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A comprehensive overview of amplitude modulation (am), a fundamental technique in electronic communication. It delves into the mathematical representation, frequency spectrum, and power calculation of am signals. The document also explores different types of am generation, including low-level and high-level modulation, and discusses the concept of sidebands and their significance in am transmission. Additionally, it examines independent sideband (isb) and vestigial sideband (vsb) modulation techniques, highlighting their advantages and disadvantages. The document concludes with a detailed explanation of the applications of am in various communication systems, including radio broadcasting, portable radios, and television transmission.
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Amplitude modulation Amplitude modulation is a process by which the wave signal is transmitted by modulating the amplitude of the signal. It is often called AM and is commonly used in transmitting a piece of information through a radio carrier wave. Amplitude modulation is mostly used in the form of electronic communication. Currently, this technique is used in many areas of communication, such as in portable two- way radios, citizens band radios, VHF aircraft radios and in modems for computers. Amplitude modulation is also used to refer to medium wave AM radio broadcasting. Derivation of Amplitude Modulation Mathematical representation of Amplitude Modulated waves in time domain (๐ก)=(2๐๐๐๐ก) (modulating signal) (๐ก)=(2๐๐๐๐ก) (carrier signal) (๐ก)=โ๐ด๐+(2๐๐๐๐ก)โ๐๐๐ (2๐๐๐๐ก) (equation of Amplitude Modulated wave) Where, Am: amplitude of modulating signal
Ac: amplitude of carrier signal fm: frequency of modulating signal fc: frequency of carrier signal Therefore, above is the derivation of Amplitude Modulation. Frequency spectrum and bandwidth
Types of Amplitude Modulation Generation *Low level Amplitude Modulation *High level Amplitude Modulation Here we will understand the difference between these two types of techniques of generation of AM, with the help of block diagrams- Low Level Amplitude Modulation (Block Diagram) The image given below shows the block diagram of a Low Level Amplitude Modulation Block Diagram of Low Level Amplitude Modulation Low Level Amplitude Modulation, Block Diagram of Low Level Amplitude Modulation Now observe the image carefully- This block diagram shows 3 main blocks- #Low level AM modulator #Wideband power amplifier and #RF carrier oscillator As you can see in the diagram that low level AM modulator has two inputs. At its first input we apply the modulating signal source (message signal) and it's second input is supplied by the RF carrier oscillator. Since it is low level amplitude modulation therefore before applying the modulating signal to the low level AM modulator, we do not amplify it. In the same way, RF carrier is also not amplified. Therefore you observe here that in low level AM modulation, neither the modulating signal
#Narrow band power amplifier and #High level AM modulator As this is high level amplitude modulation, therefore it uses a high level AM modulator which operates at high level. Before applying the message signal and the carrier wave to the high-level AM modulator, both of these signals are amplified. The modulating signal is power amplified by the wide band power amplifier to preserve the sidebands of the modulating signal but the RF carrier is amplified by the narrow band power amplifier as the carrier wave has fixed frequency. Therefore narrowband power amplifier is sufficient for its amplification. Now after amplification of both the signals, they are applied to the high level AM modulator. Then this amplitude modulated signal is transmitted by the antenna. The example of high level Amplitude Modulation is the collector modulation method. So this was all about the generation of amplitude modulation with the help of low level amplitude modulator and high level amplitude modulator. Trapezoidal pattern If an oscilloscope is setup to display the modulating waveform on the horizontal axis and RF voltage on the vertical axis, it results in a trapezoidal pattern that can be used to calculate modulation index. Modulation index can be calculated as m = (bโa)/(b+a)
Note that the trapezoidal pattern depends on the horizontal axis signal being in synchronism with the modulated wave. This may not be so with modulators that use DSP techniques. Sideband Sideband A Sideband is a band of frequencies, containing power, which are the lower and higher frequencies of the carrier frequency. Both the sidebands contain the same information. The representation of amplitude modulated wave in the frequency domain is as shown in the following figure. Both the sidebands in the image contain the same information. The transmission of such a signal which contains a carrier along with two sidebands, can be termed as Double Sideband Full Carrier system, or simply DSB-FC. It is plotted as shown in the following figure. However, such a transmission is inefficient. Two-thirds of the power is being wasted in the carrier, which carries no information. If this carrier is suppressed and the power saved is distributed to the two sidebands, such a process is called as Double Sideband Suppressed Carrier system, or simply DSBSC. It is plotted as shown in the following figure.
SSB Generation method Filter system illustrates the block diagram of this method.
Phase Shift Method The phasing method of SSB generation uses a phase shift technique that causes one of the sidebands to be concealed. A block diagram of a phasing-type SSB generator is shown in Fig.
๏ท Balanced Modulators - This circuit drives the carrier input of a balanced mixer with the baseband signal. When the carrier input is nulled, a double sideband suppressed carrier (DSB-SC) output is produced containing just the AM sidebands. ๏ท Carrier Suppression - Start with a conventional AM signal. Use precise filtering such as a notch filter to remove the carrier, leaving only the sidebands. Stringent suppression is required. Either approach requires high linearity and low distortion modulation to avoid leakage between sidebands which can interfere with demodulation. ISB Transmission Block Diagram Vestigial Sideband (VSB) Modulation Definition : Vestigial Sideband (VSB) modulation is a modulation technique which allows transmission of one sideband in addition with a part or vestige of the other. It is basically a compromise between DSB-SC and SSB modulation. VSB technique was introduced to overcome the drawbacks of SSB modulation. As SSB modulation requires accurate frequency response of the filter to transmit only one sideband completely. Thus by using VSB modulation one can simplify the design of the filter to a great extent. Also, the SSB modulation does not allow transmission of extremely low-frequency signal accurately. Hence VSB technique is used. Usually, VSB technique is used in TV transmission as television signal are extremely low- frequency signals. Generation of Vestigial sideband (VSB) modulated wave Letโs now have a look at the figure shown below for the generation and transmission of Vestigial sideband signal:
Here, from the above figure, it is clear that a VSB signal is generated at the output of the sideband filter employed in the circuit. However, further amplification of the signal is needed in order to transmit it to longer distances. The balanced modulator used here produces the DSB-SC signal which is fed to the sideband filter. The filter is designed such that it transmits one sideband including vestige (some part) of the other. Thus producing a VSB signal. Description of frequency domain Letโs have a look at the spectrum of the message signal and VSB signal It is already clear that in vestigial sideband modulation some eliminated portion of USB is compensated by the vestige of LSB. The bandwidth of the Vestigial sideband signal is given by BW = fm + fv Performance comparison of Vestigial sideband (VSB) modulation with DSB-SC and SSB modulation We are already aware of the fact that an amplitude modulated signal consists of two sidebands along with a carrier. However, systems utilizing suppression of the signal eliminates sometimes carrier only and sometimes one sideband along with the carrier. Due to this, there exist some comparison points between them. ๏ท A system utilizing a conventional amplitude modulation technique increases both power consumption and bandwidth. However, DSB-SC and SSB systems reduce the power consumption. Among all VSB system is best when it comes to bandwidth utilization. ๏ท In case of DSB-SC and SSB modulation techniques, the filter must be highly efficient in order to suppress the carrier and desired sideband. On contrary, VSB does not need such accurate filtering characteristics.