NATIONAL COLLEGE OF SCIENCE AND TECHNOLOGY Amafel Bldg. Aguinaldo Highway Dasmariñas City, Cavite Assignment # 4 TRANSMISSION LINESAgdon, Berverlyn B. July 29, 2011Communications 1 / BSECE 41A1 Score: Engr. Grace Ramones Instructor
WHAT IS AM, AMPLITUDE MODULATION (AM)?Amplitude modulation is a type of modulation where the amplitude of the carrier signal is varied inaccordance with the information bearing signal.The envelope, or boundary, of the amplitude modulated signal embeds the information bearing signal. Anonlinear device is used to combine the carrier and the modulating signal to generate an amplitudemodulated 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.AM or amplitude modulation is used for modulating a radio signal to carry sound or other information.In order that a steady radio signal or "radio carrier" can carry information it must be changed ormodulated in one way so that the information can be conveyed from one place to another. There are anumber of ways in which a carrier can be modulated to carry a signal - often an audio signal and themost obvious way is to vary its amplitude.Amplitude modulation, AM is the oldest form of analogue modulation. It was first used at the beginningthe twentieth century and it is still in use today. Currently amplitude modulation is primarily used forbroadcasting, but it is still used for some forms of two way radio communications. Its main radiocommunications use is for local aviation related VHF two way radio links. It is sued for ground to airradio communications as well as two way radio links for ground staff as well.Amplitude modulation basicsWhen amplitude modulated signal is created, the amplitude of the signal is varied in line with thevariations in intensity of the sound wave. In this way the overall amplitude or envelope of the carrier ismodulated to carry the audio signal. Here the envelope of the carrier can be seen to change in line withthe modulating signal.Amplitude modulation, AM is the most straightforward way of modulating a signal. Demodulation or theprocess where the radio frequency signal is converted into an audio frequency signal is also very simple.It only requires a simple diode detector circuit. The circuit that is commonly used has a diode thatrectifies the signal, only allowing the one half of the alternating radio frequency waveform through. Acapacitor is used to remove the radio frequency parts of the signal, leaving the audio waveform. This canbe fed into an amplifier after which it can be used to drive a loudspeaker. As the circuit used fordemodulating AM is very cheap, it enables the cost of radio receivers for AM to be kept low.
TYPES OF AMPLITUDE MODULATIONIn amplitude modulation, the amplitude of the carrier signal is varied by the modulating signal. Thereare mainly three basic types of amplitude modulation schemes as described below: 1. 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 the Lower Sideband. Therefore the DSB-LC signal may be written as v(t) = Vcsin ct + cos ( c - m)t - cos( c+ m)t The bandwidth of the modulated wave is twice that of the information signal bandwidth. 2. 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. The DSB-SC signal may be written as v(t) = VUSB(t) + VLSB(t) = cos ( m + c )t + cos ( c - m) t Bandwidth of the modulated wave is twice that of the information signal bandwidth. 3. 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. The SSB-SC signal may be written as v(t) = VUSB(t) = cos ( m + c ) t OR v(t) = LSB(t) = cos ( c - m) t Either the Upper sideband or the Lower Sideband is transmitted. Here the bandwidth bandwidth is equal to the information signal bandwidth.Apart from these three, the other types of amplitude modulations are: 4. Single sideband Full Carrier. This could be used as compatible AM broadcasting system with DSB-FC receivers. 5. 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. 6. Independent Sideband Emission: Two independent sidebands, with a carrier that is most commonly suppressed or attenuated is used here. It is used in HF point-to -point radiotelephony, in which more than one channel is required. 7. Vestigial Sideband: Here a vestige or trace of the unwanted sideband is transmitted, usually with the full carrier. This is used in video transmission. 8. Lincompex: This is an acronym that stands for linked compressor and expander. it is used commercial HF radio telephony.
POWER IN AMPLITUDE MODULATION During the modulation process the carrier power remains constant. It is only needed as areference during the demodulation process. This means that the sideband power is the useful section ofthe signal, and this corresponds to (50 / 150) x 100%, or only 33% of the total power transmitted. The total power of the transmitted signal varies with the modulating signal, whereas the carrierpower remains constant. In AM wave the transmission carriers contain most of the power The utilisation of power by an amplitude modulated signal is very poor. When the carrier ismodulated sidebands appear at either side of the carrier in its frequency spectrum. Each sidebandcontains the information about the audio modulation. To look at how the signal is made up and therelative powers take the simplified case where the 1 kHz tone is modulating the carrier. In this case twosignals 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 ofthe sidebands is equal to half that of the carrier. This means that each sideband is just a quarter of thetotal power. In other words for a transmitter with a 100 watt carrier, the total sideband power would be50 watts and each individual sideband would be 25 watts.
MODULATION INDEXIt is often necessary to define the level of modulation that is applied to a signal. A factor or index knownas the modulation index is used for this. When expressed as a percentage it is the same as the depth ofmodulation. 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 the depthof modulation) otherwise the envelope becomes distorted and the signal will "splatter" either side ofthe wanted channel, causing interference and annoyance to other users.Amplitude modulation requires a high frequency constant carrier and a low frequency modulationsignal.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, forming what iscalled a modulation envelope.