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)Olaño, Reymart June 29, 2011Communications 1 / BSECE 41A1 Score: Engr. Grace Ramones Instructor
AMPLITUDE MODULATION Amplitude modulation is a type of modulation where the amplitude of the carrier signal is varied in accordance with the information bearing signal. The envelope, or boundary, of the amplitude modulated signal embeds the information bearing signal. A nonlinear device is used to combine the carrier and the modulating signal to generate an amplitude modulated signal. The output of the nonlinear device consists of discrete upper and lower sidebands.The output of a nonlinear device does not vary in direct proportion with the input. Amplitude Modulation is abbreviated AM. Amplitude 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 of the carrier frequency. The instantaneous amplitude (overall signal power) varies depending on the instantaneous amplitude of the modulating data.
POWER IN AMPLITUDE MODULATION The total power of the transmitted signal varies with the modulating signal, whereas the carrier power remains constant. 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 made up and the relative powers take the simplified case where the 1 kHz tone 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 the main carrier, i.e. the sum of the powers of the sidebands is equal to half that of the carrier. This means that each sideband is just a quarter of the total power. In other words for a transmitter with a 100 watt carrier, 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 during the demodulation process. This means that the sideband power is the useful section of the signal, and this corresponds to (50 / 150) x 100%, or only 33% of the total power transmitted.
TYPES OF AMPLITUDE MODULATION In radio communication, a continuous wave radio-frequency signal (a sinusoidal carrier wave)has its amplitude modulated by an audio waveform before being transmitted. In the frequency domain, amplitude modulation produces a signal with power concentrated at thecarrier frequency and in two adjacent sidebands. Each sideband is equal in bandwidth to that of themodulating signal and is a mirror image of the other. Amplitude modulation that results in two sidebandsand a carrier is often called double-sideband amplitude modulation (DSB-AM). Amplitude modulation isinefficient in terms of power usage. At least two-thirds of the power is concentrated in the carrier signal,which carries no useful information (beyond the fact that a signal is present). To increase transmitter efficiency, the carrier can be removed (suppressed) from the AM signal.This produces a reduced-carrier transmission or double-sideband suppressed-carrier (DSBSC) signal. Asuppressed-carrier amplitude modulation scheme is three times more power-efficient than traditionalDSB-AM. If the carrier is only partially suppressed, a double-sideband reduced-carrier (DSBRC) signalresults. DSBSC and DSBRC signals need their carrier to be regenerated (by a beat frequency oscillator,for instance) to be demodulated using conventional techniques. Improved bandwidth efficiency is achieved—at the expense of increased transmitter and receivercomplexity—by completely suppressing both the carrier and one of the sidebands. This is single-sidebandmodulation, widely used in amateur radio due to its efficient use of both power and bandwidth. 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 carrier wave.This is commonly used at radio frequencies to transmit Morse code, referred to as continuous wave (CW)operation. ITU designations In 1982, the International Telecommunication Union (ITU) designated the various types ofamplitude 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
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).