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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)Pula, Rolando A. June 29,2011Communications 1 / BSECE 41A1 Score: Engr. Grace Ramones Instructor
Amplitude ModulationAmplitude modulation (AM) is a method of impressing data onto an alternating-current (AC)carrier waveform. The highest frequency of the modulating data is normally less than 10 percent ofthe carrier frequency. The instantaneous amplitude (overall signal power) varies depending on theinstantaneous amplitude of the modulating data.In AM, the carrier itself does not fluctuate in amplitude. Instead, the modulating data appears in theform of signal components at frequencies slightly higher and lower than that of the carrier. Thesecomponents are called sidebands. The lower sideband (LSB) appears at frequencies below thecarrier frequency; the upper sideband (USB) appears at frequencies above the carrier frequency.The LSB and USB are essentially "mirror images" of each other in a graph of signal amplitude versusfrequency, as shown in the illustration. The sideband power accounts for the variations in theoverall amplitude of the signal.When a carrier is amplitude-modulated with a pure sine wave, up to 1/3 (33percent) of the overallsignal power is contained in the sidebands. The other 2/3 of the signal power is contained in thecarrier, which does not contribute to the transfer of data. With a complex modulating signal such asvoice, video, or music, the sidebands generally contain 20 to 25 percent of the overall signal power;thus the carrier consumes75 to 80 percent of the power. This makes AM an inefficient mode. If anattempt is made to increase the modulating data input amplitude beyond these limits, the signalwill become distorted, and will occupy a much greater bandwidth than it should. This is called overmodulation, and can result in interference to signals on nearby frequencies.
Modulation IndexIt can be defined as the measure of extent of amplitude variation about an unmodulated maximumcarrier. As with other modulation indices, in AM, this quantity, also called modulation depth,indicates by how much the modulated variable varies around its original level. For AM, it relates tothe variations in the carrier amplitude and is defined as:So if h = 0.5, the carrier amplitude varies by 50% above and below its unmodulated level, and for h= 1.0 it varies by 100%. To avoid distortion in the A3E transmission mode, modulation depthgreater than 100% must be avoided. Practical transmitter systems will usually incorporate somekind of limiter circuit, such as a VOGAD, to ensure this. However, AM demodulators can be designedto detect the inversion (or 180 degree phase reversal) that occurs when modulation exceeds 100%and automatically correct for this effect
Types of Amplitude Modulation In the frequency domain, amplitude modulation produces a signal with power concentratedat the carrier frequency and in two adjacent sidebands. Each sideband is equal in bandwidth to thatof the modulating signal and is a mirror image of the other. Amplitude modulation that results intwo sidebands and a carrier is often called double-sideband amplitude modulation (DSB-AM).Amplitude modulation is inefficient in terms of power usage. At least two-thirds of the power isconcentrated in the carrier signal, which carries no useful information (beyond the fact that a signalis present). To increase transmitter efficiency, the carrier can be removed (suppressed) from the AMsignal. This produces a reduced-carrier transmission or double-sideband suppressed-carrier(DSBSC) signal. A suppressed-carrier amplitude modulation scheme is three times more power-efficient than traditional DSB-AM. If the carrier is only partially suppressed, a double-sidebandreduced-carrier (DSBRC) signal results. DSBSC and DSBRC signals need their carrier to beregenerated (by a beat frequency oscillator, for instance) to be demodulated using conventionaltechniques. Improved bandwidth efficiency is achieved—at the expense of increased transmitter andreceiver complexity—by completely suppressing both the carrier and one of the sidebands. This issingle-sideband modulation, widely used in amateur radio due to its efficient use of both power andbandwidth. A simple form of AM often used for digital communications is on-off keying, a type ofamplitude-shift keying by which binary data is represented as the presence or absence of a carrierwave. This is commonly used at radio frequencies to transmit Morse code, referred to as continuouswave (CW) operation. Designation of the various types of amplitude modulation as follows:Designation DescriptionA3E double-sideband full-carrier - the basic AM modulation schemeR3E single-sideband reduced-carrierH3E single-sideband full-carrierJ3E single-sideband suppressed-carrierB8E independent-sideband emissionC3F vestigial-sidebandLincompex linked compressor and expander
Power in Amplitude Modulation In amplitude modulated (AM) systems, the modulation audio is applied to the radiofrequency carrier in such a way that the total power of the transmitted wave is made to vary inamplitude, in accordance or in sympathy with the power of the modulating audio. It is a popular misconception that in an amplitude modulated system the carrier power (incase the 1 MHz signal) is made to vary with the application of the modulated audio. Contrary to this, it is in fact the total wave power which varies in amplitude, and not thecarrier power. The carrier power in an amplitude modulated system remains constant.