NATIONAL COLLEGE OF SCIENCE AND TECHNOLOGY Amafel Bldg. Aguinaldo Highway Dasmariñas City, Cavite Assignment # 2 AMPLITUDE MODULATION (Types of Amplitude Modulation) (Power in Amplitude Modulation) (Modulation Index)Lingad, Valentino June 29,2011Communications 1 / BSECE 41A1 Score: Engr. Grace Ramones Instructor
WHAT IS AM, AMPLITUDE MODULATIONIn order that a steady radio signal or "radio carrier" can carry information it must bechanged or modulated in one way so that the information can be conveyed from one placeto another. There are a number of ways in which a carrier can be modulated to carry asignal - often an audio signal and the most obvious way is to vary its amplitude.Amplitude modulation, AM is the oldest form of analogue modulation. It was first used atthe beginning the twentieth century, and it is still in use today. Currently amplitudemodulation is primarily used for broadcasting, but it is still used for some forms of two wayradio communications. Its main radio communications use is for local aviation related VHFtwo way radio links. It is sued for ground to air radio communications as well as two wayradio links for ground staff as well.Amplitude modulation basicsWhen amplitude modulated signal is created, the amplitude of the signal is varied in linewith the variations in intensity of the sound wave. In this way the overall amplitude orenvelope of the carrier is modulated to carry the audio signal. Here the envelope of thecarrier can be seen to change in line with the modulating signal.Amplitude modulation, AM is the most straightforward way of modulating a signal.Demodulation, or the process where the radio frequency signal is converted into an audiofrequency signal is also very simple. It only requires a simple diode detector circuit. Thecircuit that is commonly used has a diode that rectifies the signal, only allowing the one halfof the alternating radio frequency waveform through. A capacitor is used to remove theradio frequency parts of the signal, leaving the audio waveform. This can be fed into anamplifier after which it can be used to drive a loudspeaker. As the circuit used fordemodulating AM is very cheap, it enables the cost of radio receivers for AM to be kept low.HighlightsWhile amplitude modulation is one of the simplest and easiest forms of modulation toimplement, it is not the most efficient in terms of spectrum efficiency and power usage. As aresult, the use of amplitude modulation is falling in preference to other analogue modessuch as frequency modulation, and a variety of digital modulation formats. Yet despite thisdecrease, amplitude modulation is in such widespread use, especially for broadcasting, thatit will still be used for many years to come.
POWER IN AMPLITUDE MODULATIONAmplitude modulation efficiencyAmplitude modulation, AM has advantages of simplicity, but it is not the most efficientmode to use, both in terms of the amount of spectrum it takes up, and the usage of thepower. It is for this reason that it only has limited applications for broadcast and two wayradio communications systems.The reason for its inefficiency occurs as a result of the composition of the radio signal.When a radio frequency signal is modulated by an audio signal the envelope will vary. Thelevel of modulation can be increased to a level where the envelope falls to zero and thenrises to twice the un-modulated level. Any increase on this will cause distortion because theenvelope cannot fall below zero. As this is the maximum amount of modulation possible itis called 100% modulation.Even with 100% modulation the utilisation of power by an amplitude modulated signal isvery poor. When the carrier is modulated sidebands appear at either side of the carrier inits frequency spectrum. Each sideband contains the information about the audiomodulation. To look at how the signal is made up and the relative powers take thesimplified case where the 1 kHz tone is modulating the carrier. In this case two signals willbe found 1 kHz either side of the main carrier. When the carrier is fully modulated i.e.100% the amplitude of the modulation is equal to half that of the main carrier, i.e. the sumof the powers of the sidebands is equal to half that of the carrier. This means that eachsideband is just a quarter of the total power. In other words for a transmitter with a 100watt carrier, the total sideband power would be 50 watts and each individual sidebandwould be 25 watts. During the modulation process the carrier power remains constant. It isonly needed as a reference during the demodulation process. This means that the sidebandpower is the useful section of the signal, and this corresponds to (50 / 150) x 100%, or only33% of the total power transmitted.Not only is AM wasteful in terms of power, it is also not very efficient in its use of spectrum.If the 1 kHz tone is replaced by a typical audio signal made up of a variety of sounds withdifferent frequencies then each frequency will be present in each sideband. Accordingly thesidebands spread out either side of the carrier as shown and the total bandwidth used isequal to twice the top frequency that is transmitted. In the crowded conditions found onmany of the short wave bands today, this is a waste of space, and other modes oftransmission which take up less space are often used.
MODULATION INDEXIt is often necessary to define the level of modulation that is applied to a signal. A factor orindex known as the modulation index is used for this. When expressed as a percentage it isthe same as the depth of modulation. In other words it can be expressed as: M = (RMS value of modulating signal) / (RMS value of unmodulated signal )The value of the modulation index must not be allowed to exceed one (i.e. 100 % in termsof the depth of modulation) otherwise the envelope becomes distorted and the signal will"splatter" either side of the wanted channel, causing interference and annoyance to otherusers.
TYPES OF AMPLITUDE MODULATION1. Double Sideband Full Carrier (DSB- LC)This type of Amplitude modulation is also known as Full AM or Standard AM. Here the frequencyspectrum of th AM will have the carrier frequency, Upper sideband and the Lower Sideband.Therefore the DSB-LC signal may be written as2. Double Sideband- Suppressed Carrier (DSB-SC)In this type of amplitude modulation, both the sidebands namely Lower sideband and Uppersideband are present in the frequency spectrum but the carrier component is suppressed, hencethe name Double Sideband suppressed Carrier. The Carrier does not contain any information, so itis suppressed during modulation to obtain a better Power Efficiency.3. Single sideband- Suppressed Carrier (SSB-SC)In this type of amplitude modulation, the carrier is suppressed and it is either the Upper sideband(USB) or the Lower Sideband ( LSB) that gets transmitted. In DSC-SC the basic information istransmitted twice, once in each sideband. This is not required and so SSB-SC has an upper hand.4. Single sideband Full Carrier. This could be used as compatible AM broadcasting system withDSB-FC receivers.5. Single Sideband - Reduced Carrier: Here an attenuated carrier is reinserted into the SSBsignal, to facilitate receiver tuning and demodulation. This method is steadily replaced by SSB-SC.6. Independent Sideband Emission: Two independent sidebands, with a carrier that is mostcommonly suppressed or attenuated is used here. It is used in HF point-to -point radiotelephony,in which more than one channel is required.7. Vestigial Sideband: Here a vestige or trace of the unwanted sideband is transmitted, usuallywith the full carrier. This is used in video transmission.8. Lincompex: This is an acronym that stands for linked compressor and expander. it is usedcommercial HF radio telephony.