Design And Construction Of A Four-Stage FM Transmitter

ABSTRACT

An FM transmitter is an electronic device which, with the aid of an antenna, produces 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.

FM transmitter is basically a VHF colipits oscillator capable of transmitting sound or music to any standard FM receiver. The circuit works on a D.C source which makes it for a pure rectification. FM transmitter also has a capacitor microphone which picks up very weak sound signals.

This FM transmitter circuit uses four radio frequency stages: a VHF oscillator built around transistor BF494 (T1), a preamplifier built around transistor BF200 (T2), a driver built around transistor 2N2219 (T3) and a power amplifier built around transistor 2N3866 (T4).

A condenser microphone is connected at the input of the oscillator. Working of the 1 Watt transmitter circuit is simple. When you speak near the microphone, frequency-modulated signals are obtained at the collector of oscillator transistor T1.  The FM signals are amplified by the VHF preamplifier and the pre-driver stage

TABLE OF CONTENTS

TABLE OF CONTENT

TITLE PAGE

APPROVAL PAGE

DEDICATION

ACKNOWLEDGEMENT

ABSTRACT

TABLE OF CONTENT

CHAPTER ONE

1.0      INTRODUCTION

1.1      AIM OF THE PROJECT

1.2      PURPOSE OF THE PROJECT

1.3      OBJECTIVE OF THE PROJECT

1.4      LIMITATIONS OF THE PROJECT

1.5      SCOPE OF THE PROJECT

1.6      APPLICATION OF THE PROJECT

1.7      PROBLEM OF THE PROJECT

1.8      DEFINITION OF TERMS

1.9      PROJECT ORGANISATION

CHAPTER TWO

2.0       LITERATURE REVIEW

2.1      HISTORITICAL BACKGROUND OF THE PROJECT

2.2     REVIEW OF PARTS OF RADIO TRANSMITTER

2.3      REVIEW OF FM BROADCAST BANDS

2.4     MODULATION AND DEMODULATION

2.5     PRE-EMPHASIS AND DE-EMPHASIS

2.6     REVIEW OF MODULATION CHARACTERISTICS

CHAPTER THREE

3.0      CONSTRUCTION

3.1      BASICS OF FM TRANSMITTER

3.2      BLOCK DIAGRAM OF AN FM TRANSMITTER

3.3      FUNCTION OF FM TRANSMITTER BLOCK

3.4      CIRCUIT DIAGRAM OF THE SYSTEM

3.5      CIRCUIT PARTS

3.6      CIRCUIT OPERATION

3.7      DESCRIPTION OF COMPONENTS USED

CHAPTER FOUR

4.0      RESULT ANALYSIS

4.1      CONSTRUCTION PROCEDURE AND TESTING

4.2      CASING AND PACKAGING

4.3      ASSEMBLING OF SECTIONS

4.4      TESTING OF SYSTEM OPERATION

4.5      INSTALLATION OF THE COMPLETED DESIGN

4.6      SYSTEM TROUBLESHOOTING METHOD

4.7      COST ANALYSIS

CHAPTER FIVE

5.1       CONCLUSION

5.1      RECOMMENDATION

5.2      BIBLIOGRAPHY

CHAPTER ONE

1.0                                                        INTRODUCTION

In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces 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, 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. Many other types of modulation are used.

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.

1.1                            BACKGROUND OF THE STUDY

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 necessities the application of radio transmitter and receiver.

Frequency modulation (FM) is a technique for wireless transmission of information where the frequency of a high frequency carrier is changed in proportion to message signal which contains the information (Chen, 2002). FM was invented and developed by Edwin Armstrong in the 1920’s and 30’s. Frequency modulation was demonstrated to the Federal Communications Commission (FCC) for the first time in 1940, and the first commercial FM radio station began broadcasting in 1945 (Mohn, 2007).

A radio transmitter is 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 is observed which is the totality of radio communication, frequency modulation (FM) transmitter is less distorted than other wave bands like amplitude modulation and short wave band. The frequency on the tuning dial ranges from 88MHZ to 108MHZ (Louis, 2008).

In telecommunications and signal processing, frequency modulation (FM) is the encoding of information in a carrier wave by varying the instantaneous frequency of the wave. This contrasts with amplitude modulation, in which the amplitude of the carrier wave varies, while the frequency remains constant. In analog frequency modulation, such as FM radio broadcasting of an audio signal representing voice or music, the instantaneous frequency deviation, the difference between the frequency of the carrier and its center frequency, is proportional to the modulating signal. In radio transmission, an advantage of frequency modulation is that it has a larger signal-to-noise ratio and therefore rejects radio frequency interference better than an equal power amplitude modulation (AM) signal. For this reason, most music is broadcast over FM radio. However, the concept of FM is essential to a wide gamut of radio frequency wireless devices and is therefore worth studying (“The Future of Radio”, 2008).

1.1                                              PURPOSE OF THE PROJECT

The main purpose of this work is to construct a device that will allow music or other audio content from a portable media player, CD player or other portable audio system to be played through a nearby FM radio. It can either be a capability built into a portable media player, or a portable appliance that plugs into the headphone jack or proprietary output port of a portable media player or video device. The sound is then broadcast through the transmitter on an FM broadcast band frequency and picked up by the receiver. The purpose is generally to allow portable audio devices to make use of the better sound quality of a home audio system or car stereo without requiring a wired connection between them.

The FM transmitter plugs into the audio output of audio devices and converts the audio output into an FM radio signal, which can then be picked up by appliances such as car or portable radios.

1.2                                                   AIM OF THE PROJECT

The aim of this work is to design and construct an FM transmitter which transmits a frequency modulated signal to an fm radio. That is, to construct a device that can broadcast signals on an FM broadcast band and picked up by the receiver.

1.3                                             OBJECTIVE OF THE PROJECT

The objectives of this project are:

1. The objective of this project is to build and examine the workings of a FM Radio transmitter.
2. To study the circuit and determine how the different parts of the circuit function together to make an FM transmitter

• To understand about the concept of FM transmitter.

1. To be familiar with the use of design and simulation tools in the design process.
2. To be able to construct, analyze and test the complete project of FM Transmitter designed. In this part of objective the students are required to solve the problem occurred since the circuit does not work as planned earlier. Some alternative and creativity from the student are
3. To review some modern digital technologies that has been developed for effective FM signal generation.

• To show the design and components of a FM transmitter
• To show the construction process of an electronically operated system known as FM transmitter capable of transmitting a frequency modulated signal.

1.4                                          LIMITATIONS OF THE PROJECT

The relatively low power output of FM transmitters sometimes makes it unsuitable for use in some large urban areas because of the number of other radio signals. This is compounded by the fact that strong FM signals can bleed over into neighboring frequencies making the frequencies unusable with the transmitter. Removing a car’s radio antenna has been found to significantly improve transmitter reception. Some frequencies below 88.1 have even been supported as reception frequencies on some car stereos, and some indegeneous FM transmitters even take advantage of those unused frequencies which are generally more reliable as no frequency below 88.1 is used for mainstream broadcasters in the US.

Some models which connect via ports other than the headphone jack have no means of controlling the volume, which can force the sound to transmit out from the device harshly (causing over modulation, audio distortion and possible radio interference), or too low. In theory a device could use an automatic level control or audio limiter circuit to overcome this problem although there are few (if any) devices with such a facility available out on the market yet.

1.5                                                  SCOPE OF THE PROJET

• FM transmitter is basically a VHF colipits oscillator capable of transmitting sound or music to any standard FM receiver. The circuit works on a D.C source which makes it for a pure rectification. FM transmitter also has a capacitor microphone which picks up very weak sound signals, and discrete components such as: Transistor, Diode, Resistor, Capacitor and Inductor.
• This project transmits frequency on 100MHz± 5%. The transmitting distance is l00metres. It is powered with 5volt – 15volts D.C battery using suitable F.M receiver tuned to the transmitting frequency of this project. F.M Transmitter can be used as cordless microphones, mobile phone and for public address purposes.
• This project work covers the design and construction of FM transmitters for quality audio transmission and explains some of the modern trends in FM signal generation, highlighting their prospects. It also covers the advantages these technologies offer over traditional radio broadcasting and brings to light various distinguishing features possessed by these technologies

1.6                                    APPLICATION OF THE PROJECT

• FM transmitters used in many different environments. Some of the more popular uses are churches, sporting events, fitness centers, homes, cars, correctional facilities and holiday light shows.

1.7                                           SIGNIFICANCE OF THE STUDY

The project signifies a lot in the electronic communication system which telecommunication is the vital aspect which is usually demonstrated through radio communication system the frequency modulation transmitter is applied in a lot of instance frequency modulation is used in FM radio stations scattered all over the country whose advantage is paramount compared to its counterpart AM modulation frequency modulation transmitted is equally used in a miniaturized from as wireless morpheme.

This project is an alternative for people especially for workers in a company to listen or  record  a  telephone  conversation  while  informing  all  involved   parties.   There are many legitimate reasons for wanting to broadcast a FM radio that acts as a receiver. This is very important for information or entertainment lines and everybody who wants to hear the message.

1.8                                                  DEFINITION OF TERMS

FM    –                  Frequency Modulation

VHF  –                  Very High Frequency (30MHz to 300MHz)

UHF  –                  Ultra High Frequency (300MHz to 3GHz)

VFO  –                  Variable Frequency Oscillator

VCO –                  Voltage Controlled Oscillator

PLL  –                  Phase Locked Loop

Oscillator –                    device that generates a frequency

1.9                                              PROBLEM OF THE PROJECT

Each project usually will face several limitations that might occur while running its process of the project. There are several limitations occur along the implementation of this project:

• The electronic components with the recommended value such, as transistor is hard to get due to matching in the circuit of FM
• The process of mounting the component at Project Circuit Board (PCB) is hard to do since the tracks at the circuit board are very
• The process of tuning to the proper frequency to broadcast at FM radio is hard to get since the problem it’s related with the components and the circuit

1.10                                      PROJECT WORK ORGANISATION

The various stages involved in the development of this project have been properly put into five chapters to enhance comprehensive and concise reading. In this project thesis, the project is organized sequentially as follows:

Chapter one of this work is on the introduction to an fm transmitter. In this chapter, the background, aim, purpose, significance, objective limitation and problem of an fm transmitter were discussed.

Chapter two is on literature review of an fm transmitter. In this chapter, all the literature pertaining to this work was reviewed.

Chapter three is on design methodology. In this chapter all the method involved during the design and construction were discussed.

Chapter four is on testing analysis. All testing that result accurate functionality was analyzed.

Chapter five is on conclusion, recommendation and references.

1 Watt FM Transmitter Project

This FM transmitter circuit uses four radio frequency stages: a VHF oscillator built around transistor BF494 (T1), a preamplifier built around transistor BF200 (T2), a driver built around transistor 2N2219 (T3) and a power amplifier built around transistor 2N3866 (T4).

A condenser microphone is connected at the input of the oscillator. Working of the 1 Watt transmitter circuit is simple. When you speak near the microphone, frequency-modulated signals are obtained at the collector of oscillator transistor T1.

The FM signals are amplified by the VHF preamplifier and the pre-driver stage. You can also use transistor 2N5109 in place of 2N2219. The preamplifier is a tuned class-A RF amplifier and the driver is a class-C amplifier. Signals are finally fed to the class-C RF power amplifier, which delivers RF power to a 50-ohm horizontal dipole or ground plane antenna.

Use a heat-sink with transistor 2N3866 for heat dissipation (Note: or 2N4427 because it works better at 12 V and delivers up to 1 watt RF power). Carefully adjust trimmer VC1 connected across L1 to generate frequency within 88-108 MHz. Also adjust trimmers VC2 through VC7 to get maximum output at maximum range.

Regulator IC 78C09 provides stable 9V supply to the oscillator, so variation in the supply voltage will not affect the frequency generated. You can also use a 12V battery to power the circuit. Assemble the circuit on a general purpose PCB. Install the antenna properly for maximum range.

Coils L1 through L5 are made with 20 SWG copper-enamelled wire wound over air-cores having 8mm diameter. They have 4, 6, 6, 5 and 7 turns of wire, respectively.

Note: This transmitter is meant only for educational purposes. use of this transmitter with outdoor antenna is illegal in most parts of the world.

FM Transmitter circuit diagram

Source: Electronics For You Magazine