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)Tagasa, Jerald A. June 29,2011Communications 1 / BSECE 41A1 Score: Engr. Grace Ramones Instructor
AMPLITUDE MODULATIONAmplitude modulation is a type of modulation where the amplitude of the carrier signal is varied in accordancewith the information bearing signal.The envelope, or boundary, of the amplitude modulated signal embeds the information bearing signal.The total power of the transmitted signal varies with the modulating signal, whereas the carrier power remainsconstant.A nonlinear device is used to combine the carrier and themodulating signal to generate an amplitude modulatedsignal. The output of the nonlinear device consists ofdiscrete upper and lower sidebands.The output of a nonlinear device does not vary in directproportion with the input.Amplitude Modulation is abbreviated AM.
TYPES OF AMPLITUDE MODULATION Double sideband (DSB), single side band (SSB), vestigial side band, is common modes used on all amateurand commercial bands. All of these are correctly referred to as amplitude modulation. The reference to doubleside band as AM and single side band as SSB on some amateur transceivers gives credence to the misconceptionthat SSB is not an amplitude modulated wave when in fact it is. Double Sideband Full Carrier (DSB- LC). This type of Amplitude modulation is also known as Full AM orStandard AM. Here the frequency spectrum of the AM will have the carrier frequency, Upper sideband and theLower Sideband. Double Sideband- Suppressed Carrier (DSB-SC).In this type of amplitude modulation, both the sidebandsnamely Lower sideband and Upper sideband are present in the frequency spectrum but the carrier component issuppressed, hence the name Double Sideband suppressed Carrier. The Carrier does not contain any information,so it is suppressed during modulation to obtain a better Power Efficiency. Single sideband- Suppressed Carrier (SSB-SC) . In this type of amplitude modulation, the carrier issuppressed and it is either the Upper sideband (USB) or the Lower Sideband ( LSB) that gets transmitted. In DSC-SCthe basic information is transmitted twice, once in each sideband. This is not required and so SSB-SC has an upperhand. Single sideband Full Carrier. This could be used as compatible AM broadcasting system with DSB-FCreceivers. Single Sideband - Reduced Carrier: Here an attenuated carrier is reinserted into the SSB signal, tofacilitate receiver tuning and demodulation. This method is steadily replaced by SSB-SC. Independent Sideband Emission: Two independent sidebands, with a carrier that is most commonlysuppressed or attenuated is used here. Vestigial Sideband: Here a vestige or trace of the unwanted sideband is transmitted, usually with the fullcarrier. Lincompex: This is an acronym that stands for linked compressor and expander. A3E double-sideband full-carrier - the basic AM modulation scheme R3E single-sideband reduced-carrier H3E single-sideband full-carrier J3E single-sideband suppressed-carrier B8E independent-sideband emission
POWER IN AMPLITUDE MODULATION The utilisation of power by an amplitude modulated signal is very poor. When the carrier is modulatedsidebands appear at either side of the carrier in its frequency spectrum. Each sideband contains the informationabout the audio modulation. To look at how the signal is made up and the relative powers take the simplified casewhere the 1 kHz tone is modulating the carrier. In this case two signals will be found 1 kHz either side of the maincarrier. 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 means that eachsideband is just a quarter of the total power. In other words for a transmitter with a 100 watt carrier, the totalsideband power would be 50 watts and each individual sideband would be 25 watts. The total power of the transmitted signal varies with the modulating signal, whereas the carrier powerremains constant.
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 is called amodulation envelope.The modulation index is defined as the ratio of the modulation signal amplitude to the carrier amplitude. 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 the carrierand modulation signals. In the above example, they are just single frequencies, but normally the basebandmodulation 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 the frequencydomain.