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)Bani, Arviclyn C. June 29, 2011Communications 1 / BSECE 41A1 Score: Engr. Grace Ramones Instructor
AMPLITUDE MODULATIONThe process or result of the process whereby the amplitude of a carrier wave is changed inaccordance with a modulating wave. This broad definition includes applications using sinusoidalcarriers, pulse carriers, or any other form of carrier, the amplitude factor of which changes inaccordance with the modulating wave in any unique manner.Practical examples of amplitude modulation (AM) include AM radio broadcasting, single-sideband transmission systems, vestigial-sideband systems, frequency-division multiplexing,time-division multiplexing, phase-discrimination multiplexing, and reduced-carrier systems.Amplitude modulation is also defined in a more restrictive sense to mean modulation in whichthe amplitude factor of a sine-wave carrier is linearly proportional to the modulating wave. AMradio broadcasting is a familiar example. At the radio transmitter the modulating wave is theaudio-frequency program signal to be communicated; the modulated wave that is broadcast is aradio-frequency, amplitude-modulated sinusoid.In AM the modulated wave is composed of the transmitted carrier, which conveys noinformation, plus the upper and lower sidebands, which (assuming the carrier frequency exceedstwice the top audio frequency) convey identical and therefore mutually redundant information. J.R. Carson in 1915 was the first to recognize that, under these conditions and assuming adequateknowledge of the carrier, either sideband alone would uniquely define the message. Thiseventually led to the development of single-sideband (SSB) and vestigial-sideband (VSB)modulation. Apart from a scale factor, the spectrum of the upper sideband and lower sideband isthe spectrum of the modulating wave displaced, respectively, without and with inversion by anamount equal to the carrier frequency.
TYPES OF AMPLITUDE MODULATIONDouble-Sideband Amplitude Modulation Amplitude- modulation that results in two sidebandsand a carrier is often called double-sideband amplitude modulation (DSB-AM). Amplitudemodulation is inefficient in terms of power usage. At least two-thirds of the power is concentratedin the carrier signal, which carries no useful information (beyond the fact that a signal is present).Double-Sideband Suppressed-Carrier- To increase transmitter efficiency, the carrier can beremoved (suppressed) from the AM signal. This produces a reduced-carrier transmission ordouble-sideband suppressed-carrier (DSBSC) signal. A suppressed-carrier amplitude modulationscheme is three times more power-efficient than traditional DSB-AM.Double-Sideband Reduced-Carrier - If the carrier is only partially suppressed, a double-sideband reduced-carrier (DSBRC) signal results. DSBSC and DSBRC signals need their carrierto be regenerated (by a beat frequency oscillator, for instance) to be demodulated usingconventional techniques.Single sideband Full Carrier- This could be used as compatible AM broadcasting system withDSB-FC receivers.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.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.Vestigial Sideband- Here a vestige or trace of the unwanted sideband is transmitted, usuallywith the full carrier. This is used in video transmission.Lincompex- This is an acronym that stands for linked compressor and expander. it is usedcommercial HF radio telephony.
MODULATION INDEX Amplitude modulation is one of the earliest radio modulation techniques. The receivers used tolist to AM-DSB-C are perhaps the simplest receivers of any radio modulation technique; which may bewhy that version of amplitude modulation is still widely used today. Amplitude modulation (AM) occurs when the amplitude of a carrier wave is modulated, tocorrespond to a source signal. In AM, we have an equation that looks like this: Fsignal(t) = A(t)sin(ωt) We can also see that the phase of this wave is irrelevant, and does not change (so we dont eveninclude it in the equation).
POWER IN AMPLITUDE MODULATIONAmplitude modulation, AM has advantages of simplicity, but it is not the most efficient mode to use,both in terms of the amount of spectrum it takes up, and the usage of the power. It is for this reasonthat it only has limited applications for broadcast and two way radio communications systems.The reason for its inefficiency occurs as a result of the composition of the radio signal. When a radiofrequency signal is modulated by an audio signal the envelope will vary. The level of modulation can beincreased to a level where the envelope falls to zero and then rises to twice the un-modulated level. Anyincrease on this will cause distortion because the envelope cannot fall below zero. As this is themaximum amount of modulation possible it is called 100% modulation.Even with 100% modulation the utilisation of power by an amplitude modulated signal is very poor.When the carrier is modulated sidebands appear at either side of the carrier in its frequency spectrum.Each sideband contains the information about the audio modulation. To look at how the signal is madeup and the relative powers take the simplified case where the 1 kHz tone is modulating the carrier. Inthis case two signals will be found 1 kHz either side of the main carrier. When the carrier is fullymodulated i.e. 100% the amplitude of the modulation is equal to half that of the main carrier, i.e. thesum of the powers of the sidebands is equal to half that of the carrier. This means that each sideband isjust 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. During themodulation process the carrier power remains constant. It is only needed as a reference during thedemodulation 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.