = N^2 r^2 3.2 228r + 254L3.18 CAPACITOR
A capacitor temporarily stores charge or electricity in the form
of electrostatics .this should not be confused with the function of
a battery ,which chemically generate electricity a capacitor is
said to be like a water storage tank while the battery is like the
central heating pump .pumping the water round the radiator
.capacitor ,like resistor ,are so widely used that book are written
about them .so capacitors are used in storing small amour of
electrical energy they are used in smoothing [decoupling ]power
supplies , removing of voltage spikes from supplies etc
CHAPTER FOUR4.10 DESIGN AND CONSTRUCTION OF MODULE4.11 COMPLETE
CIRCUIT DIAGRAM
Fig 4.1 complete circuit diagramDesign Specification
The design specification is a detailed description of the
expected characteristics of the designed FM transmitter.
Modulation Type :
FM
Frequency of Operation: about 104MHz
Antenna Type:
wire cord of a few centimeters longRange Obtained in Free space:
Up to 100metersWorking Voltage : 3 volts(DC)
4.11.1 STAGE ANALYSIS OF EACH SECTIONThis section examines the
stage by stage analysis of the module with their respective circuit
diagrams. 4.11.1.1 THE TRANSDUCER SECTION
The current ,I, flowing into the microphone is given by ohms
law
V = I R from eqxn 3.1R=33k
V=3 volts
I=V/R
I=3/33k
I= 90Amps4.11.1.2 THE AUDIO PRE-AMPLIFIER SECTION
From the circuit diagram above,Rb=1M
Rc= 10k
Vcc= 3volts
hE= 60from the equation for the collector feedback biased
transistor,
Vcc=Ic Rc + (Ic/hE)Rb..4.1Making Ic the subject of the formula,
we obtain
Ic = Vcc/( Rc + Rb/hfE)..4.2Ic= 3/10k + 1M/60Ic= 0.1mAmps
Now from this relationship,
hE = Ic/Ib.4.3substituting the values of Ic and hfE
Ib=Ic/ hE
Ib= 0.1mA/60
Ib= 1.7Amps4.11.1.3 THE RF OSCILLATOR SECTION
using the above transistor characteristics and component
values,the resistors 47k and 100k both constitute a voltage divider
network
therefore the voltage across the 100k resistor, Vb, is given
as
Vb = [100k/100k+ 47k] 3volts
Vb= 2.0volts
From fundamental transistor equation, we know that
Vb = Ve + Vbe..4.4Where Vbe=0.6volts for a silicon transistor by
standardVe=20.6=1.4volts
Where Ve is the voltage across the emitter. The current, Ie,
across the emitter is given by
Ie=Ve/Re.4.5Ie= 1.4/39kIe=36Amps
Now Ie is approximately = Ic=36 Amps
Therefore, using equation 4.3 .and noting hE=60
Ib= Ic/ hE
Ib= 36 A/60
Ib= 60 Amps Determination of the tank circuit parameters L =
Inductance
C = Capacitance=47pF
XL = Inductive reactance
Xc = Capacitive reactance
XL = 2fL
Xc = 1
2fCResonance occurs at XL=XcAt resonance,
2fL = 1 from equation 2.4 2fC
Making the subject of the formula, we obtain
f = 1 as in equation 2.5 2LC
This is the resonant carrier frequency of a Colpitt
OscillatorFrom equation 3.2L(H) = N^2 r^2 228r + 254LGiven the
dimensions of the inductor coil to be as follows,N=number of turns
= 5r = average coil radius= 0.16mmL = length of
coil=0.5mmsubstituting these values into the expression, we
obtain
L= 25(2.25)
228(0.16) + 254(0.5)
L= 0.05Hfrom equation 2.5 Above , substituting the values for L
and Cwe obtain, the resonant frequency to be
= 1/2 0.05H(47pF = 100MHzthis is thus the carrier frequency of
the parallel L-C network
4.12 COMPONENT JUSTIFICATIONThis section describes the
importance of using each of the electronic component that
constitute the circuit diagram.
For the transducer section, the electret microphone was used as
the input transducer because of its high sensitivity. The 33k
resistor limits the amount of current entering the electret
microphone. This consequently stabilises the gain of the microphone
and maintains good stability of the sensitivity.
In the pre amplifier circuit and oscillator stages, the BC848
transistor was utilized because of its high frequency response
characteristics.
The capacitors were used as coupling and filter networks to the
various stages of the circuitry. The parallel L-C tank oscillator
was chosen due to its ability to generate a stable sine wave at the
carrier frequency, a better performance at high frequency
generation of signal and its availability in the market. A flexible
cord was used as the antenna due to the miniature nature of the
circuit and under impedance matching considerations was seen to
best suit this project work,
A 3volts DC battery was used as the power supply for this
circuitry because of its ability to produce a steady current and
its ready availability. 4.13 TEST AND ANALYSISIt was observed
during the testing of this project with a radio receiver that the
transmitted signal produced a large squeal. This unwanted
phenomenon, which was due to the value of the limiting current
resistor being too low(22k), was fixed by using a 33k resistor
across the electret microphone. This ensured adequate
stability.
Also observed during testing was the transmitter frequency was
at about 104MHz contrary to the anticipated 89-90MHz. this
situation resulted from the variations made to the inductor wire
during construction which affected the inductance and consequently
the carrier frequency. Also worth mentioning is the observation
that touching of the inductor coil caused the frequency to drift by
a reasonable amount. In addition, The main area of instability is
the oscillator part. Shielding the oscillator helps in part to
counter this and an extension of the antenna length increased the
range of signal propagation.
4.14 BILL OF ENGINEERING MEASUREMENT
S/NITEM SPECIFICATIONQTYUNIT COSTTOTAL COST
1BC848 NPN TRANSISTOR
hE=60 2 N500.00 N1,000.00
2RESISTORS
33K,100K,47K,10K,1M,390
6 N50.00 N300.00
3CAPACITORS 22n,100n,1n,10p,22n,4p76 N150.00 N900.00
4ELECTRET MICROPHONE
1 N500.00 N500.00
5VOLTAGE CONTROLLED
(L-C) OSCILLATOR
1 N1,000.00 N1,000.00
6PLASTIC CASING
1 N2,000.00 N2,000.00
7 SWITCH
1 N50.00 N50.00
8VERO BOARD
1 N100.00 N100.00
9 1.5VOLTS DC BATTERY
2 N50 N100
10 FLEXIBLE CORD YARD N50 N50
TotalN6,000.00
CHAPTER FIVE5.10 CONCLUSION
The FM transmitter is essentially a design and implementation
project. To approach a project like this, a parallel path has to be
taken in regards to the theory and the practical circuitry. For a
successful completion of any project, these paths must meet and
this only happens when they are fully understood. Ipso- facto, a
good grounding in the basics of communication theory and analogue
designs cannot be over emphasized before approaching a project like
this. To start off, looking at block diagrams or basic transmitter
was a necessity even if it seemed abstract and obscure. The
underlying meaning of each block can be found out individually.
Which is what made the overall project challenging and
rewarding.
5.11 REMARKSThe design used for this project is essentially
quite a simple one and it is this simplicity which partly brings it
down when it comes to the overall reliable performance. The main
area of instability is the oscillator part. Shielding the
oscillator helps in part to counter this.
5.20 REFERENCES
5.20.1 Modern Communications, Miller Gary M, Tata McGraw Hill,
1999, New York(2nd Edition)5.20.2 Electronic Communications:
modulation and Transmission, Schoebeck, Robert, Tata McGraw
Hill,2002, New York
5.20.3 Electronic Communication System, Kennedy and Davies,
Fourth Edition, Tata McGraw Hill
5.20.4 Fundamentals of Reliable Circuit Design, Alexander Mel,
Longman Press,2001, Texas5.20.5 A Textbook Of Electrical Technology
By B.L Theraja, A.K Theraja, S. Chand publishers India (24th
Edition)
5.20.6 www. radiocommunications.com5.20.7
www.ask.com/transmitters/frequency modulation
Receiver
Communication
channel
Transmitter
RF
amplifier
Station
selection
RF oscillator
Audio pre-amplifier
Transducer
OSCILLATOR
Oscillator
Modulator
Audio
amplifier
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