Andrew RobsonTom HopwoodTracey Desalu

Alex Ha

Year 1 Media Technology

TECH 1002 Studies in Media Technology

The AM Receiver and Audio Amplifier Project

Andrew ClayThursday 2pm-3pm

CONTENTS

Page 2 - ContentsPage 3 - IntroductionPage 4 - AM radio receiver – equipment and components Page 5 - AM radio receiver – methodPage 6 - AM radio receiver – tuning, demodulation, and amplificationPage 7 - AM radio receiver – testing and resultsPage 8 - Audio Amplifier – equipment and components Page 9 - Audio Amplifier – testing, results, transistor and loudspeakerPage 10 - Audio Amplifier - loudspeaker Page 11 - AM transmission demonstrationPage 12 - ConclusionPage 13 - Bibliography

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INTRODUCTION

The aim of this project is to gain an understanding of how an AM radio and an Audio Amplifier

works, by building two circuits from electrical components supplied. The experience of doing this will

give a greater understanding to how a radio receives a signal and what it does to this to create an

audio signal that can be heard.

With the use of ongoing research and practical lab work, the theory of AM reception of a receiver

will be explained through a description of any results gained by testing these circuits.

Attention will be drawn to particular components of the circuits and how they operate within the

system and will also include a description of the two key components in the amplifier circuit and how

they work.

Finally, an explanation will be given to how AM transmission works paying attention to a

demonstration that was given in the university labs.

This report is also given as an online report using a number of resources on the Internet. By visiting

the website shown below, access to this report will be given.

https://sites.google.com/site/radio2radioproject/home

Throughout the project, members of the group were asked to research ideas and theory with

regards to AM radio and amplification.

This research can be found in the Group Wiki section of ‘Studies in Media Technology’ under the

heading – Radio Technology – Group Research

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AM Radio Receiver

Aims and Objectives

The aim is to gain practical experience of the AM radio reception system relating to the technique of

amplitude modulation by building and testing an AM radio.

The theory of AM reception will be applied to the actual sub-system of the receiver and results

gained by the testing will be explained.

Equipment and Components used –

Soldering equipment set

Printed circuit board

IC MK484, BC548B transistor, 150pF variable tuning capacitor, 100mm long

ferrite rod, 2.5m of 0.315mm enamelled copper wire, two 100k resistors, two

1k resistors, one 270R resistor, miniature slide switch, two 100nF capacitors,

one 10nF capacitor, one 47uF capacitor, headphone socket, AA battery

holder.

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Method

Using the enamelled copper wire and the ferrite rod, the aerial was first to be constructed.

Wrap a piece of card around the ferrite rod, to ensure movement of the wire and then wind the wire

around this approximately 55 turns, taking care not to overlap the windings. Use insulating tape to

keep the wire in place while you are doing this. The quality of reception may depend on the care

taken at this stage. Leave approximately 100mm of wire at each end of the coil to attach to the

PCB.

Using the soldering equipment, the resistors were soldered into place, followed in order by the small

capacitors, transistor, Integrated circuit (MK484), and the variable tuning capacitor. Connecting wire

was attached to the headphone socket, which was then soldered onto the PCB. The ends of the

aerial were rubbed with emery paper to ensure they were free of enamel and then soldered into

place. Finally, the AA battery holder was attached.

Testing and Results

With the radio circuit now completed, place an AA battery into the battery holder and plug some

headphones into the headphone socket. With power going to the circuit and by altering the value of

the variable capacitor, the frequency that the circuit responds to can now pick up a radio station.

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TUNING

The circuit that has now been built has 3 different sections, each doing a different job. Firstly the

variable capacitor picks up one signal from thousands that are given off from a transmitter. It is this

transmitter that has received the electrical signal from the radio station, which started off as an

audio signal (such as a DJ voice or music) and then combined with a carrier wave which has the job

of transmitting this signal as an electromagnetic wave. It is the carrier waves peaks that are made

bigger and smaller and therefore its amplitude (or size) is increased. This is Amplitude Modulation.

The radio circuits TUNING section (variable capacitor & ferrite rod) picks up this signal.

Fig 1 shows a diagram of a signal (audio) and the carrier wave (or sine wave) and then the

combining of the two (modulation)

Fig 1

DEMODULATION

After the radio circuit has received the electrical signal, this needs to be changed back into audio.

Because this signal was first MODULATED as it combined with the carrier wave, it now needs to be

DEMODULATED. This is where the original sound is extracted form the carrier wave and is done by

the Integrated Circuit MK484 component.

AMPLIFICATION

The signal (or sound) that has been removed from the carrier wave is very weak and needs to be

amplified so we can hear it. The BC548 transistor does this and by sending this audio signal to

speakers or headphones can be heard clearly.

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Due to time constraints, although the radio worked and a radio station was picked up the frequency

at which this station was operating on was unknown. However, according to past experiments and

other groups, the station would have been one of two local stations.

837 Asian Network or 1260 Sabras

Referring to http://frequencyfinder.org.uk/tc/leicester.html and looking at radio stations in the local

area, it shown that these two stations and their transmission signals were being sent from a

transmitter less than a mile away at Freeman’s Common (Leicester).

As a group we asked why it was only these two stations we could pick up when a number of them

on the list of stations were in between the frequency range of 837MHz and 1260Mhz.

A number of reasons could determine this –

The quality of the coil aerial that was made

The strength of signal that the aerial was trying to pick up

The location of the transmitter in relation to where the radio circuit is

Weather conditions

Buildings can affect a signal received, so being inside may make a difference

Both the local Asian radio stations mentioned are being transmitted locally, which has a low

transmission rate, so those stations will only be available in a local area unless transmitted digitally.

According to http://www.mediumwaveradio.com/uk.php BBC Asian Network transmits at 50 Watts

and Sabras 290 Watts.

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Audio Amplifier

Aims and Objectives

The aim is to gain experience of an audio amplification system by means of building a circuit with

electrical components and results gained by testing it with the radio circuit will be explained.

Equipment and Components used –

Soldering equipment set

Printed circuit board

IC TBA 820M (Integrated circuit), three 1K resistors, two 470R resistors, one

1R resistor, two 330uF electrolytic capacitors, one 10uF electrolytic capacitor,

one 150pF ceramic capacitor, one 100nF polyester capacitor, one 47nF

polyester capacitor, one 8Ohm speaker, 100mm of speaker cable, one slide

switch, one PP3 battery snap, one 3.5mm plug to plug cable, one 3.5mm

stereo jack socket

.

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Method

Solder all the electrical components into place ensuring ones with positive and negative connections

are fixed correctly. Solder connecting wire onto the 8 Ohm speaker and connect to the circuit. The

final component to be fitted is the TBA820M, which can be placed in the IC socket. Attach the

battery and connect to the radio circuit using the 3.5mm plug to plug cable.

Testing and Results

When the audio amplifier circuit is completed, connect to a battery and to the radio circuit. At first no

signal was amplified which may have been to do with the quality of soldering etc…but after a

moment a faint signal was heard. The circuit constructed is capable of 1.2 Watts output, but despite

the correct procedure in the soldering and positioning of components, the audio signal amplified

was very faint. The fine-tuning of the radio didn’t make a lot of difference to this, although a signal

was coming through which proved that it was working.

The job of an amplifier (in this case) is to receive an electrical signal from a radio, which has been

de-modulated and already amplified, and to amplify it even more so it can be heard through a

loudspeaker.

Transistor and Loudspeaker

The main component in the circuit is a transistor, which takes the input current (small current) and

allows it to control the size of the output current. This output current needs constant power to

continually flow which is provided by a power supply (battery or mains). It is the output current that

is the amplified signal.

Amplifier circuits can contain many transistors to do this or these can be combined into an

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integrated circuit, and therefore a single component. In the case of the amplifier circuit made in this

project a TBA820M integrated circuit was used in conjunction with an 8 Ohm loudspeaker to

produce the sound, which has been converted from an electrical signal.

It is this electrical (audio) signal coming from the amplification circuit, which flows through a fixed

magnet and an electromagnetic (or voice coil). The signal is an alternating current, and therefore

changing direction all the time caused by polarity of the magnets and in turn moves the voice coil

quickly, back and forth.

This will then vibrate the air in front of the speaker and create sound waves that will travel through

the air to the ear.

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AM Transmission Demonstration

Equipment

Function Generator (which generates the signals)

Oscilloscope

A wire aerial

An AM radio receiver

The Aim

To generate an AM signal and pick it up on the radio and to prove that is how an AM transmission signal is produced.

Testing and Results

The Carrier wave used was 600 KHz (which is in the medium wave band), and combining it

with an audio information signal, produced inside the generator, which is 500Hz, then

modulated this carrier wave.

The results of this could be seen on the Oscilloscope. The line on the outside was the

500Hz signal, and the one on the inside moving rapidly was the 600KHz original wave.

The last thing that needed to be done was to prove that the audio information signal is being

transmitted, by receiving it on a radio. The radio was switched on and then tuned to 600KHz

in the AM band. A sound could then be heard which was the 500Hz signal. If it were a

human voice, the amplitude would change so it wouldn’t be the same regular sound. To

prove that was what was being picked up, the signal was switched off and then the sound

produced from the radio stopped.

This was proof that what was being transmitted from the generator, via the aerial, was being

picked up on the radio.

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CONCLUSION

Two simple electronic circuits were built and the testing of the results gained from these helped to explain their inner workings.

Looking at the radio circuit, and starting with an audio signal at a radio station its journey was followed through a number of stages, which were -

Modulation Amplification Transmission Tuning Demodulation Amplification

The results obtained by testing the circuit and actually hearing what radio station (or frequency) was picked up, helped with the explanation to how this whole system works.

The building of an audio amplifier and the testing gave experience of how this system also works, paying particular attention to the two key components, the transistor (or IC component) and the loudspeaker. By doing this an understanding of how these actually work was gained.

The explanation of AM Transmission was given after a demonstration was shown in the university labs. The objective here was to prove the theory of AM modulation for a radio transmission.

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BIBLIOGRAPHY

Woodford, Chris (2007) ‘Radio’ at http://www.explainthatstuff.com/radio.html (accessed 27th January 2010)

Radio communication at http://www.ofcom.org.uk/sitefinder/glossary/how/ (accessed 27th January 2010)

Wikipedia (2010) Carrier Wave at http://en.wikipedia.org/wiki/Carrier_wave (accessed 27thJanuary 2010)

Brain, Marshall ‘How Radio Works’ at http://electronics.howstuffworks.com/radio8.htm (accessed 27th January 2010)

Frequency Finder at http://frequencyfinder.org.uk/tc/leicester.html#am_top (accessed 29th January 2010)

Harris, Tom ‘How Amplifiers Work’ at http://electronics.howstuffworks.com/amplifier.htm (accessed 29th January 2010)

AW ‘Audio Amplifiers 582’ at http://www.howeverythingworks.org/pages.php?topic=audio_amplifiers&page=2 (accessed 29th January 2010)

SatCure ‘How do Transistors Work?’ at http://www.satcure-focus.com/tutor/page4.htm (accessed 29th January 2010)

eHow ‘ How do Amplifiers Work?’ at http://www.ehow.com/how-does_4587836_amplifiers-work.html (accessed 29th January 2010)

DJ Society ‘ Everything you wanted to know about speakers’ at http://www.djsociety.org/Speaker_1.htm (accessed 30th January 2010)

Harris, Tom ‘How Speakers Work’ at http://electronics.howstuffworks.com/speaker6.htm (accessed 30th January 2010)

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‘Image of Carrier Wave’ at http://www.yourdictionary.com/images/computer/_PMMOD.GIF (accessed 27th January 2010)

‘Image of a Speaker’ at http://p-hardware.blogspot.com/2008/04/speaker.html (accessed 30th January 2010)

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