Introduction_to_AM_-_Module_4.pdf

AM Concepts (Introduction to Amplitude Modulation) An Introduction For successful transmission and reception of intelligence by the use of radio waves, two processes are essential: modulation demodulation Modulation Process: the baseband voice, video or digital signal modifies...

AM Concepts (Introduction to Amplitude Modulation) An Introduction For successful transmission and reception of intelligence by the use of radio waves, two processes are essential: modulation demodulation Modulation Process: the baseband voice, video or digital signal modifies another, higher-frequency signal called the carrier, which is usually a sine wave process of impressing a low frequency information signals onto a high-frequency high carrier signal the resultant wave is called the modulated carrier wave and is done at the transmitting station Demodulation Process: the process of separating or recovering the signal from the modulated carrier wave just the opposite of modulation and is performed at the receiving end An Introduction What is a Carrier Wave? it is a high-frequency frequency undamped radio wave produced by RF Oscillators constant amplitude and travel with the velocity of light they cannot produce any sound in the loudspeaker of a receiver carry the signal (audio or video) from transmitting station to the receiving station An Introduction Need for Modulation three main hurdles in the process of direct transmission of AF signals: relatively short range if everybody started transmitting these low-frequency low signals directly, mutual interference will render all of them ineffective size of antennas required for their efficient radiation would be large For efficient radiation of a signal: The minimum length of an antenna is one quarter wavelength (/4)( the antenna length L is connected with the frequency of the signal by the relation L = 75 x 10^6 / f (in meters) solution: Modulation – enables a low-frequency frequency signal to travel very large distances through space with the help of a high frequency carrier wave these carrier waves need reasonably-sized sized antennas and produce no interference with other transmitters operating in the same area An Introduction Radio Broadcasting An Introduction Modulation the process of combining an audio frequency (AF) signal with a radio frequency (RF) carrier wave the AF signal is also called a modulating wave and the resultant wave produced is called modulated wave during modulation, some characteristics of the carrier is varied in time with the modulating signal and is accomplished by combining the two An Introduction Methods of Modulation The mathematical expression for a sinusoidal carrier wave is the waveform can be varied by any of its following three factors or parameters: Ec – the amplitude fc – the frequency  - the phase Amplitude Modulation AM Concepts the amplitude of the carrier wave is varied in proportion to the instantaneous amplitude of the information signal or AF signal obviously, the amplitude of the carrier wave is changed but not its frequency Process of AM the function of the modulator is to mix these two waves in such a way that (a) is transmitted along (b) all stations broadcasting on the standard broadcast band (550-1605kHz) use AM modulation Amplitude Modulation AM is a relatively inexpensive, low-quality quality form of modulation that is used for commercial broadcasting of both audio and video signals. AM is also used for two-way way mobile radio communications, such as citizens band (CB) radio. AM Modulators non-linear linear devices with two inputs and one output Radio Frequencies (RFs) frequencies that are high enough to be efficiently radiated by an antenna and propagated through free space Information Signal may be a single frequency or more likely consists of range of frequencies Modulator the information acts on or modulates the RF carrier producing a modulated waveform the modulated waveform from an AM modulator is often called an AM envelope Amplitude Modulation The AM Envelope DSBFC or conventional AM or simply AM Vc sin (2fct) Vm sin (2fmt) Vam(t) the shape of modulated wave the repetitio rate of the envelope is equal to the frequency of the modulating signal and that the shape of the envelope is identical to the shape of the modulating signal Amplitude Modulation AM Frequency Spectrum and Bandwidth Output Envelope – a complex wave Dc voltage the carrier frequency sum (fc + fm) and difference (fc - fm) frequencies the cross products are displaced from the carrier frequency by an amount equal to the modulating signal frequency AM signal spectrum contains frequency components spaced fm Hz on either side of the carrier the effect of modulation is to translate the modulating signal in the frequency domain so that it is reflected symmetrically about the carrier frequency Amplitude Modulation Frequency Spectrum of an AM DSBFC wave the AM spectrum extends from fc – fm(max) to fc + fm(max) lower sideband (LSB) the band of frequencies between fc - fm(max) and fc (LSF) upper sideband (USB) the band of frequencies between fc and fc + fm(max) (USF) Bandwidth (B) the difference between the highest upper side frequency and the lower side frequency , or two times the highest modulating signal frequency B = 2fm(max) Amplitude Modulation Example: For an AM DSBFC modulator with a carrier frequency fc= 100kHz and a maximum modulating signal frequency fm(max) = 5kHz, determine: frequency limits for the upper and lower sidebands bandwidth upper and lower side frequencies produced when the modulating signal is a single-frequency 3kHz tone draw the output spectrum Amplitude Modulation Phasor Representation of an Amplitude-Modulated Amplitude Wave Phasor Addition an AM envelope is produced from the vector addition of the carrier and the upper and lower side frequencies the two side frequencies combine and produce a resultant components that combines with the carrier vector usf c lsf  c the phasors for the carrier and the upper and lower side frequencies combine, sometimes in phase (adding) and sometimes out of phase (subtracting) Amplitude Modulation Phasor Addition Producing an AM Envelope the maximum positive amplitude of the envelope occurs when the carrier and the upper and lower side frequencies are at their maximum positive values at the same time ( +Vmax = VC + Vusf + Vlsf) the minimum positive amplitude of the envelope occurs when the carrier is at its maximum positive value at the same time that the upper and lower side frequencies are at their maximum negative values ( +Vmin = VC - Vusf - Vlsf) the maximum negative amplitude occurs when the carrier and the upper and the lower side frequencies are at their maximum negative values at the same time ( -Vmax =- VC - Vusf - Vlsf) the minimum negative amplitude occurs when the carrier is at its maximum negative value at the same time that the upper and the lower side frequencies are at their maximum positive values ( -Vmin = -VC + Vusf + Vlsf) Amplitude Modulation Phasor Addition Producing an AM Envelope