ESTD - 1999 - ISO 9001:2000 SECTOR – 4, ROURKELA – 769002 ( AFFILIATED TO BPUT , ROURKELA )

CERTIFICATE

This is to certify that Sonali Mishra, Balaram Panda, Sanyasi Barad, Pulkeshu Dash, Gaurav Singhdeo,Sambit Padhan, Ipsa Nayak students of Padmanava College of Engineering, Sector-4, Rourkela-002, successfully completed a project “Low Power AM Transmitter” in 7th semester at Department of Electronics & Telecommunication Engineering.

This report has not been submitted to any other Organization & does not form part of

any Course undergone by then, for the award of B-Tech Degree.

Head of Dept (ETE) Project GuideProf (Dr).K.C.Mohapatra Ms Suman Bala

By:-Sonali Mishra 0401216004Balaram Panda 0401216105Sanyasi Barad 0401216106Pulkeshu Dash 0401216107Gaurav Singhdeo 0401216108Sambit Padhan 0401216109 7 Th Semester Ipsa Nayak 0401216004 Electronics & Telecommunication Engineering

Acknowledgement Here I would like to extend my heartiest obligations to the following person for their sincere help and co-operation during the project presentation without whose help this report would have never been completed.

Much credit and heartful thanks are owed to Prof. (Dr).K.C.Mohapatra HOD of Electronics Department of my Institution, Padmanava College of Engineering, Rourkela for encouraging and allowing me to present the project on the topic “Low Power AM Transmitter “at our department premises for

the partial fulfillment of the requirements leading to the award of B-Tech degree.

I am indebted to Lect. Suman Bala for assisting me to prepare the project.

Also I would like to express my deep sense of gratitude and thanks to all the faculty member of Electronics department of PCE for their kind co-operation and assistance throughout the modeling of the project.

Last but not the least I would like to extend a special word of thanks to all my friends and well wishers for giving me a helping hand whenever needed and for making the project a grand success.

Sonali Mishra 0401216004Balaram Panda 0401216105Sanyasi Barad 0401216106Pulkeshu Dash 0401216107Gaurav Singhdeo 0401216108Sambit Padhan 0401216109 Ipsa Nayak 0401216004

CONTENTS

1) Objective.

2) Introduction.

3) Materials & Equipments.

4) Experimental Procedure.

5) Working.

6) Testing the Transmitter.

7) Conclusion.

8) Bibliography.

Objective

The goal of this project is to build a simple AM radio transmitter and to test its broadcast range with a radio receiver.

In this project, we will make a simple low - power broadcast circuit, using a crystal oscillator integrated circuit and an audio transformer. We can connect the circuit to the headphone jack of a portable music player (e.g. mp3, CD or cassette tape player). We'll see that we can receive the signal through the air with an AM radio receiver. Although the circuits used in radio stations for AM broadcasting are far more complicated, this nevertheless gives a basic idea of the concept behind a broadcast transmitter.

Introduction

Electromagnetic (EM) radiation is pretty much all around us. For e.g., light is electromagnetic radiation and so are x-rays. When we listen to an AM or FM radio station, the sound that we hear is transmitted to our radio by the station using EM radiation as carrier—radio waves.

Electromagnetic radiation is a propagating wave in space with electric and magnetic components. In a vacuum, electromagnetic waves travel at the speed of light.

Electromagnetic waves such as light, x-rays, and radio waves are classified by their frequency or wavelength. For e.g. EM radiation at frequencies between about 430 THz and

750 THz can be detected by the human eye and are perceived as light.

EM radiations at frequencies ranging from 3 Hz to 300 GHz are classified as radio waves. Radio waves are divided into many sub-classifications based on frequency. AM radio signals are carried by medium frequency (MF) radio waves (530 to 1710 KHz), and FM radio signals are carried by very high frequency (VHF) radio waves (88 to108 MHz).

So how does a radio wave carry sounds such as voice or music to your radio receiver?

The radio station broadcasts a carrier wave at the station’s assigned frequency. The carrier wave is modulated (varied) in direct proportion to the signal (e.g., voice or music) that is to be transmitted. The modulation can change either the amplitude or the frequency of the carrier wave. The “AM” in AM radio stands for “amplitude modulation," and the “FM” in FM radio stands for “frequency modulation." A radio receiver removes the carrier wave and restores the original signal (the voice or music). Figure 1, below shows graphically how amplitude modulation works.

Figure 1. Illustration of amplitude modulation of a carrier wave by a signal. The top diagram shows a carrier wave at a set frequency and amplitude and a signal to be broadcasted. The signal is used to modulate the amplitude of the carrier wave. The bottom diagram shows the resulting output signal . Note how the peaks of the output trace (its envelope) follow the form of the input signal.

Materials and Equipment

To do this experiment we will need the following materials and equipment:

Crystal oscillator

The oscillator is the heart of the transmitter. It has four leads, but we only use three of them. When the power is connected to two of the leads, the voltage on third lead starts jumping betw-een 0 volts and 5 volts, one million times each second.

Solder less breadboard, 1000 ohm to 8 ohm audio transformer

The other main part is the audio transformer. In this circuit it is used as a modulator. The modulator changes the strength of the radio waves to match the loudness of the music or voice we want to transmit.

The transformer has two leads on one side, (red and white in the picture) and three leads on the other side (blue, black and green in the picture). The two leads are the low impedance side of the transformer, (the 8 ohm side). The three leads are the high impedance side (the 1000 ohm side). The middle of the three leads is called the center tap, and we won't be using it in this circuit.

1/8 inch mono phone plug, A 6 V AA battery holder (holds four batteries), Four 1.5 V AA batteries, A set of alligator jumpers, Jumper wires for breadboard.

Experimental Procedure

Figure 2, below, shows the connections we need to make to build the circuit. The transformer isolates our music player

from the rest of the circuit, and also amplifies the signal from our music player. The amplified signal from the secondary coil of the transformer modulates the power to the oscillator chip (+ power at pin 14 and − power at pin 7). A wire connected to the oscillator output (pin 8) serves as the antenna for broadcasting the amplitude-modulated radio wave.

Figure 2. Simple AM transmitter circuit diagram. The square corner of the oscillator corresponds to pin 1. The pins are numbered according to standard positions for a 14-pin integrated circuit.

Building the Circuit!

1. Two alligator jumpers were used to connect the terminals of the phone plug.

2. The other ends of the alligator clips were connected to the 8 ohm side of the transformer (red and white wires).

3. The oscillator was inserted across the gap in the breadboard, so that pins 1 and 7 were on one side of the gap, and pins 8 and 14 were on the other. We can identify pin 1 of the oscillator because it is next to the square corner (the other three corners are rounded).

4. The breadboard was used to connect the + (red) and − (black) terminals of the battery holder and the 1000 ohm side of the transformer (blue and green wires) as shown in the diagram and in the picture below. Note that the 1000 ohm side of the transformer has a center tap (black wire) which is not used in this project.

5. A long jumper wire was connected to the output of the crystal oscillator (pin 8) which serves as the antenna.

6. The connections were doubly-checked to make sure that all of the connections corresponded to the circuit diagram.

This picture is a detailed view of the completed circuit on the breadboard. On the Radio Shack audio transformer, the blue and green wires are the 1000 ohm side, and the red and white wires are the 8 ohm side.The 8 ohm side of the transformer is connected to the phone plug (yellow oval). The 1000 ohm side of the transformer is connected to the positive terminal of the battery pack ("+6 V") and the oscillator, pin 14 (blue oval). The black wire from the transformer is a center tap from the 1000 ohm side and is not used in this project. Pin 7 of the oscillator is connected to the negative terminal of the battery pack ("ground"). The wire from pin 8 of the oscillator is the antenna.

WORKING

The oscillator is connected to one end of a long wire antenna. It alternately applies 9 volts of electricity to the end of the wire, and then 0 volts, over and over again, a million times each second.

The electric charge travels up and down the wire antenna, causing radio waves to be emitted from the wire. These radio waves are picked up by the AM radio, amplified, and are used to make the speaker cone move back and forth, creating sound.

The sound source (CD player or tape recorder) is normally connected to drive a speaker or earphone. It drives the speaker by emitting electricity that goes up and down in power to match the up and down pressure of the sound waves that were recorded. This moves the speaker in and out, recreating the sound waves by pushing the air in and out of our ears.

Sound waves

In our transmitter, the sound source is connected to the transformer instead of to a speaker.

The transformer is connected to the power supply of the oscillator. The sound source causes the transformer to add and subtract power from the oscillator, just as it would have pushed and pulled on the speaker.

As the power to the oscillator goes up and down, the power of the electricity in the antenna goes up and down also. The voltage is no longer simply 9 volts. It is now varying between 0 volts and 10 volts, because the power from the

transformer adds and subtracts from the power of the battery.

Power into antenna

The varying power in the antenna causes radio waves to be emitted. The radio waves follow the same curves as the waves in the antenna. However, because the transmitter and the receiver are not connected, the receiver does not know what the transmitter is using for the value of zero. All the receiver sees is a radio wave whose amplitude is varying. In the receiver, zero is the average power of the wave. This makes the wave look like this:

Radio waves in free space

If we sent this wave to the earphone, we would hear nothing, because the average power is zero. This is why our crystal radio has a diode.

The diode does a neat little trick. A diode only lets electricity flow in one direction. This means that the part of the graph where the power is rising up from zero can get through the diode, but the part where the power is going down from zero is blocked.

Electrical signal after the diode

All those little peaks of power happening a million times per second are too fast for human ears, and too fast for the earphone to reproduce. But since they are all pushing on the earphone diaphragm, all those little pushes add up, and the earphone moves. Since some of the little pushes are stronger than others (taller black bars in the illustration) they move the earphone more than the weaker ones. We hear this variation as sound.

Sound waves reaching our ears

The sound is a faithful reproduction of the original sound wave at the transmitter.

Testing the transmitter

We are now ready to test the transmitter.

The phone plug is plugged into the earphone jack of convenient sound source, such as a transistor radio, tape player, or CD player.

The batter is plugged into the batter clip.

The transmitter is placed near an AM radio, and is tuned to 1000 Khz, so that we can hear the sound source in the AM radio. The volume controls on the sound source are adjusted and on the AM radio to get the best sound.

Without any connection to any antenna or a good ground connection, the transmitter will only transmit to a receiver a few inches away. To get better range, the ground wire is clipped to a good ground, such as a cold water pipe, and the antenna to a long wire, like the one we used for the crystal radio.

CONCLUSION

A Transmitter plays a very vital role in radio communication and the use of crystal provides it with a great deal of precision. The use of this crystal makes life much easier for both, sender and receiver. This project is meant for the complete practical understanding of different types of modulation techniques which are being applied to a weak baseband signal.

This project also beautifully defines how electromagnetic radiations help carrier radio waves to transmit different information carrying signals through long distances.

Bibliography

This site has cool way of explaining electromagnetic phenomena. Electromagnetic radiation and waves:Goldman, M.V., et al., date unknown. "Electromagnetic Waves," Physics-2000, University of Colorado, Boulder [accessed April 10, 2006] http://www.colorado.edu/physics/2000/waves_particles/index.html.

Amplitude modulation: o This webpage has an applet that lets you play with

carrier and modulating signal to produce AM waves:Nyack, C.A., 1996. "Amplitude Modulation," Cuthbert Nyack [accessed April 10, 2006] http://cnyack.homestead.com/files/modulation/modam.htm

o Wikipedia contributors, 2006a. "Amplitude Modulation," Wikipedia, The Free Encyclopedia [accessed April 10, 2006] http://en.wikipedia.org/w/index.php?title=Amplitude_modulation&direction=next&oldid=44559258.

Information on crystal oscillators: Wikipedia contributors, 2006b. "Crystal Oscillator," Wikipedia, The Free Encyclopedia [accessed April 10, 2006] http://en.wikipedia.org/w/index.php?title=Crystal_oscillator&oldid=46562927.

Information on AM (medium wave) radio:Wikipedia contributors, 2007. "Medium wave," Wikipedia, The Free Encyclopedia [accessed January 24, 2007] http://en.wikipedia.org/w/index.php?title=Mediumwave&oldid=102931548.