CHAPTER ONE 1.10
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 1
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.
2
Communication channel
Transmitter
Receiver
Fig 1.0 Block Diagram of a Communication System
1.11
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 TWO 2.10 2.11
LITERATURE REVIEW 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 3
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 WAVES
As 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 4
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 5
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.
6
2.12
TRANSMITTER
The 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.
7
• 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 earth’s ionosphere to these frequencies.
Transmitting aerial
Audio amplifier
microphone
Oscillator
Fig 2.0
Modulator
Generalized block diagram of an FM transmitter
2.12.1 BASIC BUILDING BLOCKS OF AN FM TRANSMITTER Listed below are the basic building blocks of an FM transmitter. 2.12.1.1
Transducer
The function of any transducer is to convert energy from one form to another For this project, an electret microphone is being used. 8
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 Oscillators
An 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 9
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.
OSCILLATOR
Signal output
DC power input Fig 2.2 Block Diagram of an oscillator Moreover, the frequency of the output is determined by the passive component used in the oscillator and can be varied at will. Electronic 10
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.
11
Fig 2.3 Damped Oscillation waveform 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. 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 Oscillations Oscillations 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 12
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. 13
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 = 2πfL ………………………………………………2.1 14
Xc = 1
……………………………………………….2.2
2πfC Resonance occurs at XL=Xc …………………………………2.3 At resonance, 2πfL =
1
…………………………………………………2.4
2πfC Making ƒ the subject of the formula, we obtain f= 1
....................................................................................2.5
2π√LC 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 Loss These 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
15
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+ φ) ………………………………………….2.6 Where: e is the instantaneous value of the sine wave, called the carrier; E is its maximum amplitude, w is the angular velocity φ is its phase relation with respect to some reference value.
16
Any of these last three characteristics or parameters (ε, w, and φ) 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 Techniques
17
Basically, 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.
18
• 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 19
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)
Station selection
RF amplifier
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 MODULATION 20
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 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 21
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 + Φ)
from eqxn 2.6
Where X= instantaneous value (of voltage or current) A= (maximum) amplitude w= angular velocity, radians per second (rads/sec) Φ= 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 Φ 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 22
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
23
t
-V Frequency modulation
The modulating or audio signal is described as: Va = A sin 2πƒat……………………………. 2.8 Where A represents the maximum amplitude, ƒa 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 + ∆F2πfat…………….. ………………2.9 The frequency modulated wave will have the following description: V = A sin (2πFc + ∆F2πfat) }…….. …………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 =2∆F……………..2.11 It can be shown that the equation for the frequency modulated wave can be manipulated into: 24
V = A sin {2πfct + (∆F/ƒa) cos 2πƒat} ……………………….2.12 It must be noted that in this equation, the cosine term is preceded by the ∆F/ƒa. This quantity is called the modulation index and is indicated as “Mƒ” Modulation index = Mƒ = ∆F/ƒa …………………2.13 2.16.2
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.
25
• 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 MODULATION
Note 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 26
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 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 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 transmitter
2.18
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.
27
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-µs pre-emphasis in FM gives a noise rejection at least 24dB better than AM.
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,
Input audio
Transducer
to antenna
Audio preamplifier
28
RF oscillator
Fig 3.1 A block diagram of an FM transmitter
The 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 29
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 UNIT The 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.
30
3.16
RESISTOR
For 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 ………………………………………………..3.1 Where 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
INDUCTOR
The 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=2πf) which reflects the fact that the voltage leads the current.
31
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(μH) can be computed using this relationship. L = N^2 r^2
……………………………………………3.2
228r + 254L 3.18
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 FOUR 32
4.10
DESIGN AND CONSTRUCTION OF MODULE
4.11
COMPLETE CIRCUIT DIAGRAM
Fig 4.1 complete circuit diagram
33
Design 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 long
Range Obtained in Free space: Up to 100meters Working Voltage : 3 volts(DC)
4.11.1
STAGE ANALYSIS OF EACH SECTION
This section examines the stage by stage analysis of the module with their respective circuit diagrams. 34
4.11.1.1
THE TRANSDUCER SECTION
The current ,I, flowing into the microphone is given by ohms law V = I R from eqxn 3.1 R=33k V=3 volts I=V/R I=3/33k I= 90µAmps 4.11.1.2
THE AUDIO PRE-AMPLIFIER SECTION
35
From the circuit diagram above, Rb=1MΩ Rc= 10kΩ Vcc= 3volts hƒE= 60 from the equation for the collector feedback biased transistor, Vcc=Ic Rc + (Ic/hƒE)Rb……………………………..4.1 Making Ic the subject of the formula, we obtain Ic = Vcc/( Rc + Rb/hfE)……………………………..4.2 Ic= 3/10k + 1M/60 36
Ic= 0.1mAmps Now from this relationship, hƒE = Ic/Ib…………………………………………….4.3 substituting the values of Ic and hfE Ib=Ic/ hƒE Ib= 0.1mA/60 Ib= 1.7µAmps
4.11.1.3
THE RF OSCILLATOR SECTION
37
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 = [100kΩ/100k+ 47k] 3volts Vb= 2.0volts From fundamental transistor equation, we know that Vb = Ve + Vbe……………………………………..4.4 Where Vbe=0.6volts for a silicon transistor by standard Ve=2—0.6=1.4volts Where Ve is the voltage across the emitter. The current, Ie, across the emitter is given by Ie=Ve/Re……………………………………………….4.5 38
Ie= 1.4/39k Ie=36µAmps Now Ie is approximately = Ic=36 µAmps Therefore, using equation 4.3 .and noting hƒE=60 Ib= Ic/ hƒE Ib= 36 µA/60 Ib= 60 µAmps • Determination of the tank circuit parameters L = Inductance C = Capacitance=47pF XL = Inductive reactance Xc = Capacitive reactance XL = 2πfL Xc = 1 2πfC Resonance occurs at XL=Xc At resonance, 2πfL =
1
from equation 2.4
2πfC Making ƒ the subject of the formula, we obtain f= 1
as in equation 2.5
2π√LC This is the resonant carrier frequency of a Colpitt Oscillator 39
From equation 3.2 L(µH) = N^2 r^2 228r + 254L Given the dimensions of the inductor coil to be as follows, N=number of turns = 5 r = average coil radius= 0.16mm L = length of coil=0.5mm substituting these values into the expression, we obtain L= 25(2.25) 228(0.16) + 254(0.5) L= 0.05µH from equation 2.5 Above , substituting the values for L and C we obtain, the resonant frequency ƒ to be ƒ=
1/2π√ 0.05µH×47pF
ƒ=
100MHz
this is thus the carrier frequency of the parallel L-C network
4.12
COMPONENT JUSTIFICATION
This 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
40
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 ANALYSIS
It 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 41
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/N ITEM SPECIFICATION 1 BC848 NPN TRANSISTOR
QTY UNIT COST 2 N500.00
TOTAL COST N1,000.00
hƒE=60 2
RESISTORS
33K,100K,47K,10K,1M,390Ω 3
6
N50.00
N300.00
6 1
N150.00 N500.00
N900.00 N500.00
CAPACITORS
22n,100n,1n,10p,22n,4p7 4
ELECTRET MICROPHONE
5
VOLTAGE CONTROLLED (L-C) OSCILLATOR
1
N1,000.00
N1,000.00
6
PLASTIC CASING
1
N2,000.00
N2,000.00
42
7
SWITCH
1
N50.00
N50.00
8
VERO BOARD
1
N100.00
N100.00
9
1.5VOLTS DC BATTERY
2
N50
N100
10
FLEXIBLE CORD ½ YARD
N50
N50
Total
N6,000.00
CHAPTER FIVE 5.10
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
43
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
REMARKS
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 main area of instability is the oscillator part. Shielding the oscillator helps in part to counter this.
44
5.20 5.20.1
REFERENCES 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, Texas 5.20.5
A Textbook Of Electrical Technology By B.L Theraja,
A.K Theraja, S. Chand publishers India (24th Edition) 5.20.6
www. radiocommunications.com
5.20.7
www.ask.com/transmitters/frequency modulation
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