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)Balane, Maycen M. June 29, 2011Communications 1 / BSECE 41A1 Score: Engr. Grace Ramones Instructor
AMPLITUDE MODULATIONAmplitude modulation (AM) is a technique used in electronic communication, most commonly for transmittinginformation via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to theinformation being sent. For example, changes in the signal strength can be used to specify the sounds to bereproduced by a loudspeaker, or the light intensity of television pixels. (Contrast this with frequency modulation,also commonly used for sound transmissions, in which the frequency is varied; and phase modulation, often usedin remote controls, in which the phase is varied)In the mid-1870s, a form of amplitude modulation—initially called "undulatory currents"—was the first method tosuccessfully produce quality audio over telephone lines. Beginning with Reginald Fessendens audiodemonstrations in 1906, it was also the original method used for audio radio transmissions, and remains in usetoday by many forms of communication—"AM" is often used to refer to the mediumwave broadcast bandAmplitude modulation, AM is the oldest form of analogue modulation. It was first used at the beginning thetwentieth century and it is still in use today. Currently amplitude modulation is primarily used for broadcasting, butit is still used for some forms of two way radio communications. Its main radio communications use is for localaviation related VHF two way radio links. It is sued for ground to air radio communications as well as two way radiolinks for ground staff as well.Amplitude modulation (AM) is a method of impressing data onto an alternating-current (AC) carrier waveform. Thehighest frequency of the modulating data is normally less than 10 percent of the carrier frequency. Theinstantaneous amplitude (overall signal power) varies depending on the instantaneous amplitude of themodulating data.Amplitude modulation (AM) is also widely used to alter a carrier wave to transmit data. For example, in AM radio,the voltage (amplitude) of a carrier with a fixed center frequency (the stations channel) is varied (modulated) bythe analog audio signal.
TYPES OF AMPLITUDE MODULATION 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. Double Sideband Full Carrier (DSB- LC). This type of Amplitude modulation is also known as Full AM or Standard AM. Here the frequency spectrum of the AM will have the carrier frequency, Upper sideband and the Lower Sideband. 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. 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. Vestigial Sideband: Here a vestige or trace of the unwanted sideband is transmitted, usually with the full carrier. Independent Sideband Emission: Two independent sidebands, with a carrier that is most commonly suppressed or attenuated is used here. Lincompex: This is an acronym that stands for linked compressor and expander. Single sideband Full Carrier. This could be used as compatible AM broadcasting system with DSB- FC receivers.
POWER IN AMPLITUDE MODULATION In terms of the positive frequencies, the transmission bandwidth of AM is twice the signals original(baseband) bandwidth—since both the positive and negative sidebands are shifted up to the carrier frequency.Thus, double-sideband AM (DSB-AM) is spectrally inefficient, meaning that fewer radio stations can beaccommodated in a given broadcast band. The various suppression methods in Forms of AM can be readilyunderstood in terms of the diagram in Figure 2. With the carrier suppressed there would be no energy at thecenter of a group. And with a sideband suppressed, the "group" would have the same bandwidth as the positivefrequencies of The transmitter power efficiency of DSB-AM is relatively poor (about 33%). The benefitof this system is that receivers are cheaper to produce. The forms of AM with suppressed carriers are found to be100% power efficient, since no power is wasted on the carrier signal which conveys no information.
MODULATION INDEXIt can be defined as the measure of extent of amplitude variation about an unmodulated maximum carrier. As withother modulation indices, in AM, this quantity, also called modulation depth, indicates by how much themodulated variable varies around its original level. For AM, it relates to the variations in the carrier amplitude andis defined as:So if h = 0.5, the carrier amplitude variesby 50% above and below itsunmodulated level, and for h = 1.0 itvaries by 100%. To avoid distortion inthe A3E transmission mode, modulationdepth greater than 100% must beavoided. Practical transmitter systemswill usually incorporate some kind oflimiter circuit, such as a VOGAD, toensure this. However, AM demodulatorscan be designed to detect the inversion(or 180 degree phase reversal) thatoccurs when modulation exceeds 100%and automatically correct for this effect.Variations of modulated signal withpercentage modulation are shownbelow. In each image, the maximumamplitude is higher than in the previousimage. Note that the scale changes fromone image to the next.