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 IndexCauan, Sarah Krystelle P. June 29, 2011Communications 1 / BSECE 41A1 Score: Engr. Grace Ramones Instructor
AMPLITUDE MODULATIONModulation is the process of varying a higher frequency carrier wave to transmit information.Though it is theoretically possible to transmit baseband signals (or information) withoutmodulating it, it is far more efficient to send data by modulating it onto a higher frequency"carrier wave." Higher frequency waves require smaller antennas, use the available bandwidthmore efficiently, and are flexible enough to carry different types of data. AM radio stationstransmit audio signals, which range from 20 Hz to 20 kHz, using carrier waves that range from500 kHz to 1.7 MHz. If we were to transmit audio signals directly we would need an antennathat is around 10,000 km! Modulation techniques can be broadly divided into analog modulationand digital modulation. Amplitude modulation (AM) is one form of analog modulation.
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 information bearingsignal. A nonlinear device is used to combine the carrier and the modulating signal to generatean amplitude modulated signal. The output of the nonlinear device consists of discrete upperand lower sidebands. The output of a nonlinear device does not vary in direct proportion withthe input.AM or amplitude modulation is used for modulating a radio signal to carry sound or otherinformation.
TYPES OF AMPLITUDE MODULATIONThe modulated signal has waves at three frequencies: f c, fc – fb and fc + fb. Transmitting at allthree frequencies wastes power and bandwidth. To avoid that problem use a filter to removeone of the sidebands (usually the lower sideband, fc – fb). Use a highpass filter to remove thelower sideband signal; this process is single sideband (SSB) modulation.However, by removing one of the sidebands we lose some of the original power of themodulated signal. To maximize the power transmitted, transmit both the lower and the uppersideband. This process is double sideband (DSB) modulation. The following figure illustratesDSB.Frequency Domain View of Double Sideband – Full CarrierOne of the components of the modulated signal is the pure carrier wave. Because the carrierwave does not have any information, we can remove the carrier wave component from thesignal before we transmit it. This process is called single sideband/double sideband –suppressed carrier (SSB-SC, DSB-SC) modulation. However, we need the carrier whendemodulating the signal. Special circuits can extract information about the carrier from one ofthe sidebands; these circuits are used when demodulating SSB-SC or DSB-SC signals.We can also use amplitude modulation to send digital data. Quadrature amplitude modulation(QAM) uses four predetermined amplitude levels to determine digital bits.
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 theLower Sideband. Therefore the DSB-LC signal may be written asv(t) = Vcsin ct + cos ( c - m)t - cos( c+ m)tThe bandwidth of the modulated wave is twice that of the information signal bandwidth.Double Sideband- Suppressed Carrier (DSB-SC)In this type of amplitude modulation, both the sidebands namely Lower sideband and Uppersideband are present in the frequency spectrum but the carrier component is suppressed,hence the name Double Sideband suppressed Carrier. The Carrier does not contain anyinformation, so it is suppressed during modulation to obtain a better Power Efficiency.The DSB-SC signal may be written asv(t) = VUSB(t) + VLSB(t) = cos ( m + c )t + cos ( c - m) tBandwidth of the modulated wave is twice that of the information signal bandwidth.Single sideband- Suppressed Carrier (SSB-SC)In this type of amplitude modulation, the carrier is suppressed and it is either the Uppersideband (USB) or the Lower Sideband ( LSB) that gets transmitted. In DSC-SC the basicinformation is transmitted twice, once in each sideband. This is not required and so SSB-SC hasan upper hand.The SSB-SC signal may be written asv(t) = VUSB(t) = cos ( m + c )t ORv(t) = LSB(t) = cos ( c - m) tEither the Upper sideband or the Lower Sideband is transmitted. Here the bandwidth bandwidthis equal to the information signal bandwidth.Apart from these three, the other types of amplitude modulations are: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 -pointradiotelephony, 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.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-sideband
POWER IN AMPLITUDE MODULATIONEven with 100% modulation the utilisation of power by an amplitude modulated signal is verypoor. When the carrier is modulated sidebands appear at either side of the carrier in itsfrequency spectrum. Each sideband contains the information about the audio modulation. Tolook at how the signal is made up and the relative powers take the simplified case where the 1kHz tone is modulating the carrier. In this case two signals will be found 1 kHz either side of themain carrier. When the carrier is fully modulated i.e. 100% the amplitude of the modulation isequal to half that of the main carrier, i.e. the sum of the powers of the sidebands is equal to halfthat of the carrier. This means that each sideband is just a quarter of the total power. In otherwords for a transmitter with a 100 watt carrier, the total sideband power would be 50 watts andeach individual sideband would be 25 watts. During the modulation process the carrierpower 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.Not only is AM wasteful in terms of power, it is also not very efficient in its use of spectrum. Ifthe 1 kHz tone is replaced by a typical audio signal made up of a variety of sounds with differentfrequencies then each frequency will be present in each sideband. Accordingly the sidebandsspread out either side of the carrier as shown and the total bandwidth used is equal to twice thetop frequency that is transmitted. In the crowded conditions found on many of the short wavebands today, this is a waste of space, and other modes of transmission which take up lessspace are often used.
MODULATION INDEXIt is often necessary to define the level of modulation that is applied to a signal. A factor or indexknown as the modulation index is used for this. When expressed as a percentage it is the sameas the depth of modulation. In other words it can be expressed 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 % in terms of thedepth of modulation) otherwise the envelope becomes distorted and the signal will "splatter"either side of the wanted channel, causing interference and annoyance to other users.Amplitude modulation requires a high frequency constant carrier and a low frequencymodulation signal.A sine wave carrier is of the formA sine wave modulation signal is of the formThe high frequency carrier takes on the shape of the lower frequency modulation signal, formingwhat is called a modulation envelope.The modulation index is defined as the ratio of the modulation signal amplitude to thecarrier amplitude. whereThe overall signal can be described by:More commonly, the carrier amplitude is normalized to one and the am equation is written as:In most literature this expression is simply written as:If the modulation index is zero (mam = 0) the signal is simply a constant amplitude carrier.If the modulation index is 1 (mam = 1), the resultant waveform has maximum or 100% amplitudemodulation.
SidebandsExpanding the normalized AM equation:we obtain:where: sinωct represents the carrier represents the lower sideband represents the upper sidebandThe sidebands are centered on the carrier frequency. They are the sum and differencefrequencies of the carrier and modulation signals. In the above example, they are just singlefrequencies, but normally the baseband modulation signal is a range of frequencies and hencetwo bands are formed.As a side point, note that multiplication in the time domain causes addition and subtraction inthe frequency domain.