TYPES OF AMPLITUDE MODULATION 1. Double Sideband Full Carrier (DSB- LC) This 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 Lower Sideband. Therefore the DSB-LC signal may be written as v(t) = Vcsin ct + cos ( c - m)t - cos( c+ m)t The bandwidth of the modulated wave is twice that of the information signal bandwidth. 2. Double Sideband- Suppressed Carrier (DSB-SC) In this type of amplitude modulation, both the sidebands namely Lower sideband and Upper sideband are present in the frequency spectrum but the carrier component is suppressed, 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. The DSB-SC signal may be written as v(t) = VUSB(t) + VLSB(t) = cos ( m + c )t + cos ( c - m) t Bandwidth of the modulated wave is twice that of the information signal bandwidth. 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 is transmitted twice, once in each sideband. This is not required and so SSB-SC has an upper hand. The SSB-SC signal may be written as v(t) = VUSB(t) = cos ( m + c )t OR v(t) = LSB(t) = cos ( c - m) t Either the Upper sideband or the Lower Sideband is transmitted. Here the bandwidth bandwidth is equal to the information signal bandwidth. 4. Single sideband Full Carrier. This could be used as compatible AM broadcasting system with DSB-FC receivers. 5. Single Sideband - Reduced Carrier: Here an attenuated carrier is reinserted into the SSB signal, 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 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. 7. Vestigial Sideband: Here a vestige or trace of the unwanted sideband is transmitted, usually with the full carrier. This is used in video transmission. 8. Lincompex: This is an acronym that stands for linked compressor and expander. it is used commercial HF radio telephony.
POWER IN AMPLITUDE MODULATION In AM wave the transmission carriers contain most of the power The utilisation of power by an amplitude modulated signal is very poor. When thecarrier is modulated sidebands appear at either side of the carrier in its frequencyspectrum. Each sideband contains the information about the audio modulation. To lookat how the signal is made up and the relative powers take the simplified case where the1 kHz tone is modulating the carrier. In this case two signals will be found 1 kHz eitherside of the main carrier. When the carrier is fully modulated i.e. 100% the amplitude ofthe modulation is equal to half that of the main carrier, i.e. the sum of the powers of thesidebands is equal to half that of the carrier. This means that each sideband is just aquarter of the total power. In other words for a transmitter with a 100 watt carrier, thetotal sideband power would be 50 watts and each individual sideband would be 25watts.
MODULATION INDEXIt is often necessary to define the level of modulation that is applied to a signal. A factoror index known as the modulation index is used for this. When expressed as apercentage it is the same as the depth of modulation. In other words it can beexpressed 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 % interms of the depth of modulation) otherwise the envelope becomes distorted and thesignal will "splatter" either side of the wanted channel, causing interference andannoyance to other users.Amplitude modulation requires a high frequency constant carrier and a low frequencymodulation signal.
AMPLITUDE MODULATIONModulation is the process of varying a higher frequency carrier wave to transmitinformation. Though it is theoretically possible to transmit baseband signals (orinformation) without modulating it, it is far more efficient to send data by modulating itonto a higher frequency "carrier wave."Amplitude modulation is a type of modulation where the amplitude of the carrier signal isvaried in accordance with the information bearing signal.The envelope, or boundary, of the amplitude modulated signal embeds the informationbearing signal. A nonlinear device is used to combine the carrier and the modulatingsignal to generate an amplitude modulated signal. The output of the nonlinear deviceconsists of discrete upper and lower sidebands. The output of a nonlinear device doesnot vary in direct proportion with the input.AM or amplitude modulation is used for modulating a radio signal to carry sound orother information.
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)Lasco, Ben Rajnish E. June 29,2011Communications 1 / BSECE 41A1 Score: Engr. Grace Ramones Instructor