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‘Modulation: Modulation is the process of changing the characteristics (Amplitude, Frequency, Phase) of cartier signal according to the instantaneous value of the modulating signal. is a low-frequency signal (voice 0-4 KHz and video 0-6 MHz) it cannot be transmitted over a long distance. Carrier signal is a high-frequency signal used to transmit a low-frequency modulating signal to allong distance. : 1. Amplitude Modulation (AM) - AM DSB FC/AM DSB SC/SSB/VSB 2. Angle Modulation - i. Phase Modulation (PM) - NBPM/WBPM i. Frequency Modulation (FM) - NBFM/WBFM ‘Modulating signal m (1) (Message signal, Base band signal, Demodulated signal) - Low-frequency signal Information carrying signal. m (t)= Vm Sin wm tor Vm Cos wm t Carrier signal c (t)- High-frequency signal used to carry the information-carrying signal i.e, modulati signal. c(t)= Ve sin we t or Ve Cos we t ‘Amplitude modulation (AM): AM is the process of changing the amplitude of the carrier signal according to the modulating signal. Frequency modulation (FM): FM is the process of changing the frequency of the carrier ‘signal according to the modulating signal. Phase modulation (PM): PM is the process of changing the phase of the carrier signal according to the modulating signal. Need for Modulation (or) Advantages of Modulation: 1, Reduction in the height of the antenna: For efficient transmission, antenna size=\/2 We knowC= fA, & C = 3x10*8m/s For example if f= SkHz, then M2 = C/2f = 3x10*8/2x5x10*3=30km Itis impossible to construct antenna of 30km height. I= 10MHz (high frequency), \/2 This antenna height can be practically achieved. 2. Multiplexing: Several me are ted over the common without interference using modulation. 3. Adjustment of Band Width: Signal to noise ratio can be improved since itis the function of Band Width. 4, Ease of Radiation: Due to modulation, signals are translated to higher frequencies. It becomes ‘easy to design amplifier circuits and antenna systems at increased frequencies. ‘Amplitude modulation (AM/DSB FC): In Amplitude modulation, the amplitude of the ca according to the instantaneous value of the modulating signal. Expression for AM ‘is changed Bandwidth of AM: BW=2fm The carer signal lt) =Vesiuct ee The message sgl mt) -V- srt eRe: The AM signal, Vgng= Vt Vn SIM yt cretion sey *a =H (L4msinogs) where m, Ye f= Va Ste. t (1m, sinayt) sinoet ‘The instantaneous value of AM signals Sit = Nes my Vesinutsin aot ‘Modulation index / Depth of modulation /% modulation: Its the parameter which indicates the depth of modulation (or) measure of modulation. Indicates the amount that the carrier signal is modulated. The modulation index ranges from me 0 to 1. It is defined as the ratio of the amplitude of the modulating signal to the amplitude of the carrier signal. m=Vm/Ve or m = (Vmax - Vin)/(Vmax + Vin) Degrees of modulation: There are 3 degrees of modulation » m<1 under modulation + m=1 critical modulation + m>1 over modulation( distortion) | Silabend wer 2 figs Teel fomer 9) Bence Tel Por TM bor (meme Powe faery Seb) He Caplin) BARES oly Sidehanda ave Prsenh theft Re = Fat i eet ay $s 24 a3 pg a4 . 43) ae gz gan 2s 3 al. & bg 2 Get Bs . re 3 ‘ Z ai i & Ail ; HT WU : ie Hogg Be ‘P g2 Ps rf y i L ik pt u u Pods Li is tat hs aad 47 a Bix ge | cag dar 2f iit ae u a Pes ge aa a ae qs - Modulation performed for a message signal with one frequency component. Multi-tone modulation -Modulation performed for a message signal with more than one frequency component Power and current relation in AM The Current relation in AM Is pea re(ve war(ueey where Pt - total modulated power, Pc-un modulated carrier power and m- modulation index. / where It - total modulated current, ic-un modulated carrier current and m-modulation index. Advantages of AM: AM wave can travel a long distance, It covers a larger area than FM. Disadvantages: Poor performance Inthe presence of ole, Inefficient ue ef anomiter power, Wastagen Band Width Generation of AM Waves (AM modulators): AM modulators: Nonlinear modulator (or) low level modulator (Square law modulator, Balanced modulator) Linear modulator (or) high level modulator (Collector modulator) Low level modulation: AM generation is at low power level, Modulation takes place prior to the output element of the final stage of the transmitter. High level modulation: AM generation is at high power level. Before ‘modulation, the carrier and the modulating signal are amplified to an adequate power level. Modulation takes place in the last RF amplifier stage of the transmitter. Efficiency of high level modulation is greater than low level. Nonlinear modulators: It makes use of the non linear characteristics of nonlinear device. (diode, transistor, FET) To make the device to operate in the nonlinear V1 characteristics (diode, transistor, FET) the input is kept low. Linear modulator: It makes use of the linear of the VI characteristics of diode, The input is kept high to operate the device in the linear region of VI characteristics of diode. ALA Gaveier Signed oy Segnarnett ja Amplitele modulated by a mee: Sigel of Aceene wit hee antewn Renjelonts pe gre ition, B Ek: i Bednar omer, nba owne, Sibant on, affinny FE and Hak Freswaney spect, ad cob a 5 ean amente a Doge [or ea efires] 2 es wpe oscar SR ane Some ee hsp # 2 ye Weer Rew OO he hohe uy © ty Owe Bh, = Be tine foo e-etnns Sd © Caertes Nowe fee AE = SH BL a has ree fe Se Eee Sireme = DSB SC -Double sideband suppressed carrier system: DSB SC contains only sidebands and the carrier is suppressed. Transmitting power Is saved but bandwidth remains the same as AM. It can be generated by the product of Carrier and the message signal Derive the expression for DSBSC: Product modulator generates DSB SC i and message signal produces DSB SC Product modulator ‘muttiplication of carrier signal Cartier signal c(t}= Ve sinut Message signal mi S(t}=Vc Vm sinwimt sin, t {Vm [c0s(w-ty}t costae Wn)t] 2 sidebands et) st) 050 C signat Messane signal Bandwidth of DSB SC: fm, BW remains same as AM. ‘Advantages: DSB - SC is more efficient in transmitted power as compared to DSBFC. Better signal to noise ratio as compared to SSB. Disadvantage: BW remains same as AM even though carrier is suppressed. Detection: Demodulation or detection is the process by which the original modulating signal is recovered from ‘the modulated signal. ttis the reverse process of modulation. ‘Coherent detection: The modulating m(t) can be recovered from DSB - SC by first multiplying the locally ‘generated carrier. The phase and frequency of the locally generated carrier and carrier at the transmitter must bbe exactly coherent in phase and frequency otherwise the detected signal will be distorted. It consists of product modulator followed by an LPF. The product modulator multiples the DSB SC ‘modulated signal and the locally generated carrier. The output of product modulator is applied to the LPF to allow the modulating signal only. Operation: st) ve) Vil The input signal can be DSB SC ar SSB Sc aa) Post |] |, tetemutisied by cally generated carrier DSBSCsignal |_modulator Vth m(t}A “= 2 , # toed 2 i 2. 2 4S 2 = © Rly =e = 42742 aa i ¥ Bs | Rs = 2 2 2 Fy [= Lee 3 2 3 1 g2 43 so: = se a z = zh lI ze &* C4 
