The AM Receiver and Audio Amplification Project


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A Year 1 Media Technology report looking at AM Transmission and Audio amplification.
This written report was submitted as well as an online version which can be found at
January 2010

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The AM Receiver and Audio Amplification Project

  1. 1. Andrew Robson Tom Hopwood Tracey Desalu Alex Ha Year 1 Media Technology TECH 1002 Studies in Media Technology The AM Receiver and Audio Amplifier Project Andrew Clay Thursday 2pm-3pm CONTENTS
  2. 2. 2 Page 2 - Contents Page 3 - Introduction Page 4 - AM radio receiver – equipment and components Page 5 - AM radio receiver – method Page 6 - AM radio receiver – tuning, demodulation, and amplification Page 7 - AM radio receiver – testing and results Page 8 - Audio Amplifier – equipment and components Page 9 - Audio Amplifier – testing, results, transistor and loudspeaker Page 10 - Audio Amplifier - loudspeaker Page 11 - AM transmission demonstration Page 12 - Conclusion Page 13 - Bibliography 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
  3. 3. 3 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. 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 AM Radio Receiver Aims and Objectives
  4. 4. 4 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. 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
  5. 5. 5 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. 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
  6. 6. 6 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.
  7. 7. 7 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 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 BBC Asian Network transmits at 50 Watts and Sabras 290 Watts. Audio Amplifier Aims and Objectives
  8. 8. 8 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 . 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
  9. 9. 9 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 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
  10. 10. 10 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. AM Transmission Demonstration
  11. 11. 11 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. CONCLUSION
  12. 12. 12 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. BIBLIOGRAPHY Woodford, Chris (2007) ‘Radio’ at (accessed 27th January 2010)
  13. 13. 13 Radio communication at (accessed 27th January 2010) Wikipedia (2010) Carrier Wave at (accessed 27th January 2010) Brain, Marshall ‘How Radio Works’ at (accessed 27th January 2010) Frequency Finder at (accessed 29th January 2010) Harris, Tom ‘How Amplifiers Work’ at (accessed 29th January 2010) AW ‘Audio Amplifiers 582’ at topic=audio_amplifiers&page=2 (accessed 29th January 2010) SatCure ‘How do Transistors Work?’ at (accessed 29th January 2010) eHow ‘ How do Amplifiers Work?’ at does_4587836_amplifiers-work.html (accessed 29th January 2010) DJ Society ‘ Everything you wanted to know about speakers’ at (accessed 30th January 2010) Harris, Tom ‘How Speakers Work’ at (accessed 30th January 2010) ‘Image of Carrier Wave’ at (accessed 27th January 2010) ‘Image of a Speaker’ at (accessed 30th January 2010)