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)Lopera, Jericho James L. June 29, 2011Communications 1 / BSECE 41A1 Score: Engr. Grace Ramones Instructor
Amplitude Modulation (AM)In amplitude modulation, the instantaneous amplitude of a carrier wave is varied in accordance with theinstantaneous amplitude of the modulating signal. Main advantages of AM are small bandwidth and simpletransmitter and receiver designs. Amplitude modulation is implemented by mixing the carrier wave in anonlinear device with the modulating signal. This produces upper and lower sidebands, which are the sumand difference frequencies of the carrier wave and modulating signal.The carrier signal is represented byc(t) = A cos(wct)The modulating signal is represented bym(t) = B sin(wmt)Then the final modulated signal is[1 + m(t)] c(t)= A [1 + m(t)] cos(wct)= A [1 + B sin(wmt)] cos(wct)= A cos(wct) + A m/2 (cos((wc+wm)t)) + A m/2 (cos((wc-wm)t))Because of demodulation reasons, the magnitude of m(t) is always kept less than 1 and the frequency muchsmaller than that of the carrier signal.The amplitude modulation is one of the simplest and easiest forms of modulation to implement, it is notthe most efficient in terms of spectrum efficiency and power usage. As a result, the use of amplitudemodulation is falling in preference to other analogue modes such as frequency modulation, and a variety ofdigital modulation formats. Yet despite this decrease, amplitude modulation is in such widespread use,especially for broadcasting, that it will still be used for many years to come.
Modulation IndexAmplitude modulation requires a high frequency constant carrier and a low frequency modulation signal. A sine wave carrier is of the form A sine wave modulation signal is of the formThe high frequency carrier takes on the shape of the lower frequency modulation signal, forming what iscalled a modulation envelope.The modulation index is defined as the ratio of the modulation signal amplitude to the carrieramplitude. whereThe overall signal can be described by:More commonly, the carrier amplitude is normalized to one and the am equation is written as: In most literature this expression is simply written as: If the modulation index is zero (mam = 0) the signal is simply a constant amplitude carrier. If the modulation index is 1 (mam = 1), the resultant waveform has maximum or 100% amplitude modulation.SidebandsExpanding the normalized AM equation:we obtain:where: sinωct represents the carrier represents the lower sideband represents the upper sidebandThe sidebands are centered on the carrier frequency. They are the sum and difference frequencies of thecarrier and modulation signals. In the above example, they are just single frequencies, but normally thebaseband modulation signal is a range of frequencies and hence two bands are formed.As a side point, note that multiplication in the time domain causes addition and subtraction in thefrequency domain.
Types of Amplitude ModulationIntroduction to Types of Amplitude ModulationIn amplitude modulation, the amplitude of the carrier signal is varied by the modulating signal. There aremainly three basic types of amplitude modulation schemes as described below: Double Sideband Full Carrier (DSB- LC) Double Sideband- Suppressed Carrier (DSB-SC) Single sideband- Suppressed Carrier (SSB-SC)Apart from these three, the other types of amplitude modulations are: Single sideband Full Carrier. This could be used as compatible AM broadcasting system with DSB-FC receivers. Single Sideband - Reduced Carrier : Here an attenuated carrier is reinserted into the SSB signal, to facilatate receiver tuning and demodulation. This method is steadily replaced by SSB- SC. Independent Sideband Emission : Two independent sidebands, with a carrier that is most commonly suppressed or attenuated is used here. It is used in HF point-to -point radiotelephony, in which more than one channel is required. Vestigial Sideband : Here a vestige or trace of the unwanted sideband is transmitted, usually with the full carrier. This is used in video transmission. Lincompex : This is an acronym that stands for linked compressor and expander. it is used commercial HF radio telephony.The type of amplitude modulation used depends upon the application it is used for.Double Sideband full CarrierThis type of Amplitude modulation is also known as Full AM or Standard AM.Here the frequency sepectrum of th AM will have the carrier frequency, Upper sideband and the LowerSideband. Therefore the DSB-LC signal may be written asv(t) = Vcsin ct + cos ( c - m)t - cos( c + m)tThe bandwidth of the modulated wave is twice that of the information signal bandwidth.Double Sideband- Suppressed CarrierIn this type of amplitude modulation, both the sidebands namely Lower sideband and Upper sideband arepresent in the frequency spectrum but the carrier component is suppressed, hence the name DoubleSideband suppressed Carrier. The Carrier does not contain any information, so it is suppressed duringmodulation to obtain a better Power Efficiency.The DSB-SC signal may be written asv(t) = VUSB(t) + VLSB(t) = cos ( m + c ) t + cos ( c - m) tBandwidth of the modulated wave is twice that of the information signal bandwidth.Single Sideband- Suppressed CarrierIn 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 is transmittedtwice, once in each sideband. This is not required and so SSB-SC has an upper hand.The SSB-SC signal may be written asv(t) = VUSB(t) = cos ( m + c )t ORv(t) = LSB(t) = cos ( c - m) tEither the Upper sideband or the Lower Sideband is transmitted. Here the bandwidth bandwidth is equalto the information signal bandwidth.
Amplitude modulation efficiency/POWER IN Amplitude modulationTo look at how the signal is made up and the relative powers take the simplified case where the 1 kHztone is modulating the carrier. In this case two signals will be 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 themain carrier, i.e. the sum of the powers of the sidebands is equal to half that of the carrier. This meansthat each sideband is just a quarter of the total power. In other words for a transmitter with a 100 wattcarrier, the total sideband power would be 50 watts and each individual sideband would be 25 watts.During the modulation process the carrier power remains constant. It is only needed as a reference duringthe demodulation process. This means that the sideband power is the useful section of the signal, and thiscorresponds to (50 / 150) x 100%, or only 33% 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 kHztone is replaced by a typical audio signal made up of a variety of sounds with different frequencies theneach frequency will be present in each sideband. Accordingly the sidebands spread out either side of thecarrier as shown and the total bandwidth used is equal to twice the top frequency that is transmitted. In thecrowded conditions found on many of the short wave bands today, this is a waste of space, and othermodes of transmission which take up less space are often used.