Amplitude Modulation [PDF]

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1. AMPLITUDE MODULATION & DEMODULATION AIM: To generate the amplitude modulated signal and also calculate the modulation index. APPARATUS: 1. 2. 3. 4. 5. 6. 7. 8. 9.



Bread board trainer. Regulated Power Supply (0-30v) C.R.O Function generator Decade inductance box Resistors 68k, 10k, 12k , 3.3k Capacitors 0.001F, 1, 10, 0.1 Transistor SL 100. Diode OA-79



CIRCUIT DIAGRAM:



XSC1



VCC



24V G A



VCC



B



T



R4 12kohm



R1 68kohm C1



C4



D1



3 Q1 BC107BP 1uF



6



DIODE_VIRTUAL



1 2 Vc



0.001uF



10V 7.07V_rms 25kHz 0Deg



C3 R2



4



10kohm 5



0



L1 4.05mH



10uF Vm 10V 7.07V_rms 1000Hz 0Deg



R3 3.3kohm



OA79



C2



7



0.01uF



C5



m0dulated signal



demodulated signal



0.01uF



THEORY:



Amplitude modulation is a change in the signal voltage with all other factors remaining constant. I.e. the amplitude of a carrier signal is varied by the modulating signal voltage whose frequency is lower than that of carrier. Am defined as the system of modulation in which the amplitude of the carrier is made proportional to the instantaneous amplitude of the modulating voltage. The carrier voltage Vc and modulating voltage Vm can be represented by Vc= Vc Sin Wct,Vm= VmSinWmt.



The modulation m is given by m=Vm/Vc.



1. The change in the amplitude of the wave is caused by, and is directly proportional to the polarity and amplitude of the message signal. 2. The zero level of the message signal corresponds to the maximum level of the un modulated carrier signal. When the message signal goes positive ,the total signal is made greater in amplitude than the un modulated by an amount equal two the message signal. When the message signal goes negative, the total voltage is made less than the un modulated carrier wave by an amount equal to the message signal. 3. The +ve and –ve peaks of the modulated wave are mirror images of each other for conditions of Vmax and Vmin.



4. When the level of the message signal is equal to the level of carrier signal, the carrier is said to be fully modulated (100% modulation). The conditions for 100% modulation is Vc+Vm=2Vc,Vc-Vm=0. Vmax-Vmin Modulation factor (m)= ---------------Vmax+Vmin i.e. the peak message signal voltage is equal to the peak carrier voltage at 100% modulation . Then the total voltage in the wave at 100% modulation must be equal to twice the un modulated carrier voltage. The total side band voltage is equal to the carrier voltage; the voltage of each side band is equal to onehalf the carrier voltage. There fore total voltage= Vc+mVc/2+mVc/2 , Which means that the to side band voltages are equal and the value between zero and one-half of the carrier voltage.



The expression for the amplitude-modulated wave is E(message ) = e = Es(max)sin(2π fs)t E(total)=Ec sin(2π fc)t- mEc/2 cos 2π (fc+fs)t + mEc/2 cos2π (fc-fs)t. Ec sin(2π fc)t represents the carrier signal, mEc/2 cos 2π (fc+fs)t is upper sideband and mEc/2 cos2π (fc-fs)t. is lower side band.



The minimum voltage to be transmitted will be the unmodulated carrier voltage Ec for m=o.The maximum voltage to be transmitted will be 2Ec for 100% modulation, with a voltage of Ec/2in the lower side band and a voltage of Ec/2 in the upper side band.



The process of extracting a base band signal from the modulated signal is known as de modulation .AM signal with large carrier are detected by using the envelope detector. The envelope detector employs the circuit that extract the envelop of the AM wave. The envelope of the AM wave is the base band signal , How ever, a low level modulated signal can be detected by using square law detector in which a device operating in the non linear region is used to detect the base band signal.



A diode operating in a linear region of its characteristics can extract the envelop detector .it is very simple and less expensive.



PROCEDURE: 1. Connections are made as per the circuit diagram shown 2. The circuit is designed such that fc =25khz, i. fc= 1/2 √LC Where C=0.01μF& L= 4.05mh 3. The output of the amplitude- modulated signal is taken across the collector and is observed on CRO. 4. The readings are noted such that keeping Vc at 10v and by varying Vm for 2v,3v& 4v,notedown 5. Vmax &Vmin. And calculate modulation index. i. Vmax-Vmin ii. Modulation index (m)= ----------------i. Vmax+Vmin



6. Now the demodulation circuit is connected across the out put of modulator circuit ,to measure the frequency & amplitude of the out put message signal. 7. The AM can be measured using trapezoidal method on CRO by connecting modulated out put to ch.2 and message signal to ch.1. 8. On CRO we observe a trapezoid with longer side L1 and shorter side L2.So m=(L1 – L2)/(L1+L2). L1



L2



m



OBSERVATIONS:



Vc



Vm



MODEL WAVE FORMS:



Vmax



Vmin



M=Vm/Vc



%modulati on



PRECAUTIONS: 1. Connections must be tight. 2. The modulation index must be calculated carefully.



RESULT: 1. The amplitude modulated waveform for different cases is observed and the modulation index is calculated. 2. The AM can also be measured using CRO by trapezoidal method. VIVA QUESTIONS: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.



Define AM and draw its spectrum? Draw the phases representation of an amplitude modulated wave? Give the significance of modulation index? What are the different degrees of modulation? What are the limitations of square law modulator? Compare linear and nonlinear modulators? Compare base modulation and emitter modulation? Explain how AM wave is detected? Define detection process? What are the different types of distortions that occur in an envelop detector? How can they be eliminated? Over modulation causes How many channels are contained in the AM broadcast band? What is the bandwidth of each of the channels in the AM broadcast band