1

CHAPTER ONE

1.10 INTRODUCTION

Generally, the signals heard on AM and FM radio, as well as the signals seen on

a television set receiving broadcasts from an antenna are carried by waves.

Radio transmitter is used mostly as a public medium, sending commercial

broadcasts from a transmitter to anyone with a radio receiver within its range, so

it is known as a point-to-multipoint medium. However, radio can also be used

for private point-to-point transmissions. Transmitters are said to be electronic

units that accepts the information signal to be transmitted and converts it into

Radio Frequency signal capable of being transmitted over long distances. Every

transmitter has three basic functions. First, the transmitter must generate a

signal of the correct frequency at a desired point in the spectrum. Second, it

must provide some form of modulation that causes the information signal to

modify the carrier signal. Third, it must provide sufficient power amplification

to ensure that the signal level is high enough to carry over the desired distance.

Personal radio communication is generally limited to short distances usually a

few kilometres, but powerful transmitters can send broadcast radio signals

hundreds of kilometres. The FM system was developed as an alternative to AM

as a system that would have a better quality than the AM transmitter due to its

benefits as; it has noise immunity, it rejects interfering signals because of the

capture effect and provides better transmitter efficiency. Though, it has its

disadvantage as it uses excessive amount of spectrum space. In FM, the carrier

amplitude remains constant, while the carrier frequency is change by

modulating signal.

Most Frequency Modulated transmitters are used in the very high frequency and

ultra high frequency range, and crystals are not available to generate those

frequencies directly. As a result, the carrier is usually generated at a frequency

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considerably lower than the final output frequency. A technique called

modulation was introduced to ensure the transmission of signals over long

distance also; the conversion of the audio signal information to a high frequency

information signal is by modulation.

1.11 AIM AND OBJECTIVES

The aim of this project is to design and construct a handheld 1watt FM

transmitter for short distance communication which is capable of transmitting

any audio signal from any audio source within the range of 100meters to a

location also capable of accepting the audio signals simultaneously of the

frequency ranges of about 102MHz to 105MHz.the intension is to design a

locally made system. This is because such systems sold in the market are

imported so it is nice to be designed in the course of producing this system.

1.12 FACTORS TO BE CONSIDERED

Several FM communications transmitters operate at relatively low power

levels, typically less than 100 watts. All the circuits use transistors even in the

very high and ultra high frequencies range. The final stages of amplification in

FM broadcast transmitters typically use large vacuum tube class C amplifiers.

However, the proposed 1 watt FM transmitter circuit is made up of four radio

frequency stages: a VHF oscillator, a pre-amplifier, a driver and a power

amplifier stage. Signals are finally fed to the class-C RF power amplifier, which

is to deliver RF power of about 9 volts dc to a proposed 1.75ft single pole

antenna. It is required to generate a frequency within a range of 102MHz-

105MHz to cover within a distance of about 100meters; a condenser

microphone is to be connected at the input of the oscillator.

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1.13 METHODS COMMONLY USED TO ACHIEVE THIS PROJECT

The basic requirement of a frequency modulating system is an output frequency

which varies with instantaneous amplitude of the modulating voltage. The other

requirement is that the un-modulated frequency should be of a constant value

and the frequency deviation should be independent of the modulating

frequency.

There are methods of frequency modulation as follows,

First, the Indirect-method of FM generation involves the use of a stable crystal

oscillator to generate the carrier signal and the use of a buffer amplifier to

isolate it from the remainder of the circuitry. This carrier signal is then applied

to a phase modulator. It is obvious that FM here is generated indirectly through

phase modulation PM. While it is not possible to vary the frequency of a crystal

oscillator directly, it is possible to vary its phase. The resulting PM signal as the

output of the modulator is the desired FM signal. Here the voice input is

amplified and processed to limit the frequency range and prevent over

deviation. The Armstrong modulator is used in this method such that an audio

signal is passed through pre-emphasis network and then an integrator of the

input signal.

Second, the Direct-method; in this method crystals are not available to generate

those frequencies directly; as a result of this the carrier is usually generated at a

frequency considerably lower than the final output frequency. To achieve the

desired output frequency, one or more frequency multiplier stages are used.

Another method of direct FM generation is the use of a reactance modulator.

This modulator is a circuit in which a transistor is made to act like a variable

reactance. The reactance modulator is placed across the LC circuit of the

oscillator and as the modulator’s reactance varies in response to an applied

audio signal, the oscillator frequency varies as well.

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However, the third technique is to use a voltage controlled oscillator. The output

frequency of the voltage controlled oscillator is proportional to the voltage of

the input signal; i.e. if audio signal is applied to the input of a voltage controlled

oscillator, the output is an FM signal.

1.14 SCOPE OF WORK

This project is limited to the construction of an FM transmitter from educational

design perspective utilizing basic discrete components and doesn’t interfere

with government regulation in bandwidth and other transmissions. Hence it is

limited to 1 watt. It is intended to be received within a distance of 100meters

and a DC battery is used to make it a very portable.

1.15 REALIZATION OF OBJECT IN BLOCK DIAGRAM

FIGURE 1.0 BLOCK DIAGRAM OF THE PROPOSED 1 WATT FM

TRANSMITTER

The FM transmitter block diagram represents the information signal in audio

form which it is transuded into electrical signals and the corresponding

processing signal is later undergone before being sent to the transmitter. The

audio input sources block represents the audio signals coming into microphones

OSCILLATOR

PRE-AMPLIFIER MICROPHONE

ANTENNA

AUDIO INPUT

SOURCES

POWER AMPLIFIER

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from the radio players, cassette players and any audio signal source that one

would like to transmit at any point in time. The next stage is the pre-

amplification stage. This pre-amplifier amplifies the input signals and does pre-

emphasis by also integrating a high-pass filter to satisfy lower frequencies of the

audio signals. For a satisfactory reproduction of music and speech for

entertainment, the frequency ranges of 102MHz to 105 MHz are recommended.

After pre-amplification, the power amplifier boosts signals efficiency and its

rate of transmission then signal is connected to the oscillator to modulate it. The

oscillator is to generate the carrier frequency within the range of 102MHz to

105MHz and this will be modulated. The carrier is coupled to the antenna.

1.16 MOTIVATION

Though this project was the initiative of my supervisor i embraced it with great

seriousness. However, the idea behind this project is to enhance my exposure to

the field of telecommunication as a communication Engineer and to help

promote effective communication in the institution due to the fact that

inadequate network is the order of the day. Also the idea is to be informed about

the activities going on both inside and outside the institution; as it is known that

information is supremacy.

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CHAPTER TWO

2.10 LITERATURE REVIEW

This chapter presents and highlights the design tends to offer in terms of

flexibility of usage as against previous and existing designs. It also discusses the

various theories employed in realizing the design and operational principles of

the components used in the design.

2.11 ELECTROACOUSTIC TRANSDUCERS (MICROPHONES)

A transducer is a device that converts energy from one form to another form.

Transducers in terms of electro acoustic converts sound wave energy to

electrical signal energy and vice versa. Transducer that converts changes in air

pressure into corresponding changes in electrical signals is called microphone.

Several varieties of microphones exist which may be classified according to the

basic principle of operation. These include its resistance, moving coil induction,

and capacitance and piezoelectric effect property.

2.11.1 Variable Resistance Microphones

An example of this class of microphone is the carbon granules microphone

which is mainly used in telephone handsets and portable radio systems. The

electrical resistance of the granules varies with pressure and results to varying

circuit current. It is characterized by a limited frequency response. If the

movement of the diaphragm takes place sinusoidal at a frequency F, then the

resistance of the granules at any given time t is given by:

Where Ro is the Resistance of granules when there is no pressure on the

diaphragm, Rt is the total resistance under signal conditions and r is the

maximum change in resistance due to sound pressure.

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However, the carbon granules microphone is small and relatively cheap, very

rugged and produces a relatively high output.

2.11.2 Capacitor Microphones

These have an excellent frequency response and frequently reduced as standard

against which others are calibrated. They are quite delicate because of the

narrow separation between the diaphragm and the back plate and the thin

diaphragm needed. An elect ret microphone is an example of such, as it is the

electrostatic equivalent of a permanent magnet and can store electrostatic

charges almost indefinitely. When an elect ret is placed between two metal

plates it forms a special kind of a changed capacitor, with charge held by the

elect ret permanently. One of the plates is used as microphone diaphragm and

made to vibrate with sound waves. Thus sound waves are converted to audio

frequency (AF) voltage signals very small and needs to be amplified. The

microphone has high internal impedance and does not require a polarizing

voltage. A field effect transistor (FET) is always included inside the microphone

capsule and used as a source follower to provide matching between the high

impedance microphones to the low impedance of the amplifier circuit. They

require high voltage supply and must be used with a signal amplifier.

2.11.3 Magnetic Microphones

The variables reluctance and moving coil induction microphones are grouped.

Both types of microphones do not require bias current for their operation but

have low signal output levels. The variable reluctance microphone are not

common as their diaphragm has to be of a magnetic material and which is

difficult to construct as it not being too rigid and having interfering mechanical

resonance. It has high source impedance. The moving coil induction

microphones or dynamic microphone as it is commonly known has the same

structure as the moving coil loudspeaker. Its diaphragm is light and has a

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characteristic very linear frequency response. It is used mainly for music

recording and broadcasting.

2.11.4 Piezoelectric Microphones

These are also known as crystal microphones. They are self generating

microphones and quite rugged and provide reasonable good frequency response.

Recently the crystal has been replaced with ceramics materials and at time are

called ceramic crystal microphones.

2.12 PRE-AMPLIFIER

This is better described as the audio amplifier. It is sometimes called a low

frequency amplifier. It is basically designed to amplify electrical signals of

about 20Hzto20KHz. The two principal types of audio amplifiers are the

voltage and the 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 pre-amplifier

circuit was employed since the audio signal from the microphone is quite weak

and requires amplification. Also by definition the pre-amplifier following the

microphone is called microphone amplifier.

2.13 POWER AMPLIFIER

It is also known as the radio frequency (RF) 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 conditions.

Amplifiers are classified into various classes depending on the position of their

operating point in its output characteristic. This is dependent on the biasing of

the amplifier.

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2.13.1 Class-A Amplifiers

This are weak signals amplifiers, such as the kind used in a microphone pre-

amplifier, they are always Class-A amplifiers. They are linear meaning that the

shape of the output wave is a faithful, but magnified, reproduction of the shape

of the input wave. To obtain a class-A operation with a bipolar transistor, the

bias must be such that, with no signal input, the device operates near the middle

of the straight-line portion of the collector current (IC)versus base current (IB)

curve. This is usually for an NPN transistor. For PNP reverse the polarity signs

with a JFET and MOSFET, the bias must be such that, with no signal input, the

device is near the middle of the straight-line part of the drain current (ID) versus

gate voltage (EG) curve. This is usually for an N-channel device. For P-channel,

reverse the polarity signs.

2.13.2 Class-AB Amplifiers

The operation of class-AB amplifier occurs when a bipolar transistor is biased

slightly above the point where the no-signal base current becomes zero (cut-

off), or when an FET is biased slightly above the point where the no-signal gate

current becomes zero(pinch off), the input signal drives the device into the non-

linear part of the operating curve. In class-AB amplifiers, the input signal might

cause the device to go into cut-off or pinch-off for a small part of the cycle.

Whether or not this happens depends on the actual bias point and the strength of

the input signal. If the bipolar transistor or FET is never driven into cut-

off/pinch-off during any part of the signal cycle, the circuit is class-AB1

amplifier. If the device goes into cut-off/ pinch-off for any part of the cycle, the

circuit is a class-AB2 amplifier. In any class-AB amplifier the output waveform

differs in shape from the input waveform. If the signals are modulated, such as

in a voice radio transmitter, the data impressed on the signal will emerge

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undistorted anyway. Class-AB operation is commonly used in RF power

amplifier systems.

2.13.3 Class-B Amplifiers

When a bipolar transistor is biased exactly at cut-off, or when a FET is biased

exactly at pinch-off, an amplifier is said to be working in class-B. Here there are

no collector or drain current when there is no signal, as a result of this the

energy is saved compared to the class-A and class-AB circuits. When there is an

input signal, current flows in the device during exactly half the cycle.

Sometimes two bipolar transistors or FETs are used in a class-AB or class-B

circuit, one for the positive half of the cycle and the other for the negative half.

In this way distortion is eliminated. This is a push-pull amplifier and is

commonly used in audio applications. The class-B scheme can be used for RF

power amplification. The output wave has a shape that is much different from

that of the input wave and this produces harmonics in addition to the signals at

the fundamental frequency. This can be a problem, but it can be overcome by a

resonant circuit in the output. If the signal is modulated, the modulation

envelope is not distorted.

2.13.4 Class-C Amplifiers

A class-C amplifier comes to operation when a bipolar or FET can be biased by

past cut-off or pinch-off and it can work as a power amplifier if the drive is

sufficient to overcome the bias and cause the device to conduct during part of

the cycle. A class-C RF power amplifier is a non-linear for signal envelopes in

which the amplitude varies over a continuous range. An example is the standard

amplitude modulation (AM) signal. The class-C circuit will work ;properly only

for a signal whose amplitude is constant, or else has only two states called

on/off, high/low, mark/space. Continuous wave (CW) radio telegraphy, radio

teletype (RTTY) and frequency modulation (FM) are examples of such signals.

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A class-C RF power amplifier needs substantial driving power in order to

overcome the cut-off or pinch-off bias that is applied to its base or gate when

properly operated, however, it can work with high efficiency and is still used in

some broadcast transmitters.

2.14 OSCILLATORS

An electric oscillator may be defined as one of the following;

(i) An unstable amplifier

(ii) A circuit that produces an output which varies its input with time

(iii) A circuit which converts DC energy to AC energy at very high

frequency

These definitions exclude electromagnetic alternators which convert mechanical

or heat energy into electrical energy. An oscillator differs from an amplifier in

one basic aspect, in that the oscillator do not require an external signal either to

start or to 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 about100MHz- 104 MHz

oscillator frequencies.

DC POWER

SIGNAL OUTPUT

FIG. 2.1 BLOCK DIAGRAM OF AN OSCILLATOR

OSCILLATOR

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Moreover, the frequency of the output is determined by the passive components

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.

2.14.1 Sinusoidal or Harmonic Oscillators

These are oscillators which can produce an output having sine wave forms and

produce any of the following oscillations; damped or un-damped oscillations.

2.14.2 Non-Sinusoidal or Relaxation Oscillators

These are oscillators which produce an output which has square, rectangular or

saw-tooth wave form.

Oscillations whose amplitudes keeps decreasing or decaying with time are

called damped oscillations. Ultimately, 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.

Oscillations whose amplitude remains constant that is those that do not change

with time are called un-damped oscillations. They 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.

In addition, oscillators can be described also as an electronic circuit whose

function is to produce an alternating electromotive force (EMF) of a particular

13

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, and

colpitts oscillator. etc. For the purpose of this project the colpitts oscillator is

used.

Capacitors and inductors are the two components 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 other frequency. 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.

2.15 RESISTOR:

For a resistor, according to ohm’s law, the voltage dropped across it is

proportional to the amount of current flowing through it. ie.

Where voltage across the resistor

Current flowing through the resistor and

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Resistance of the resistor

Any current waveform across the 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 applications. 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 acts as filters, they are used to set operating currents and signal 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.

2.16 INDUCTOR:

The voltage across an inductor leads the current through it by 90degress. 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 (

) which reflects the fact that the voltage leads the current.

Given the dimensions of an inductor coil such as average radius of coil(r) in

inches, number of turns of the coil(N), length of the coil(L) in inches, the

inductance in Micro Henrys( ) can be computed using this relationship.

⁄

2.17 CAPACITOR:

A capacitor temporarily stores charge or electricity in form of electrostatics.

This is not to 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. i.e pumping the water round the

15

radiator. Capacitor like resistor are so widely used that books are written about

them. So capacitors are used in storing small amount of electrical energy, they

are used in smoothing i.e decoupling power supplies, removing of voltage

spikes from supplies etc.

2.18 TRANSISTOR:

A transistor is defined as a semiconductor device obtained by sand witching a P

or N type semiconductor material between a pair of the same type of

semiconductor materials. We have two types according to

(i) P-N-P is obtained by sand witching an N-type material between two

P-type materials.

(ii) N-P-N transistor is obtained by sand witching a P-type material

between two N-type materials.

Transistors have three regions or terminal as follows.

(i) Emitter (E):

It emits (supplies) electrons if it is an N-type region and holes, if it is a

P-type region. The electrons or holes emitted constitute emitter

current ( ). The emitter-base junction is always forward biased.

When grounded it is called common emitter.

(ii) Collector(C):

It collects electrons (or holes emitted by the emitter. It constitutes a

current called collector current( ), which flows through the load. The

collector base junction is always reversed biased. When grounded, it is

known as common collector.

(iii) Base (B):

The region between emitter and collector is the Base. The base

provides the path for blow of electrons or holes from emitter to

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collector. In the case of an N-P-N transistor, when electrons from

emitter (N) enter into base (P), they recombine with holes in the base

region similarly, in case of a P-N-P transistor holes entering from (P)

into the base (N), try to recombine with electrons of the base region.

Thus, in both the transistor a current is constituted called base current

( ), which is however, very small. When grounded is termed

common base.

2.19 DETERMINATION OF RESONANT FREQUENCY

A resonant circuit comprises an inductor and a capacitor in parallel or in series

XL

XC ⁄

Where:

Capacitor

XL

XC

Resonance occurs when

XL=XC 2.3

At resonance,

2 ⁄

Making f the subject of formula, i have;

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⁄

√ √ ⁄

√ ⁄

This is the resonant carrier frequency of a colpitts oscillator. The tank or

resonant circuit has three main specifications, namely: Bandwidth, Quality

factor or Q-factor, Insertion loss. These parameters define the pass band, shape

and loss of the tank circuit response.

2.20 MODULATION

Modulation is a process of superimposing information contained in a lower

frequency electronic signal into higher frequency signal. The higher frequency

is called the carrier 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.

( )

Where:

The instantaneous value of the sine wave called the carrier

The maximum amplitude

The angular velocity

The phase relation is with respect to some reference value. Any of this last

three characteristics or parameters (e, ) of the carrier may be varied by

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the modulating signal, giving rise to amplitude, frequency or phase modulation

respectively in this project, frequency modulation is considered.

2.20.1 NEED FOR MODULATION

However, it is needed due to the following reason.

(i) For efficient radiation and reception of radio waves. The transmitting

and receiving antenna must have height will be too long to be realized.

(ii) 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 an un-modulated carrier cannot be used to

convey information.

(iii) By standard, the bandwidth for commercial quality speech is 30Hz to

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.

2.21 TYPES OF MODULATION TECHNIQUES

Basically, there are two types of modulation namely Amplitude Modulation and

Angle Modulation. Angle Modulation is further divided into frequency and

phase modulation. They are each briefly discussed below.

2.21.1AMPLITUDE 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 intensity of the carrier waves is changed while the

frequency remains constant remains constant.

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2.21.1.1 LIMITATIONS OF AMPLITUDE MODULATION

Although theoretically highly effective, Amplitude Modulation suffers from the

following draw backs;

(i) Noisy Reception: In an AM wave, the signal is in the amplitude

variations of the carrier practically all the natural and manmade noises

consists of electrical amplitude disturbances. As a radio receiver

cannot distinguish between amplitude variations that represent noise

and those that contain the desired signal, therefore reception is

generally noisy.

(ii) Low Efficiency: In amplitude modulation, useful power is in the side

bands as they contain the signal.

(iii) 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.

(iv) 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 band

width 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 5 KHz which is

hardly sufficient to reproduce the music properly.

2.21.2 PHASE MODULATION

Here, the phase of the sinusoidal carrier is made to vary with the instantaneous

value of the modulating audio signal or the information signal changes the

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phase of the waves with the frequency and the amplitude kept constant.

However; phase modulation varies with the modulating frequency.

2.21.3 FREQUENCY MODULATION

In this case the frequency of the carrier wave is varied in proportion to the

modulating signal. During modulation only the frequency of the carrier varies as

it increases positively with increase in modulating voltage.

The carrier amplitude is kept constant and thus the associated power of the

modulated wave is constant and this is a vital advantage over amplitude

modulation.

2.22 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 a

large distance to the 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.

2.22.1 NECESSITY FOR 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 audio frequencies. If the modulated wave after amplification is

directly fed to the speaker 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 move 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.

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Receiver antenna loud speaker (no sound)

FIG. 2.2 BLOCK DIAGRAM OF DEMODULATION PROCESSES.

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

then amplified and fed to the speaker for conversion into sound.

2.23 THEORY OF FREQUENCY AND PHASE MODULATION

Frequency 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 most direct way to get FM is to apply the

audio signal to a varactor in a tuned oscillator an example is known as reactance

modulation. The varying voltage across the varactor causes its capacitance to

change in accordance with the audio wave form. The changing capacitance

results in variation of the resonant frequency of the induction-capacitance (LC)

tuned circuit, causing a swing in the frequency generated by the oscillator.

A direct way to get FM is to modulate the phase of the oscillator signal. Any

change in the instantaneous phase of a sine-wave RF carrier causes a change in

its instantaneous frequency. The first practical system was put forward in 1936

as an alternative to A.M in an effort to make radio transmissions more 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. When the phase modulation is used, the audio signal must be

Station

selection

RF

Amplifier

22

processed, adjusting the amplitude-versus-frequency response of the audio

amplifiers; otherwise, the signal sounds unnatural when it is heard at the output

of an FM receiver.

2.23.1 FREQUENCY DEVIATION

Assuming the moment that the carrier of the transmitter is at its resting

frequency ie state of no modulation of 100MHz and we apply a modulating

signal. The amplitude of the modulating signal will cause the carrier to shift

from this resting frequency by a certain amount. If we increase the amplitude of

the modulating signal we will increase the deviation. A maximum of 75 KHz is

allowed as specified by the Federal Communication Council. If we remove the

modulation, the carrier frequency shifts back to its initial resting frequency

100MHz. However, the deviation of the carrier is proportional to the amplitude

of the modulating voltage. The deviation in comparison to the amplitude of the

modulating voltage is called the Deviation Ratio.

For most FM voice transmitters, the deviation is standardized at 5 KHz for

commercial broadcast. The deviation obtainable by means of direct FM is

greater, for a given oscillator frequency than the deviation that can be obtained

using phase modulation. Deviation can be increased by a frequency multiplier.

When an FM signal is passed through a frequency multiplier, the deviation is

multiplied along with the carrier frequency. In FM High-Fidelity music

broadcasting and in some other applications the deviation is much greater than

5 KHz. This is called wide band FM, as opposed to narrowband FM discussed

above. To obtain the optimum fidelity the deviation for an FM signal should be

at least equal to the highest modulating audio frequency. ie 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. Thus for voice signals 5 KHz is more than

enough. For music, a deviation of at least 15 KHz to 20 KHz is needed. The

23

ratio of the frequency deviation to the highest modulating audio frequency is

called the Modulating Index. Ideally, the ratio should be between 11 and 21.If

it is less than11, the signal sounds muffled, and the efficiency sacrificed, but

increasing it beyond 21broadens the bandwidth without providing much

improvement in the signal quality.

Frequency Modulation can also be described as a 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 methods of

modulation are the Amplitude Modulation (AM) and the 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.

2.23.2 DESCRIPTION OF MODULATION SYSTEMS

The general equation of an un-modulated wave or carrier may be written as

( ).................................................................................2.6

Where Instantaneous value (of voltage or current)

Maximum Amplitude

Angular Velocity in radians per seconds (rads/sec).

Phase Angle in radians.

It is noted that the 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.

24

Amplitude Modulation is achieved when the amplitude is varied.

Phase Modulation is obtained altering the phase angle .

Frequency Modulation is achieved by varying the frequency of the carrier.

It is assumed that the modulating signal is sinusoidal. This signal has two

important parameters which must be represented by 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 un modulated value, called the deviation, is made proportional to the

instantaneous amplitude of the modulating voltage. The rate at which the

frequency variation; or changes 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 are. 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 below noting 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.

25

FIG. 2.3 A MODULATING SIGNAL

FIG. 2.4 FREQUENCY MODULATED SIGNAL

The modulating or audio signal is described as

Where represents the maximum amplitude,

Represent the frequency of the audio signal,

26

Represent time and

Represent the instantaneous value of the modulating signal voltage.

The carrier frequency F, will vary around a resting FC (carrier frequency) thus

The frequency modulated wave will have the following description.

( )

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.

Frequency deviation

Carrier swing frequency deviation

It can be proven that the equation for the frequency modulated wave can be

transformed into:

( ⁄ )

It is noted that in the above equation (2.12), the cosine term is the preceded by

the ⁄ . This quantity is called the modulation index and is indicated or

represented as “MF”.

Modulation

Index ⁄

27

2.23.3 ADVANTAGES OF FREQUENCY MODULATION

(i) The efficiency of transmission is very high

(ii) It gives high fidelity reception

(iii) It gives noiseless reception as discussed before, noise is said to be a

form of amplitude variations and a FM receiver will reject such

signals.

(iv) The operating range is quite large.

2.23.4 APPLICATIONS OF FREQUENCY MODULATION

The five major categories in which FM is used are as follows:

(i) Non-commercial broadcast at 88MHz to 90MHz

(ii) Commercial broadcast with 200 KHz channel bandwidth at 90MHz to

108MHz.

(iii) Television audio signals with 50 KHz channel bandwidth at 54MHz to

88MHz, 174MHz to 216MHz and 470 MHz to 806MHz.

(iv) Narrow band public service channels from 108 MHz to 174MHz and

in excess of 806MHz.

(v) Narrow band amateur radio channel at 29.6MHz, 52MHz to 53MHz,

144MHz to 147.99MHz, 440MHz to 450MHz and in excess of

902MHz.

(vi) 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 band. FSK is also used on VHF for data transmission at 4800bps

using the Direct Frequency Modulation (DFM) technique, or G3RUH

Modulation at 9600bps.

(vii) Digital-AFSK: Audio Frequency Shift Keying (AFSK) 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

28

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-

emphasised audio is used for the demodulation.

2.24 NOISE AND FREQUENCY MODULATION

There are several other forms of modulation particularly associated with digital

communications 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 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:

(i) Adjacent channel interference: FM offers not only an improvement in

the signal to noise ratio but also better discrimination against other

interfering signals, no matter what their source is. Also wideband FM

broadcasting channel occupies 200 KHz of which only 180 KHz is

used and the remaining 20 KHz guard band goes a long way towards

reducing adjacent channel interference even further.

(ii) Co-channel interference-capture 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 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 forms of interference. This applies even if the other

transmitter operates at the same frequency as the desired transmitter.

29

2.25 PRE-EMPHASIS AND DE-EMPHASIS

Noise 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 pre-

emphasis in FM gives a noise rejection of at least 24db better than AM.

30

CHAPTER THREE

3.10 METHODOLOGY

The 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.

AUDIO INPUT ANTENNA

FIG. 3.1 A BLOCK DIAGRAM OF A 1 WATT FM TRANSMITTER

However, there are several methods of generating Frequency Modulation as

follows, but for the purpose and success of this project, the Direct-method of

Frequency Modulation generation was implemented.

3.12 Direct-Method of Frequency Modulation Generation

In this method crystals are not available to generate those frequencies directly;

as a result of this the carrier is usually generated at a frequency considerably

lower than the final output frequency. To achieve the desired output frequency,

one or more frequency multiplier stages are used. Another method of direct FM

generation is the use of a reactance modulator. This modulator is a circuit in

which a transistor is made to act like a variable reactance. The reactance

modulator is placed across the LC circuit of the oscillator and as the

modulator’s reactance varies in response to an applied audio signal, the

oscillator frequency varies as well. Others include the indirect-method of FM

Transducer

(microphone)

Audio

pre-amplifier

Power

amplifier

RF oscillator

31

generation which involves the use of a stable crystal oscillator to generate the

carrier signal and the use of a buffer amplifier to isolate it from the remainder of

the circuitry. And the use of voltage controlled oscillator. The output frequency

of the voltage controlled oscillator is proportional to the voltage of the input

signal; i.e. if audio signal is applied to the input of a voltage controlled

oscillator, the output is an FM signal.

The various components used in the construction of this project include:

resistors, transistors, capacitors, potentiometer and inductors.

The major sections that constitute this design are:

(i) The power supply unit

(ii) The audio pre-amplification unit

(iii) The power amplification unit

(iv) The RF oscillator unit

(v) The antenna

(vi) The indicator

3.12 THE POWER SUPPLY UNIT:

This unit consists of a 9 volts DC battery .The power supply ensures the circuit

functions effectively. To an extent, it determines the carrier frequency of the

oscillator circuit.

3.13 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 can 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 state is from

the base of the transistor while the output is from the collector. The capacitors at

32

this unit serve as a coupling unit filter networks and frequency determination of

input signal.

3.14 THE POWER AMPLIFIER UNIT:

The function of this stage is to amplify the carrier frequency or signal from the

pre-amplification stage to the desired power output level. It serves as a boost of

signal coming into it or as a driver. This stage consists of NPN transistor,

common emitter configuration with voltage divider biasing, biasing resistors.

The input to this stage is from the base of the transistor, while the output is from

the collector, which goes to the parallel resonant circuit or tank circuit.

3.15 RF OSCILLATOR UNIT:

This unit consists of a parallel resonant circuit or tank which is responsible for

producing the carrier wave upon which the intelligence signal is to be

superimposed for modulation.

3.16 THE ANTENNA UNIT:

The antenna is responsible for the transmission of the modulated signal through

space. For this project, the antenna is 1.75ft. Single pole antenna, It should be

noted that extending the length of the antenna consequently extends the range of

signal transmission as observed during testing.

3.17 THE INDICATOR:

This section consists only of a Light Emitting Diode whose function is to

indicate power supply to the rest of the component in the circuit.

For the design and construction of this project, some fundamental components

were used. An insight into their properties and their characteristic behaviour

relevant to the design under consideration are discussed below.

33

CHAPTER FOUR

4.10 DESIGN AND CONSRUCTION OF MODULE

FIG.4.1 CIRCUIT DIAGRAM OF SYSTEM

4.11 DESIGN SPECIFICATION

The design specification is a detailed description of the expected characteristics

of the designed FM transmitter.

(1) MODULATION TYPE: FM

(2) FREQUENCY OF OPERATION: 104.7 MHz

mic 25k4.7

220k

4.7k 1nF

10k

2-15pF

3.3pF

100

2N2219A

10k1nFBC547C

ANTENNA

4.7

5-30v

1k

VR1C1

R1

Q1 C4R5

R6

Q1

C6

0.2C5R4

C3R2

R3

C2

34

(3) ANTENNA TYPE: Single Pole Antenna

(4) RANGE IN FREE SPACE: About 100meters

(5) WORKING VOLTAGE: 9 volts (DC)

4.11.1 STAGE ANALYSIS FOR EACH SECTION

This section examines the stage by stage analysis of the module with their

respective circuit diagrams.

4.11.1.1 THE TRANSDUCER SECTION (MICROPHONE)

FIG 4.2 THE TRANSDUCER STAGE

The current, , flowing into the microphone is given by ohms law

...............................................................................................................3.1

25k, 9 volts

mic25k

GND

INPUT TO PRE-AMP STAGE

FROMAUDIO SOURCESTO MIC

VR1

35

⁄

⁄

⁄

4.11.1.2 THE AUDIO PRE-AMPLIFIER SECTION

FIG. 4.3 THE AUDIO PRE-AMPLIFIER STAGE

From the circuit diagram above,

, , , E ,

4.7

220k

4.7k

BC547C

GND

9V

INPUT TOPOWER AMP STAGE

INPUT FROM

TRANSDUCER (MIC)C1

R1

R2

Q1

36

Where is the collector current and is the base current.

From the equation for the collector feedback biased transistor,

( ) 4.1

Making the subject of formula, we obtain

( ⁄ ) ⁄

( ⁄ )

Now from this relationship,

E ⁄

Substituting the values of and E

⁄

⁄

37

4.11.1.3 THE POWER AMPLIFIER SECTION

FIG. 4.4 THE POWER AMPLIFIER STAGE

Using the above transistor characteristics and component values, the resistors

10 each both constitute a voltage divider network. Therefore the voltage

across the 10 resistor, is given as

( ⁄ )

( )

( )

1nF10K

100

2N2219A

1nF

4.7

10k

9V

GND

INPUT TOOSCILLATOR STAGE

INPUT FROMPOWER AMP STAGE

3.3pF

1k

C2

C3R3

R4

C4 R5

R6

Q1 C6

38

From the fundamental transistor equation, we know that

Where for a 2N2219A N-P-N transistor and silicon transistor

by standard

( )

Where is the voltage across the emitter, the current, across the emitter is

given as

⁄

However, noting that approximately,

Therefore, using equation and noting

39

4.11.1.4 THE RF OSCILLATOR STAGE

FIG. 4.5 THE RF OSCILLATOR STAGE

DETERMINATION OF THE TANK CIRCUIT PARAMETERS

⁄

Resonance is said to occur when

At resonance,

2-15pF

ANTENNA

9V

INPUT TOOSCILLATOR STAGE

C50.2

40

⁄ From equation

Making the subject of the formula, we obtain

√ ⁄ as in equation . This is the resonant carrier frequency of a

Colpitt Oscillator.

From equation

( )

⁄

Obtaining the dimensions of the inductor coil to be as follows,

Number of turns

Average radius of coil

Length of coil

Substituting these values into the expression, we obtain

⁄

⁄ = ⁄

.

41

From equation in the previous page, substituting for the values of and

we obtain, the resonant frequency to be

√ ⁄

√ ⁄

√ ⁄

⁄

⁄

MHz. this is thus the carrier frequency of the parallel - network.

4.12 COMPONENT JUSTIFICATION

This section describes the importance of using each of the electronic

components that constitutes the circuit diagram.

For the transducer section, the electrets microphone was used as the input

transducer because of its high sensitivity. The 25k potentiometer limits the

amount of current entering the electrets microphone. This consequently

stabilizes the gain of the microphone and maintains good stability of the

sensitivity.

In the pre-amplifier, power amplifier circuit and oscillator stages, the BC547C

and 2N2219A respectively were used utilized because of its high frequency

response characteristics. The capacitors were used as coupling 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

42

performance at high frequency generation of signal and its availability in the

market.

A single pole antenna was used as the antenna due to the miniature nature of the

circuit and under impedance matching considerations was seen to be the best

suit for this project work.

A 9 volt DC battery was used as the power supply for this circuitry because of

its ability to produce a steady current and its ready availability.

43

4.13 BILL OF ENGINEERING MEASUREMENT

S/N ITEM SPECIFICATION QUANTITY UNIT

COST

TOTAL

COST

1 N-P-N TRANSISTORS

BC547C; h E

2N2219A; h E

1

1

₦10

₦150

₦10

₦150

2 RESISTORS(fixed wire wound)

220kΩ,4.7kΩ,10kΩ,10kΩ,100Ω,1kΩ

6

₦5

₦30

3 POTENTIOMETER

25kΩ

1

₦20

₦20

4 CAPACITORS

4.7 F( Electrolytic)

1nF,1nF,3.3pF(Ceramics)

2-15pF(Variable Capacitor)

2

3

1

₦20

₦20

₦300

₦40

₦60

₦300

5 ELECTRET MICROPHONE 1 ₦100 ₦100

6 SWITCH 1 ₦50 ₦50

7 VERO BOARD(DOTTED) 1 ₦50 ₦50

8 9 VOLTS DC BATTERY 2 ₦50 ₦100

9 SINGLE POLE ANTENNA 1 ₦50 ₦50

10 PLASTIC CASING 1 ₦100 ₦100

11 BATTERY CONNECTOR 1 ₦50 ₦50

12 INDICATOR 1 ₦5 ₦5

13 CONNECTING WIRE 2 ₦20 ₦40

14 MIC PORT 1 ₦50 ₦50

15 MISCELLANEOUS - - ₦3000

TOTAL COST

₦4,205

44

4.14 COMPLETE CIRCUIT DIAGRAM

FIG.4.6 COMPLETE CIRCUIT DIAGRAM OF MODULE

mic 25k4.7

220k

4.7k 1nF

10k

2-15pF

3.3pF

100

2N2219A

10k1nFBC547C

ANTENNA

4.7

5-30v

1k

VR1C1

R1

Q1 C4R5

R6

Q1

C6

0.2C5R4

C3R2

R3

C2

45

CHAPTER FIVE

5.10 TEST AND OBSERVATION

It was observed that during the testing of this project with a radio receiver that

the transmitted signal produced a large squeal, an external microphone port was

improvised for the microphone due to imbalance biasing, which gave better

performance. Also observed during testing that the transmitter frequency was at

104.7 MHz contrary to the anticipated and calculated 102.7MHz.This situation

resulted from variable capacitor the variations made to the inductor wire during

construction which affected the inductance and consequently the carrier

frequency. Also on maximum power the power transistor heats up at power

amplifier stage, to solve this issue a heat sink is needed for the transistor, 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. The use of a voltmeter was used to test for voltages at various

points also an oscilloscope was used to test for frequencies of oscillations and

also the range was put in to consideration.

46

CHAPTER SIX

6.10 CONCLUSION

The simple FM transmitter was designed with a view which includes using the

basic concepts and theories of FM transmission in the propagation of

information. Attempts were made to put in a simple and straight forward design.

This design was tested and found to reach about 100 meters of range and its

frequency of operation of about 104.7 MHz in agreement with my objective.

The design and construction procedures involved in the organization of this

system and the casing was also presented. I have attempted to use minimum

components to achieve optimum transmission range and portability. The

selection of the modified electronic circuitry components was carefully done

considering these factors; their ability to perform their required function without

interruption during the operational life and their economic ability. However, this

project is a success due to these satisfactions and the tests carried out.

6.11 RECOMMENDATION

The 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 project has succeeded to a great extent, in exposing the very

basic principles involved in FM transmission and reception.

The following recommendation was made based on the experience in doing the

project and the problem encountered in the course of the work.

Modular design should be used, for analysis of each module should be done

cautiously, accurate measurement should be done to determine the values of

capacitor and wire wound inductor. Every variable capacitor and inductor

should be aligned properly to produce the required threshold voltage above the

system noise in the RF section.

47

6.12 REFERENCES

1. A Textbook of Electrical Technology by B.L Theraja, A.K Theraja,

S.Chand publishers India (24th

Edition)

2. Electronic Communications: modulation and transmission, Schoebeck,

Robert, Tata McGraw Hill, 2002, New York

3. Electronic Communications System, Kennedy and Davies Fourth Edition,

Tata McGraw Hill

4. Electronic Principles Devices & Circuits M.L.ANAND S.CHAND First

Edition, 2000

5. Fundamentals of Reliable Circuit Design, Alexander Mel, Longman

Press, 2001, Texas

6. Mastering Electronics, Whitehead R.J “” McGraw Hill, New York, 1988,

3rd

Edition chapter 9 &10.

7. Modern Communications, Miller Gary M, Tata McGraw Hill, New York

(2nd

Edition)

8. Radio Communication Concepts: Analogue, Carson, John Wiley & Sons,

Inc; 1990

9. RF Circuit Design, Boswick, SAMS, 1982.

10. Solid State Radio Engineering, Krauss, Bo stain, Raab, John Wiley&

Sons, Inc; 1980.

11. The ARRL Handbook, The American Radio Relay League, Inc., 1994.