A TRANSMITTER
CHAPTER ONE
1.1 INTRODUCTION
A transmitter is an electronic device used in telecommunications to produce radio waves in order to transmit or send data with the aid of an antenna. The transmitter is able to generate a radio frequency alternating current that is then applied to the antenna, which, in turn, radiates this as radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating current, the antenna radiates radio waves. In addition to their use in broadcasting, transmitters are necessary component parts of many electronic devices that communicate by radio, such as cell phones, wireless computer networks, Bluetooth enabled devices, garage door openers, and two-way radios in aircraft, ships, and spacecraft, radar sets, and navigational beacons. The term transmitter is usually limited to equipment that generates radio waves for communication purposes; or radiolocation, such as radar and navigational transmitters. Generators of radio waves for heating or industrial purposes, such as microwave ovens or diathermy equipment, are not usually called transmitters even though they often have similar circuits. The term is popularly used more specifically to refer to a broadcast transmitter, a transmitter used in broadcasting, as in FM radio transmitter. This usage usually includes the transmitter proper, the antenna, and often the building it is housed in.
A transmitter can be a separate piece of electronic equipment, or an electrical circuit within another electronic device. A transmitter and receiver combined in one unit is called a transceiver. The term transmitter is often abbreviated "XMTR" or "TX" in technical documents. The purpose of most transmitters is radio communication of information over a distance. The information is provided to the transmitter in the form of an electronic signal, such as an audio (sound) signal from a microphone. The transmitter combines the information signal to be carried with the radio frequency signal which generates the radio waves, which is often called the carrier. This process is called modulation. The information can be added to the carrier in several different ways, in different types of transmitter. In a frequency modulation (FM) transmitter, it is added by varying the radio signal's frequency slightly.
The antenna may be enclosed inside the case or attached to the outside of the transmitter, as in portable devices such as cell phones, walkie-talkies, and garage door openers. In more powerful transmitters, the antenna may be located on top of a building or on a separate tower, and connected to the transmitter by a feed line, that is a transmission line.
A glance into an electronic equipment such as a radio or television set or a computer, it would not be surprising if one felt discouraged about the study of electronics because of the seemingly complex nature of the circuits. However, while an electronic system may have a large number of component parts, there are only few of them that build up the system of the frequency modulated (FM) radio transmitter and other electronic equipment, according to Duncan (1983), the major component parts (devices) include: diodes, transistors resistors, capacitors, inductors, switches and transducers. Those used in integrated circuits (transistors, diodes, resistors and capacitors) have the same action as their discrete (separate) counterparts. The only difference is the use of space, that is, size. Different systems are built from a fairly small number of basic circuits consisting of these devices connected in certain ways so as to perform particular functions as may be desired. These electronic systems include: the power supply, oscillator, pre-amplifiers, power-amplifiers, frequency modulation, antenna system, modulator system, multiplexing system and mixer stages. Figure 1 shows the stages of a transmitter.
Fig 1.1 BLOCK DIAGRAM OF A TRANSMITTER
1.2 Aim and objectives
The aim of this project is to design and construct short range FM transmitter.
With the following objectives:
1) Design the schematic diagrams and identify the various components for the transmitter.
2) Assemble the identified components of the transmitter and construct the FM transmitter
1.3 Scope of the project
The design is only for frequency modulated (FM) transmitter and will only cover a short range this is due to cost implication and rules from the regulatory body that regulates the issuing of licenses for broadcasting.
1.4 Significance of the project
The short range frequency modulated (FM) transmitter will be of immense benefit to local communities in that if developed and mass-produced by the production industries, it will ease their communication services. It will be cheap enough to be procured and maintained, so that when installed in a fairly central place it could be used in local information services. It will also serve the school in terms of communication, when FM receivers are placed in various classes and offices on campus they can easily listen to the information passed via the FM transmitter.
CHAPTER TWO
2. REVIEW OF RELATED LITERATURE
2.1 INTRODUCTION
Information transmission is very vital to human life just as the early men used sticks to produce sound which indicates the location of each other as they wander about also down to the middle era when town crises come into play for the same information propagation to be transmitted from one point to another with the aid of radio communication which necessitates the application of radio transmitter and receiver.
A radio transmitter is a device whose major function is to send information (intelligence) from one point to another in most cases the information to be transmitted are voice, music and code signals. However the transmission of radio signal is done with the aid of electrical resonance; this is when the frequency of the receiver is equal to the incoming one from the transmitter resonance. The frequency on the tuning dial for FM band ranges from 88MHZ to 108MHZ. In transmitter configuration a sound is fed at the microphone which is the transducer that converts the physical sound into the electrical signal which is usually amplified by the fist electronic circuit (audio frequency amplifier class A) whose output is fed into a modulator which fields a frequency modulated output fed into the final stage (radio frequency amplifier class C) then to the antenna which radiates the radio signal into the atmosphere.
2.2 TYPES OF RADIO TRANSMITTERS
This section discusses two types of Radio transmitter. Am transmitter and FM transmitter.
2.2.1 AM TRANSMITTER
Alley (1988) observed that an AM transmitter has two principal design types-low level modulation and high level modulation. The low level modulated transmitter is modulated at its low power stages requiring low modulation power. The high level modulated transmitter, which usually accomplishes modulation at the anode of the output power amplifier, requires modulation power to be equal to about 50% of the carrier's power. In order to amplify faithfully and reproduce the modulation at low levels. The power amplifiers must be linear (Johnson, 1988). These are usually class B linear amplifiers, which are much less efficient than the class C amplifiers employed in the higher level modulated carrier amplifiers. Because it operates over a larger band of frequencies. The higher power modulator must have linear power amplification to achieve low distortion. Johnson (1988) observed that low power radio frequency oscillator whose frequency is very accurately controlled (since it determines the final carrier frequency) is the exciter for the transmitter. The exciter is followed by several stages of power amplification, which are required to drive the final power output stage. In low-level modulated transmitters, all the amplifiers following the modulated stage are tuned to the same frequency. Those ahead of the modulate stage may be used to double or triple the frequency of the exciter. In a high level modulated transmitter the power amplifier stages are seldom tuned to the same frequency expect for the input and output tank circuits of power amplifiers which feeds the antenna. So the modulator of a high level transmitter drives its input from a microphone or other service of audio signal and amplifies the signal with low distortion of the order of 1% to the level which is usually half that of carrier power. Alley (1988) remarked that most high power modulators utilize push-pull in either class A or linear class C to reduce distortion by balancing even harmonics and to balance out some hum and noise components. Negative feedback is also used for these purposes.
Fig. 2.1: A figure of AF signal
Fig. 2.2: A figure of RF signal
Fig 2.3: A figure of signal carrier
2.2.2 FM TRANSMITTER
According to Johnson (1988) the transmitting frequency in FM transmitter is varied above and below the median by an amount according to the amplitude of the modulating signal and a rate determined by the modulating signal. The amplitude of the transmitted radio frequency is constant. Therefore the entire FM system is arranged to be insensitive to amplitude disturbances. He observed that FM transmitters are similar to AM transmitters except that the AM modulation amplifying system is dispensed with and the exciter must be a variable frequency source. One method of modulating the frequency at the exciter utilizes a reactance in the frequency determining section in the oscillator exciter. The reactance value is changed electrically or electronically in accordance with the low-frequency modulating signal (Black, 1988). The remainder of the transmitter as an AM transmitter is made up of frequency doubling and tripling stages of power amplification and a power output stage.
Carrier Stabilization is a more difficult problem in FM transmitter than for AM transmitters (Lyon, 1988). Many schemes have been used, including those used for carrier automatic frequency control in receivers. Another means of frequency stabilization involves heterodyning a high-frequency crystal generated wave with a lower frequency well established, tank circuit oscillator, modulated by push-pull reactance tube. The FM transmitter monitoring differ from AM transmitter because the means of detecting frequency modulation are different from the means of detecting amplitude modulation. (Johnson, 1998). A frequency discriminator tuned to the relevant frequency must be used to reconstitute the modulation at the transmitter monitoring position.
Fig. 2.4: A figure of FM signal
2.3 Development in Transmitter Design
Transmitter research for several years was devoted mainly to increasing the power of spark transmitters and by 1910, the emphasis was gradually shifting to the development of high power continuous wave transmitters. Before 1905 a few individuals suspected that single-frequency sinusoidal continuous waves might have advantages in radio communication over the intermittent damped waves radiated by a spark transmitter, whose power is diffused over a substantial range of frequencies. Three lines of research were made in the next decade towards achieving the goal of high power continuous wave generators, improvement of spark transmitters, and development of the alternator (ac generator) to produce radio frequency power and development of the arc oscillator as a radio-frequency generator(Chipman, 2000).
Between 1900 and 1913 much was done to make a spark transmitter have an output that was nearly like a continuous wave. The introduction of tuned circuits helped considerably, but their benefit was restricted by persisting low-resistance in the spark gap, indicating a need for more rapid spark extinction. Improvements towards the goal of approximating continuous wave were made by using multiple spark gaps. It was first used by Right in 1894, rotary gap was used by Tesla in 1896 and cooled gaps or quenched gap used by Max Wien in 1906 (Chipman, 2000: 164). Alternators that produced power at frequencies of a few kilohertz were made by Elihu Thomson in 1889 and by Tesla in the 1890s. Fassender patented the idea of using an alternator as a radio transmitter in 1901. He collaborated with Charles P Steinmetz in the design of 1 -KW, 10 kilohertz machine that was delivered to him in 1903. Fassender used a 1-KW, 50 Kilohertz machine in making his Christmas Eve radiobroadcast in 1906. Alexanderson designs a 50KW-25Kilohertz transmitter for the U.S. Navy in 1917. The arc oscillator which basically consisted of a dc carbon arc connected to a resonant circuit was patented by Elihu Thomson in the 1890s. Thornton’s operated in a strong magnetic field, but it had one water-cooled metal electrode and the arc burned in hydrogen. Transmitters using arc oscillators to generate radio frequencies were never very widely used in their original form because they required close supervision, he went on. The triode's ability to act as an oscillating signal generator was discovered almost simultaneously by Armstrong De Forest with Charles Logwood, Herbert Van Etten and others between 1912- 1913. The transmitting power of early triodes was limited to less than 100 watts, because their glass envelop could not withstand the high temperature resulting from wasted energy dissipated as heat. Using fused quartz envelops, C.F. Elwell produced 2- KW triode oscillator in 1920. he Klystron created by the brothers (Ruse11 and Sigurd Varian) at Stanford University in1939 and the travelling wave tube invented by Rudolph Kompfner in 1944. Since the World War II these three devices have retained their status as the only generators of high power microwaves from a few tens of watts to a few megawatts. Compact, reliable and inexpensive solid-state generators of low power microwaves began to appear in the late 19GOs. After the invention of the maser in 1954, the maser concept was extended from microwaves to optical frequency range, where it acquired the laser. The first operating laser, emitting a,beam of visible red light, was developed by Theodore 1-1. Maiman in 1960. According to Chipman (2000) Radio telegraphy, radio broadcasting, short-wave radio telephony, and all other forms of radio communication in use up to 1935 used amplitude modulation (AM), in which the power output of the transmitter is varied in proportion to the time pattern of the information signal (code, voice or music). After prolonged experimenting, Edwin H. Armstrong demonstrated in 1935 that better protection against atmospheric, static, inter- station interference and signal fading could be obtained by using wide-band frequency modulation (FM) in which the frequency of the transmitter is varied by the information signal. In the face of powerful opposition from the large established AM industry and from the federal communications commission, Armstrong persisted with public presentations until the Yankee Network in Boston and station WDRC in Connecticut started FM in 1939. Three year later 40 FM stations were broadcasting and there are now hundreds of FM stations. Singleton (1988) corroborated this history in the development of radio. Up till the present day, information service in the local communities is done by the town criers who move about from place to place each time they have any message to pass to I ' the people'. On special occasions drums are beaten to give the infornmtion desired. But now, advances in electronics have continued to make life much easier to mankind. It has given us pocket calculators, digital watches, heart pacemakers, computers for industry, commerce and scientific research. Also, we have automatically controlled production processes, instant viewing of events on our television screens over long distances and radio transmission. . Duncan (1983) stated that all these advances were made possible because we have learnt to build complete circuits containing thousands of electronic parts on a tiny wafer I' of silicon,'which may not be more than 5t1-m square and 0.5mm thick.
2.4 Review of terms related to project
The following terms relate to the topic of this project.
1. Transmission
2. Modulator
3. Amplifier
4. Antenna
5. Transducer
6. Modulation and Demodulation
7. Amplitude modulation
8. Frequency modulation
2.3.1 Transmission
Transmission is the electrical transfer of a signal, message, or other form of intelligence from one location to another. Traditionally, transmission has been one of the two major disciplines of telecommunication. Switching is the other principal specialty. Switching establishes a connection from user X to some distant user Y. Simplistically, we can say that transmission is responsible for the transport of the signal from user X to user Y. In the old days of telephony, these disciplines were separate with strong demarcation between one and the other but this demarcation is fast disappearing. For example, under normal circumstances in the PSTN, a switch provides network timing that is vital for digital transmission. One very important type of transmission is the baseband transmission. This is the transmission of a raw electrical signal. This type of signal is very similar to the 1s and 0s transmitted electrically from a PC. Another type of baseband signal is the alternating current derived from the mouthpiece of a telephone handset (subset). Here the alternating current is an electrical facsimile of the voice sound wave impinging on the telephone microphone. Baseband transmission can have severe distance limitations. We will find that the signal can only be transmitted so far before being corrupted one way or another. For example, a voice signal transmitted from a standard telephone set over a fairly heavy copper wire pair (19 gauge) may reach a distant subset earpiece some 30 km or less distant before losing all intelligibility. This is because the signal strength is so very low that it becomes inaudible. To overcome this distance limitation, we may turn to carrier or radio transmission. Both transmission types involve the generation and conditioning of a radio signal. Carrier transmission usually implies (not always) the use of a conductive medium such as wire pair, coaxial cable, or fiber-optic cable to carry a radio or light-derived signal. Radio transmission always implies radiation of the signal in the form of an electromagnetic wave. We listen to the radio or watch television. These are received and displayed or heard as the result the reception of radio signals.
2.3.2. Modulator
Any device or circuit by means of which a desired signal is impressed upon a higher frequency periodic wave known as carrier is called modulator. The modulator may vary the amplitude, frequency or phase depending on the preference of modulation (Alley, 1988). Modulator is any device for effecting the process of modulation. In radar, a device for generating a succession of short pulses which cause a transmitting value to oscillate at each pulse. Modulation in itself is the process or the result of the process in which some characteristics of one wave is varied in sympathy with some characteristics of another wave.
2.3.3Amplifier
The job of an amplifier is to produce an output which is an enlarged copy of the input. The amplifier is a device whose output is a magnified version of its input and which draws its power from sources other than the input signal. We have audio, balanced (push-pull), booster, bootstrap, buffer, casad D, direct coupled, direct current, distributed, feedback, group le following types of amplifier; in, cascade-class B, class C, class ~ led anode, grounded cathode, grounded grid, head, intermediate frequency, linear magnetic, operational, parametric, paraphrase, power, push-pull, push-push, quiescent, radio frequency, regenerative, servo, single-ended, stagger-tuned, transistor, tuned, video frequency, voltage and wide-band.
Faissler (1991) stated that amplifiers are the most important building blocks used in electronics. They are active devices usually having two input terminals and two output terminals. In a simple form the output is a linear function of the input, a constant multiply by the input. The amplifier may respond differently to ac inputs from the dc inputs. The most important properties of an amplifier are its (a) gain (b) input impedance (c) output impedancce and (d) the frequency response. This refers only to signals without any biases but assumes linearity over some input range of voltages. That is doubling the input voltage doubles the output also. He said that gain is a complex number because the input and output may differ in phase and amplitude. The current and power gains can be described in the same way. The input impedance describes the loading effect of the input, the amplifier will have on a signal source. The output impedance is the Thevenin's equivalent impedance of the amplifier which affects its ability to provide output current. The frequency response describes the performance of the amplifier as a function of the input frequency. Both the input and output impedances are resistive while the gain changes depend on frequency. An amplifier that has two inputs and responds to the difference between the two inputs is called a differential input or a difference amplifier. If two outputs of an amplifier are 180' out of phase, it is said to have a differential output or double-ended output. Active attennuator amplifier do not have gains greater than 1 (Halkias 1988).
Power Amplifier ; This is the final stage in multistage amplifiers, such as audio amplifier and radio transmitter designed to deliver appreciable power to the load. Power amplifiers may be employed to supply power ranging from a few watts in an audio amplifier to many thousands of watts in a radio transmitter. Klock (1988: 30) explained that in audio amplifiers the load is usually the dynamic impedance presented to the amplifier by a loud speaker and the problem is to maximize the power delivered to the load over a wide range of frequencies. The power amplifier in a radio transmitter, according to Klock, operates over a relatively narrow band of frequencies with the load essentially a constant impedance. The mode of operation of power amplifiers is denoted by classes A, AB, B and C. He observed that class C (operation is limited to radio frequencies with a tuned load. The other classes may be used for audio and high frequency operations.
A class "A" operation amplifier is used when the amount of power to be transferred to the load is relatively small, that is, less than 10w. The amount of harmonic distortion introduced into the load voltage can be kept small by operating transistors or tubes within the nearly linear region of their characteristics. Class A operation has relatively little use because the conversion efficiency, efficiency of a power amplifier, is low, Alley (1988). However, for usual operating conditions, the efficiency is on the order of 10% (Grob, 1982). If the power amplifier were required to deliver 10w with 10% efficiency, the tube or transistor would have to be capable of dissipating an average power of 100w. Further more the power supply must be capable of supplying the power dissipated as heat plus useful power delivered to the load.This poses an unnecessary burden upon the power supply. Grob pointed out that other classes of operation have higher conversion efficiency and are therefore used for power amplifiers requiring higher power delivery. Klock (1988) held that Class AB is an improvement in the conversion efficiency that can be obtained by using class AB operation. However, while a class A amplifier can be operated single-ended (one output transistor or tube), a class AB amplifier must be operated push-pull. He stressed that in class AB operation, the transistor or tube current does not flow for the codilete cycle of the input voltage. In a single ended circuit this would introduce excessive distortion. Class B: This class is often used for the power amplifier in an audio amplifier. The amplifier in this class must be push-pull circuit. Theoretically, with ideal transistors or tubes, the class B amplifier can have a conversion efficiency of 78.5% practical, the efficiency is on the order of 50%. This is an appreciable improvement over the class A operation. The load is usually transfomer coupled to the two transistors or tubes operating in push-pull. For maximum power transfer, (Klock, 1988) stated that the dynamic load impedance presented to the amplifier is determined by the amount of hannonic distortion that can be tolerated. Use of more sophisticated circuitry than this as considered in an elementary presentation of a push-pull amplifier operating in class B can produce nearly distortionless power amplification. He said that this is of prime importance in the final amplifier stages of a high fidelity audio amplifier. Vacuum tube power amplifier can operate in class B with an appreciable amount of grid current flowing for small portion of the cycle of input sinusoidal signal. This imposes additional requirements upon the driving stage of the amplifier. If the equivalent circuit of the I driver has' too large equivalent output impedance, the flow of grid current through this impedance will cause a distortion of the grid wave form. This class of operation is usually encountered in driver circuits for class B amplifiers operating in the radio frequency region where high Q circuit counteract the effects of grid current. Audio operation is usually restricted to class B operation because the usual form of phase inverter circuitry has a large output impedance. In radio frequency operation, the transformer phase inverter can be used with tuned circuit and air-core or powdered-iron slug coils because the operation is essentially at one frequency Class "C": Because the collector or plate current flows for less than one-half cycle of the input, sinusoidal signal, this class of operation is restricted to radio-frequency operation where tuned load is employed. The load is usually the input impedance of an antenna or of an antenna matching network. This was the observation of (Alley, 1988). The load voltage will be nearly sinusoidal, even though the current is following in pulses because of the relatively sharp tuning of the load. This phenomenon allows the amplification of large amounts of power at conversion efficiencies as high as 80%. This is extremely important for applications requiring delivery of large amount of power to the load. The driving source must usually be employed to deliver power to the base circuit or grid circuit of the power amplifier in many cases as much as 10% of the power delivered to the load. This requirement is not excessive.
2.3.4. Antenna
An antenna (aerial) according to A Dictionary of Electronic 1994, is that part of a radio system from which energy is radiated or received from space. Americans called it antenna while. the British called it aerial. Some transmitters have reflectors made of sheet metal or a wire mesh, to concentrate the beam of radiation in one direction. When an ac flows into a transmitting antenna, radio wave of the same frequency (f) as the ac are radiated. If the length of the antenna is comparable with wave length (A) lambda, of the waves. Lambda can be calculated from the following equation which is true for any wave motion (Galloway 1988).
V=fπ
where V is the speed of the radio waves. This is the speed of light, namely 300million metres per second (3 x 10^8 m/s) The greater f is, the smaller will h be, since V is fixed. If f = IkHz, then h=1, if antennas are not to be too large, they must be supplied with RF currents of greater than 20kHz from the transmitter.
Antennas are conductors designed to radiate electromagnetic waves or to receive radiated waves that are present in the air or space. They are made in a wide range of sizes and shapes to serve particular applications. When there is current in it, the antenna always has an associated magnetic field in the space around it. When the intensity of the magnetic field changes or the field itself is moved and induced voltage is generated electric field. The result is two varying fields, one with magnetic flux and the other with electric lines of force. Any changing electric field will generate magnetic field while any changing magnetic field will also generate electric field.
Transducers
Microphone is an electro-acoustic transducer, which responds to sound waves and delivers essentially equivalent electrical waves. Microphone is an electro-acoustic device containing a transducer, which is actuated by sound waves and delivers electric signals proportional to the sound pressure. Microphones are usually classified with respect to the transducer principles used (Sessler, 1988). Their directional characteristics are also of interest, that is, the voltage output as a function of the direction of incidence for constant sound pressure. In addition to directional characteristics, Sessler pointed out that some other important characteristics of the microphone included open-circuit sensitivity, equivalent noise level, dynamic range, and vibration sensitivity. He defined open circuit sensitivity as the ratio of open-circuit output voltage to the sound pressure. Pressure sensitivity was defined as referring to the actual pressure acting upon the diaphragm of the microphone while the free-field sensitivity refers to the pressure that existed in the sound field before insertion of the microphone. Equivalent noise level, he said, is equal to the level of sound pressure, which generates an output voltage of the microphone corresponding to its inherent A-weighted noise voltage. He said that dynamic range is the range of sound pressure level in decibel (dB) extending from the equivalent noise level to the level where the non-linear distortion reaches 3%. He defined vibration sensitivity as the ratio of the output voltage of the microphone, as a result of acceleration of its case, to the magnitude of the acceleration.
The various types of microphone differ by the transducers used for converting the acoustic into electric signals. The most commonly used are: electrostatic, piezoelectric, dynamic, and carbon transducers. The first four transducer principles are reversible that is, the microphone can also be used as sound generator while the carbon transducer is non- reversible. Duncan (1983) stated that a good microphone should respond more or less equally to all sounds in the audio frequency range which is from about 20Hz to 20KHz. Otherwise the electrical signals it passes on for amplification and conversion back to sound by the output transducer (a loud speaker) will not be identical or nearly so to the original sound.
2.3.6.Modulation and demodulation
The process or result of the process whereby a message is changed into information bearing signals that not only unambiguously represent the message but also are suitable for propagation over the transmitting medium to the receiver is called modulation (Black, 1988). The vehicle for the propagation of electrical signals from one region in space to another is always an electromagnetic field. When the field changes with time it takes the form of a wave. At the distant end, the receiver is waiting to be informed. This is accomplished by the arrival of tl$' propagated wave, which must change in a way the receiver cannot predict. So he further defined modulation as the process whereby in response to the received wave, either the original message or information pertaining to the original message is made available in the form desired and delivered when and where it is wanted. The term's demodulation and detection are often used to denote the recovery of the wanted message from a modulation signal. Modulation is fundamental to communication. No matter how, when and where communication takes place modulation is involved. Modulation implies bandwidth occupancy. If a signal is to change in a way that cannot be predicted, it necessarily implies that the signals occupy a nonzero band of frequencies. So, the spoken word occupies a band from a few hundred to several thousand hertz. Ordinary telephony is a good example of these modulation concepts. The longitudinal sound waves generated by the spoken word constitute information-bearing signal to be communicated. The telephone transmitter, acting as a modulator changes the acoustic energy into electrical energy suitable for high speed propagation to a distant point. At the receiving end demodulation in the telephone receiver changes the electrical energy (signal) back to I. pressure wave in the air. More broadly defined, modulation is the process or result of the process whereby some parameters of one wave is varied in accordance with other waves. In the treatment of modulation, the word wave is used as a generic term intended to include such concepts as signal, voltage, current, pressure, displacement and the like, whether these are constant or changing. He explained that this broad definition of modulation may be illustrated by a familiar example of amplitude modulation which implies three fundamental concepts-modulating wave, carrier wave and the modulated wave. A modulating wave changes some parameters of the wave to be modulated, a carrier wave is the wave suitable for modulation by the modulating wave, the modulated wave has some parameters changed in accordance with the modulating wave.
Demodulation is extracting the original information-bearing signal from a carrier wave. A demodulator is an electronic circuit (or computer program in a software-defined radio) that is used to recover the information content from the modulated carrier wave.
A.Amplitude modulation
In amplitude modulation, the amplitude of a carrier is the parameter subjected to change by the modulating waves Alley maintained. He further stated that in a more restrictive sense, AM is defined to mean modulation in which the amplitude factor of the sine wave carrier is linearly proportioned to the modulating wave. The modulated wave is composed of the transmitted carrier, which conveys no information (apart from its amplitude, frequency and phase) plus the familiar upper and lower sidebands, which conveys identical and mutually redundant information.
B.Frequency modulation
This is angle modulation in which the instantaneous frequency of a sine wave carrier is caused to depart from the carrier frequency by an amount proportional to the instantaneous value of the modulating wave.
Fig 2.5: unmodulated carrier signal, message signal and Modulated carrier signal
2.3.7.NEED FOR MODULATION
1.Channel assignment (various information sources are not always suitable for direct transmission over a given channel
2.Reduce noise &interference.
3. Overcome equipment limitation
This chapter has reviewed different subjects as it pertain to radio signal transmission. The work is focused on constructing an FM transmitter that can solve the problem of information dissemination in an environment with a small area.
Chapter three
DESIGN
3.0 FM TRANSMITTER
3.1 INTRODUCTION
The FM transmitter mainly consist of pre-amplifier, FM modulator, oscillator, frequency multiplier and Power amplifier.
The figure 3.1 shows the block diagram of the FM transmitter and the required components of the FM transmitter are; microphone, audio preamplifier, modulator, oscillator, RF-amplifier and antenna. There are two frequencies in the FM signal, first one is carrier frequency and the other one is audio frequency. The audio frequency is used to modulate the carrier frequency. The FM signal is obtained by differing the carrier frequency by allowing the AF. The FM transistor consists of oscillator to produce the RF signal.
Fig.3.1: BLOCK DIAGRAM OF FM TRANSMITTER
3.2 FUNCTIONS OF THE FM TRANSMITTER BLOCK
The pre-amplifier boosts the signal level from several mili-volts(mv) to a higher enough stage for feeding into the modulator. Usually, a high pass filter network is added between the pre-amplifier and modulator stage. this high pass filter acts as pre-emphasis network to improve the signal to noise level of FM transmission at higher frequency. The pre-emphasis network is optional. However, the receiver will suffer from distortion at higher frequency of audio signal if this stage is ignored. With the carrier signal generated from oscillator, the Modulator modulates the carrier with input signals from the pre-amplifier stage.
The operating frequency of the generated FM output is still not high enough to be transmitted through space. Thus several stages of frequency multiplier are put to increase the operating frequency.
After going through a number of multipliers, the attention of signal level is compensated by the final stage power amplifier. Power amplifier restored the FM signal strength to the desired level.
The essential function of each circuit in the FM transmitter may be described as follows.
3.2.1 The Exciter
The function of the carrier oscillator is to generate a stable sine wave signal at the rest frequency, when no modulation is applied. It must be able to linearly change frequency when fully modulated, with no measurable change in amplitude. The buffer amplifier acts as a constant high-impedance load on the oscillator to help stabilize the oscillator frequency. The buffer amplifier may have a small gain. The modulator acts to change the carrier oscillator frequency by application of the message signal. The positive peak of the message signal generally lowers the oscillator's frequency to a point below the rest frequency, and the negative message peak raises the oscillator frequency to a value above the rest frequency. The greater the peak-to-peak message signal, the larger the oscillator deviation.
3.2.2 Frequency Multiplier
Frequency multipliers are tuned-input, tuned-output RF amplifiers in which the output resonant circuit is tuned to a multiple of the input frequency. Common frequency multipliers are 2x, 3x and 4x multiplication. Frequency multiplier is sometimes seen, but its extreme low efficiency forbids widespread usage. It should be noted that frequency multiplier, multiples only by whole number.
3.2.3 Power output section
The final power section develops the carrier power, to be transmitted and often has a low-power amplifier driven the final power amplifier. The impedance matching network is the same as for the AM transmitter and matches the antenna impedance to the correct load on the final over amplifier.
3.3 STAGES OF AN FM TRANSMITTER AND THEIR FUNCTIONS
3.3.1. OSCILLATION STAGE:An oscillator is a circuit which generates an AC output signal without requiring any external applied input signal. The main functions of all oscillators in this work it to produce sinusoidal wave shapes of a specific frequency and amplitude. In doing so, the stability of an oscillator is very important.
3.3.2. DC POWER SUPPLY: A DC power supply is an electric circuit that is used to convert an input AC voltage into a stable output voltage in the form of DC.The function of a DC power supply in this work is that it provides direct current.
3.3.3. MICROPHONE: A microphone is a form of an electromechanical transducer, it convert sound wave of varying air pressure into electric audio signal and voice (sound) is coupled into the system by the microphone. The function of a microphone here is to convert the pressure waves coursed by sound into vibrations within a coil which transforms the vibration.
3.3.4. MODULATOR:
3.3.5. AUDIO AMPLIFIER STAGE: The audio amplifier stage is a circuit of class – A amplifier designed to amplify very weak signal such as the audio frequency (AF) from the microphone output. This amplifier is necessary for the purpose of transmission.
3.3.6. RF AMPLIFIER: This frequency modulated signal is further amplified to a higher frequency before it is coupled to the Antenna for transmission. The class – C amplifier is used as it is the most suitable type of the amplifier for high power output at radio frequency. The function of RF amplifier in this work is that it improves (i.e. rejection of unwanted signal).
3.3.7. ANTENNA: Antenna is a device that radiates radio frequency energy in responses to an applied voltage and the associated alternating electric circuit in it and a voltage between its terminals. The main function of the antenna in this work is to radiate electromagnetic energy into space.
Fig 3.2: FM transmitter circuit
3.4 Circuit Design
This FM transmitter is surprisingly powerful despite its small component count and 9V operating voltage. It will easily transmit short range meters in the open air. It may be tuned anywhere in the FM band. The output power of FM transmitter is within the legal limits of the school environment. However, some countries may ban all wireless FM transmitters without a licence.
The first stage of the circuit is a preamplifier stage based on transistor Q1. This is a collector to base biased amplifier stage where resistor R2 sets the collector current and R1 provided the necessary collector to base bias. C1 is the input DC decoupling capacitor which couples the input audio signal to the Q1 base. C7 is the power supply bypass capacitor.
Next stage is the oscillator and modulator stage built around transistor Q2. Electrolytic capacitor C2 couples the output of the first stage to the second stage. R3 and R4 are the biasing resistors of Q2. R5 is the emitter resistor of Q2. Inductor L1 and trimmer capacitor C4 form the tank circuit which is necessary for creating oscillations. The modulated FM signal is available at the collector of Q2 and it is coupled to the antenna using capacitor C6.
3.5 The Short range FM transmitter construction
The FM transmitter was constructed on a printed circuit board PCB. Components were added to the PCB in any order. the microphone was inserted with the pin connected to the metal case connected to the negative rail (that is, to the ground or zero voltage side of the circuit). The coil is about 3mm in diameter and 5 turns. The wire is tinned copper wire, 0.61 mm in diameter. After the coil is soldered into place, the coils are spread apart about 0.5 to 1mm so that they are not touching. (The spacing in not critical since tuning of FM transmitter will be done by the trim capacitor. It is quite possible, but not as convenient, to use a fixed value capacitor in place of the trimcapacitor, and to vary the Transmission frequency is by simply adjusting the spacing of the coils. That is by varying L of the LC circuit rather than C.) Adding and removing the batteries acts as a switch.We Connected a half or quarter wavelength antenna (length of wire) to the aerial point. At an FM frequency of 106MHz these length 75cm.
Note:The led serves as an indicator, indicating that there is power supply present in the transmitter
3.6 Circuit Components
The basic components of a FM transmitter are:
i. Capacitor
ii. Resistors
iii. Transistors
iv. Diode
v. Antenna
vi. Condenser microphone
vii. Power supply (9v supply)
3.6.1 Capacitor
A capacitor is a two terminal, electrical component. Along with resistor and inductor, they are one of the most fundamental passive components we use. What makes capacitors special is their ability to store charges; they are like a fully charged electric batteries. Capacitors have all sort of critical application in a circuit. Common applications include local energy storage. Capacitance is it's unit. Not all capacitors are created equal,each caps is built to have a specific amount of capacitance. The capacitance of a capacitor tells you how much charge it can store; more capacitance means more capacity to store charge.The standard unit of a capacitance is called farad.
Fig 3.3 A FIXED CAPACITOR
3.6.2 Resistors
The resistor is a passive electrical component to create resistance in the flow of electric current. They can be found In almost all electrical networks and electronic circuits. The resistance is measured in ohms. An ohm is the resistance that occurs when a current of one ampere passes through a with a one volt drop across its terminals. The current is proportional to the voltage across the terminal ends. Resistors are used for many purposes. A few examples include delimit electric current, voltage division, heat generation, matching and loading circuits, control gain, and fix time constants. They are commercially available with resistance values over a range of more than nine orders of magnitude. They can be used to as electric brakes to dissipate kinetic energy from trains, or be smaller than a square millimeter for electronics.
Fig: 3.4 RESISTORS
3.6.3 Transistor
It is genaral purpose silicon,NPN bipolar junction transistor. it is used for amplification and switching purposges.The current gain may vary between 110 and 800. The maximum DC current gain is 800.Its equivalent transistors are 2N3904 and 2SC1815. These equivalent transistors however have different lead assignments,The variants of BC547 are 547A, 547B, and 547C. which vary in range of current gain and other characteristics. The transistor terminals require a fixed DC voltage to operate in the desired region of its characteristic This is known as the biasing. For amplification applications, the transistor is biased such that it is partly on for all input conditions" The input signal at base is amplified and taken at the emitter. BC458 is used in common emitter configuration for amplifiers" The voltage divider is the commonly used biasing mode. for switching applications, transistor is biased so that it remains fully on if there is a signal at its base 0n the absence of base signal,it gets completely off.
Fig 3.5: A TRANSISTOR
3.6.4 Inductor
inductor is a passive electronic component that storesenergy in the form of a magnetic field. In its simplest form, an inductor consistsof a wire loop or coil. The inductance is directly proportional to the number ofturns in the coil. Inductance also depends on the radius of the coil and on the type of material around which the coil is wound. For a given coil radius and number of turns, air coresresult in the least inductance. Materials such as wood, glass, and plastic - known as dielectric materials - are essentially the same as air for the purposes of inductor winding. Ferromagnetic substances such as iron, laminated iron, and powdered iron increase the inductance obtainable with a coil having a given number of turns. In some cases, this increase is on the order of thousands of times. The shape of the core is also significant. Toroidal (donut-shaped) cores provide more inductance, for a given core material andnumber of turns, than solenoidal (rod-shaped) cores. The standard unit of inductance is the henry, abbreviatedH. This is a large unit. More common units are the microhenry, abbreviated µH (1 µH =10-6H) and the millihenry, abbreviated mH (1 mH =10-3 H). Occasionally, the nanohenry (nH) is used (1 nH = 10-9 H).
Fig 3.6: AN INDUCTOR
3.6.5 Diode
A diode is a specialized electronic component with two electrodes called the anode and the cathode. Most diodes are made with semiconductor materials such as silicon, germanium, or selenium. The fundamental property of a diode is its tendency to conduct electric current in only one direction. It essentially acts as a one-way switch for current. It allows current to flow easily in one direction, but severely restricts current from flowing in the opposite direction. Most diodes allow current to flow only when positive voltage is applied to the anode.
LED- the light emitting diode helps us to indicate it there is a flow of current in the circuit.
Fig 3.7: A Diagram Diode
3.6.6 Condenser microphone
Microphone converts sound energy to electrical energy for further processing by the circuit. In this project, the microphone is connected to coupling compactor C1 to feed modulating signal to the base of the transistor Q1. With the microphone feed resistor R1 retained
Fig 3.8: A CONDENSE MICROPHONE
3.6.7 Power supply
The power supply is an indispensable part of any project. 9v battery to power the transmitter.
Fig 3.9: POWER SUPPLY
3.7 Tools and Instruments Used
1. A strip board cutter
2. A small hacksaw
3. A jackplane
4. A hammer
5. A multimeter
6. A soldering iron
7. A lead socker
8. A pair of long nose pliers
9. A light testing screw driver
3.8 Soldering board:
This, has pierced copper strips bonded to the board on one side to form the connections between the components. Leads from the components are inserted from the other side of the board and soldered in place on the copper side. When the copper strip has to be broken to prevent unwanted connections a strip board cutter is inserted into the hole where the break is required and rotated anticlockwise a few time. Alternatively an ordinary twist drill held between the fingers and rotated will do Components involved. That is the more the components, the larger the board used. However, the size of the strip board used depends on the sizes and number of the some circuits may be combined in one board when they do not require many components.
3.9 CASING
A plastic case was used to cover the FM transmitter circuit.
The plastic case was chosen to reduce interference, duct from damaging the components and to improve high quality production.
Chapter four
4. Implementation and result
4.1 Implementation
This chapter gives the details of how the short range FM transmitter was implemented and tested.
The various components of the various circuits of the system were laid out on the pcb as earlier explained and soldered. Each circuit was tested after construction for continuity. When it was confirmed that the soldering was alright, the component's terminals were then trimmed.
The circuit is basically a radio frequency (RF) oscillator that operates on a frequency of 106 MHz.. Audio signal picked up and amplified by the electret microphone is fed into the audio amplifier stage built around the first transistor. Output from the collector is fed into the base of the second transistor where it modulates the resonant frequency of the tank circuit (the 5 turn coil and the variable capacitor) by varying the junction capacitance of the transistor. Junction capacitance is a function of the potential difference applied to the base of the transistor. The tank circuit is connected in a Colpitts oscillator circuit.
The electrets microphone: an electrets is a permanently charged dielectric. It is made by heating a ceramic material, placing it in a magnetic field then allowing it to cool while still in the magnetic field. It is the electrostatic equivalent of a permanent magnet. In the electrets microphone a slice of this material is used as part of the dielectric of a capacitor in which the diaphragm of the microphone forms one plate. Sound pressure moves one of its plates. The movement of the plate changes the capacitance. The electrets capacitor is connected to a FET amplifier. These microphones are small, have excellent sensitivity, a wide frequency response and a very low cost. First amplification stage is a standard self-biasing common emitter amplifier. The 22nF capacitor isolates the microphone from the base voltage of the transistor and only allows alternating current (AC) signals to pass. The tank (LC) circuit: every transmission needs an oscillator to generate the radio Frequency (RF) carrier waves. The tank (LC) circuit, the transistor (BC547) and the feedback 5pF capacitor are the oscillator in the circuit. An input signal is not needed to sustain the oscillation. The feedback signal makes the base- emitter current of the transistor vary at the resonant frequency. This causes the emitter-collector current to vary at the same frequency. This signal fed to the aerial and radiated as radio waves. The 27pF coupling capacitor on the aerial is to minimize the effect of the aerial capacitance on the LC circuit. The name 'tank' circuit comes from the ability of the LC circuit to store energy for oscillations. In a pure LC circuit (one with no resistance) energy cannot be lost. The tank circuit does not oscillate just by having a DC potential put across it. Positive feedback must be provide.
Audio signal from the microphone is a very low level signal, of the order of milliolts. This extremely small voltage needs to be first amplified. A common emitter configuration of a bipolar transistor, biased to operate in class A region, produces an amplified inverted signal. Another important aspect of this circuit is the colpitt oscillator circuit. This is an LC oscillator where energy moves back and forth between the inductor and capacitor forming oscillations. It is mainly used for RF application. When this oscillator is given a voltage input, the output signal is a mixture of the input signal and the oscillating output signal, producing a modulated signal. In other words, the frequency of the oscillator generated circuit varies with the application of an input signal, producing a frequency modulated signal.
The drawing of the frequency modulated (FM) transmitter was accomplished. The schematic diagrams identifying the components and values of the various component circuits were produced and the various circuits were thus assembled into one composite circuit. After the testing and corrections, the circuits were found to be in order. However, the noise interference, persisted until the limiting adjustment which reduced it was made, thus solving the problem of noise in the transmitter. It should be noted that noise cannot be completely removed. The reduction in the noise level was reflected in the clearer reception of the transmitted message. This marked a remarkable improvement in the design and construction of a frequency modulated (FM) transmitter.
4.2. Testing
4.2.1. Transistor Testing
When constructing this FM transmitter testing was carried out on the transistors before, they were soldered on the printed circuit board. The conditions of the transistors needed to be verified because brand transistor like any other semiconductor device could be faulty from the shelf. Many technicians, technologists and engineers have complained that a new IC or transistor that they had fitted into a circuit was faulty. With the transistor out of the circuit, the diode junction of the transistors were checked with digital multimeter. For the NPN transistor used, the test was as follows:
The positive lead of the ohmmeter was connected to the base and the negative (common) lead was connected to the emitter. This forward biased the base/emitter junction, and recorded low reading. The negative terminal was then connected to the collector. This forward biased the base/collector junction and the ohmmeter recorded low reading. Then the ohmmeter reverse biased the two junction and both junctions read high. If one of the junctions read low in both conditions the junctions was shorted. If ohmmeter recorded high reading in both reverse and forward bias condition then the junction was open circuit and the transistor was bad.
Next, reading was made between the collector and emitter, the meter leads were reverse to make a second reading. Both of these readings should be high. If not, the transistor had a collector emitter junction short circuit short and was replaced. If the terminals were unknown, the base terminal was found by identifying which had a low reading in relation to the other two terminals. This terminal was the base. If the low readings were caused by the base being positive, the transistor was NPN. If the low readings were caused by base being negative, the transistor was PNP. The transistors was found to be in good condition after the test based on the specified criteria
4.2.2. Continuity Test
When all the components had been soldered on the printed circuit board (PCB) continuity test was carried out before the connecting the system to power supply. The test revealed open circuit faults, which could be due to:
1. A break in the circuit.
2. The failure of a component leading to it having an unusually high resistance
3. An increase in the insulation at certain points caused by dirt.
4. It also revealed that short circuit faults which were due to; the failure of a component leading to it having an abnormally low resistance.
5. The touching of uninsulated parts of the component terminal
6. The effects of moisture lying between conducting paths which create links of lower resistance than the component
Solder bridges between the terminals
Fig 4.1: FM TRANSMITTER CIRCUIT
4.2.3 Transmitter range testing
The FM transmitter was placed about 200m from an FM radio. The radio was set to 106MHz. The FM transmitter was then turned on. The winding of the coil was spread apart by approximately 1mm from each other. No coil winding was touching another winding. A small screw driver was used to tune the trim cap. The screwdriver was removed from the trim screw after every adjustment so the LC circuit is not affected by stray capacitance. Some difficulties was experienced in finding the transmitting frequency so another person tuned up and down the FM dial after every adjustment. One full turn of the trim cap covered its full range of capacitance from 6pF to 30pF. The normal FM band tunes in over about one tenth of the full range of the tuning cap. So it is best to adjust it in steps of 5 to 10 degrees at each turn. The reason that there must be at least 10 ft. separation between the radio and FM transmitter is that the FM transmitter emits harmonics; it does not only emit on one frequency but on several different frequencies close to each other.
4.3. Total cost of the project
Components
Amount (#)
R1-22kohms
50
R2-1mohms
50
R3-10kohms
100
R4-470ohms
100
R5-47kohms
100
C1-22nf
200
C2-100npf
200
C3-1nf
150
VC4-30pf
200
C5-5.6pf
100
C6-27pf
100
C7-22nf
150
L1-1mh
150
T1-BC547A
300
T2-BC547A
350
Led
100
.battery
150
Antenna
150
Miscellaneous
4000
Total
65000
Chapter five
Conclusion and recommendations
5.0 Conclusion
Consequent upon the increasing advances in communication technology, it has become very necessary to seek means of easing the problems associated with information dissemination in the local communities. A way that readily comes to mind is the use of a short-range frequency modulated radio transmitter. When FM receivers are placed in various classes and offices 100-200m away they can easily listen to the information passed via the FM transmitter at the information unit of the school.
So, far the test result of this project which is the outcome of construction procedures has revealed the successful achievement of the primary objective; the design and construction of a short range FM transmitter operating on 9V power supply. Because of the impressive good result, obtained from the usability test, the FM transmitter is now ready for either instructional or entrepreneur purposes. The successful completion of this work has indicated that practically a short range FM transmitter can be designed and constructed.
5.1 Recommendations
In order to reproduce this work there is need to be sure that:
1) Proper positioning of the components is made with specific attention to the pre- amplifier stage which must be shielded from external magnetic fields or influence.
2) Proper soldering to ensure that too little or too much solder is avoided.
3) The dumber of links are minimised as much as possible
4) Terminal pins and links are carefully trimmed and marked before breaking the tracks for separation.
5) Careful attention is given to polarised components such as transistors and capacitors.
Fig 5.1:FM Transmitter
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