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MAHALAKSHMI ENGINEERING COLLEGE TIRUCHIRAPALLI – 621213 QUESTION BANK DEPARTMENT: ECE SEMESTER: IV SUBJECT CODE / Name: EC2252 – COMMUNICATION THEORY UNIT-I AMPLITUDE MODULATION SYSTEMS PART -A (2 Marks) 1. Define Modulation Index for an AM Signal (AUC MAY 2010)(AUC MAY 2006) (AUC MAY 2013) It is defined as the ratio between message amplitude to that of carrier amplitude. ma=Em/Ec 2. Draw the circuit of an envelope detector. (AUC NOV 2007) 3. State any important spectral properties of periodic power signals (AUC NOV 2007) A signal px (t)is said to be periodic if xp(t) =xp(t+T), (1.1) for all t and some T. � ( denotes the end of definition, example, etc.) EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE Page 1 Let T0 be the smallest value of T for which this is possible. We call T0 as theperiod of xp(t). Fig. shows a few examples of periodic signals. 4. Define Amplitude Modulation. Give the expression for AM wave (AUC NOV 2007) This is defined as the modulation, in which amplitude of carrier is changed in accordance to the amplitude of the modulating signal. 5. Give the applications of SSBSC-AM. (AUC NOV 2007) Where power saving and low bandwidth requirement are important Used in land and air mobile communication, navigation and amateur radio 6. Compare low level modulation and high level modulation. (AUC NOV 2007) 7. A transmitter radiates 9 KW without modulation and 10.125 KW after modulation. Determine depth of modulation. (AUC NOV 2007) 8. A 2 MHz carrier having amplitude of 5V is modulated by a 4 KHz audio signal having amplitude of 2V. Determine the modulation index and draw frequency spectrum of amplitude modulated wave. (AUC MAY 2008) EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE Page 2 9. SSB is suitable for speech signals and not for video signals. Why? (AUC MAY 2008) Bandwidth of SSB is half that of DSB-SC AM. Thus twice the number of channels can be accommodated at a given frequency spectrum. No carrier is transmitted, hence possibility of interference with other channels are avoided. It eliminates the possibility of fading. Fading occurs due to multipath propagation of electro-magnetic waves. 10. A 500 W carrier modulated to a depth of 60 percent. Calculate the total power in modulated wave. (AUC NOV 2008) 11. Define vestigial sideband transmission. Mention its application. (AUC NOV 2008) In VSB the desired sideband is allowed to pass competently. Whereas a small portion of the undesired sideband is also allowed Application: Television broadcasting 12. Compute the bandwidth of the Amplitude modulated signal C(t)=23*cos(230000πt)[1+0.8*cos (310πt)] (AUC MAY 2009) Ma=0.8, 2πfm=310 Fm=fm=310 Fm=49.33Hz BW=2fm=98.66Hz 13. Define AM and its spectrum (AUC MAY 2009) Amplitude modulation is defined as the process in which the amplitude of the carrier signal is varied in accordance with the modulating signal or message signal. EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE Page 3 14. An AM DSB-FC wave with a peak unmodulated carrier voltage AC=10V, a load resistor RL=10Ώ and the modulation index m=1. Determine the total power of the modulated wave. (AUC MAY 2009) 15. How many AM broadcast stations can be accommodated in a 100 kHz bandwidth if t he highest frequency modulating a carrier is 5 kHz? AUC DEC 2010) (AUC MAY 2010) ANS:20AM Broadcast stations 16. What are the advantage of VSB (AUC MAY 2011) It has bandwidth greater than SSB but less than DSB system. Power transmission greater than DSB but less than SSB system. No low frequency component lost. Hence it avoids phase distortion. 17. Calculate the local oscillator frequency if incoming frequency is f1 and translated carrier frequency (AUC MAY 2011) The requiredloca oscillator frequcny (f1) is F2= f1+f2 F1=f2-f1 18. How many AM broadcast stations can be accommodated i n a 100 kHz bandwidth if the highest frequency modulating a carrier i s 5 kHz? (AUC DEC 2011) Ans: 20 AM broad cast stations 19. State the applications of FDM. (AUC DEC 2010) (AUC MAY 2011) Telemetry Commercial broadcast and television Communication network EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE Page 4 20. What is meant by frequency translation? 21. Compute the bandwidth of the amplitude modulated signal given by s(t)=23[1+0.8 cos(310t)]cos(230000πt) (AUC MAY 2012) ANS: ma=0.8 , 2πfm=310 therefroefm=49.33Hz B.W=2fm=98.66Hz 22. What are the causes of linear distortion (AUC MAY 2010)(AUC MAY 2012) Amplitude distortion Phase distortion Frequency response distortion EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE Page 5 Group delay distortion Harmonic distortion Pulse distortion 23. Compare the performance of AM and FM systems. S.No. 1. Amplitude modulation Amplitude of the carrier is varied according to amplitude of modulating signal 2. Am has poor fidelity due to narrow band width 3. Most of the power is in carrier hence less efficient 4. Noise interference is more 5. Adjacent channel interference is present 6. AM broad cast operates in MF and HF range. 7. In AM only carrier and two side bands are present. 8. The transmission equipment is simple. 9. Transmitted power varies according to modulation index. 10. Depth of modulation has limitations. It can be increased above 1. Frequency modulation Frequency of the carrier is varies according to amplitude of the modulating signal Since the band width is large, fidelity is better All the transmitted power is useful. Noise interference is minimum. Adjacent channel interference is avoided due to guard bands. FM broadcast operates in VHF and UHF range. Infinite number of sidebands are present. The transmission equipment is complex. Transmitted power remains constant irrespective of modulation index. Depth of modulation has no limitation. It can be increased by increasing frequency deviation. 25.Compare Bandwidth and power requirement in terms of carrier power for AM,DSBSC, and SSB(AUC MAY 2013) Description AM DSBSC SSB Bandwidth 2fm 2fm fm 66.68% 83% 50% 75% power requirement for 33.33% sinusoidal power requirement for 33.33% non sinusoidal EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE Page 6 PART- B 1. Discuss coherent detection of DSB-SC modulated wave with a block diagram of the detector. (AUC NOV 2006) (ii) Write about the coherent detection method in detail for DSB-SC and SSB- SC. What happens when there is phase mismatch? (8)(AUC DEC 2010) EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE Page 7 EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE Page 8 EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE Page 9 2. Draw the filtering scheme for the generation of VSB modulated wave and Explain. (AUC NOV 2007) Generation of VSB In VSB 1. One sideband is not rejected fully. EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE Page 10 2. One sideband is transmitted fully and a small part (vestige)of the other sideband is transmitted. The transmission BW is BWv = B + v. where, v is the vestigial frequency band. EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE Page 11 EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE Page 12 3. Explain the double side band suppressed carrier modulation technique (AUC NOV 2007)Explain in detail about the balanced modulator with block diagram.(AUC NOV 2007) Discuss the method of demodulation of DSBSC signal using costas loop.(AUC MAY 2008) Explain the operation of a ring modulator. State its advantages. (AUC NOV 2008) (With the help of a neat diagram, explain the generation of DSB-SC using balanced modulator. (8) (AUC MAY 2010) Generation of DSB-SC Signals The circuits for generating modulated signals are known as modulators. We shall discuss three basic modulators – nonlinear, switching and ring modulators. Conceptually, the simplest modulator is the product or multiplier modulator, Figure 1(a). However, it is very difficult (and expensive) in practice to design a product modulator that maintains amplitude linearity at high carrier frequencies. EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE Page 13 EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE Page 14 EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE Page 15 EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE Page 16 EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE Page 17 4. Explain frequency translation. (AUC NOV 2007) EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE Page 18 5. An SSB signal is generated by modulating an 800 KHz carrier by the signal m(t) = cos2000πt + 2sin1200πt. The amplitude of the carrier is Ac=10. Obtain the magnitude spectrum of the lower sideband SSB signal. (AUC MAY 2008) 6. With a neat block diagram explain the SSB transmission. (AUC NOV 2008) (ii) How SSB can be generated using Weaver’s method? Illustrate with a neat block diagram. (6) Generation of Suppressed-Carrier AM •Full-carrier AM is simple but not efficient •Removing the carrier before power amplification allows full transmitter power to be applied to the sidebands •Removing the carrier from a fully modulated AM systems results in a double-sideband suppressed-carrier transmission. 1. Phasing method 2. Filter Method Phasing method This method is a special modulation type of IQ canonical form of Generalized transmitters Filter Method The filtering method is a special case in which RF processing (with a sideband filter) is used to form the equivalent g(t), instead of using base band processing to generate g(m) directly. EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE Page 19 The filter method is the most popular method because excellent sideband suppression can be obtained when a crystal oscillator is used for the sideband filter. Crystal filters are relatively inexpensive when produced in quantity at standard IF frequencies. Weaver’s Method for Generating SSB. Single-Sideband AM EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE Page 20 •The •As two sidebands of an AM signal are mirror images of one another a result, one of the sidebands is redundant •Using single-sideband suppressed-carrier transmission results in reduced bandwidth and therefore twice as many signals may be transmitted in the same spectrum allotment •Typically, a 3dB improvement in signal-to-noise ratio is achieved as a result of SSBSC 7. (i)Draw an envelope detector circuit used for demodulation of AM and explain its operation. (10)(i)Draw an envelope detector circuit used for demodulation of Am and explain its operation.With a help of a neat diagram, explain the operation of an envelope detector. Why does negative peak clipping take place (AUC MAY 2011)(AUC MAY 2011) EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE Page 21 The charging time constant RsC is very small when compared to the carrier period 1/fc i.e., RsC<< 1/fc Where Rs= internal resistance of the voltage source. C = capacitor fc= carrier frequency i.e., the capacitor C charges rapidly to the peak value of the signal. The discharging time constant RlC is very large when compared to the charging time constant i.e., 1/fc <<RlC<< 1/W Where Rl= load resistance value W = message signal bandwidth i.e., the capacitor discharges slowly through the load resistor. Advantages: It is very simple to design It is inexpensive Efficiency is very high when compared to Square Law detector Disadvantage: Due to large time constant, some distortion occurs which is known as diagonal clipping i.e., selection of time constant is somewhat difficult Application: It is most commonly used in almost all commercial AM Radio EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE Page 22 8. Compare Amplitude Modulation and Frequency Modulation. (6)(AUC MAY 2010) Amplitude Modulation Frequency Modulation. Amplitude of the Carrier is varied in Frequency of the Carrier is varied in according to amplitude of modulation signal according to amplitude of modulation signal AM has poor fidelity due to narrow bandwidth Since the bandwidth is large fidelity is better Most of the power is in the carrier hence less All the trnasmitterd power is useful efficient Noise interference is more Noise interference is minimum Adjacent channel interference is present Adjacent channel interference avoided due to guard bands 9. With necessary diagrams and expressions explain the generation and demodulation of AM. (AUC DEC 2011)(AUC APR/MAY 2011) Switching Modulator (1/4) ◊One way to generate an AM wave: Switching Modulator. ◊ Assume carrier wave c(t) is large in amplitude and the diode acts as an ideal switch. load voltage v2(t) varies periodically between the values v1(t) and zeros at a rate equal to the carrier frequency fc. ◊ By assuming a modulating wave that is weak compared with the EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE Page 23 carrier wave, we have effectively replace the nonlinear behavior of the diode by an approximately equivalent piecewise-linear time-varying operation Unwanted component, the spectrum of which contains ◊ Delta function at 0, ±2fc, ±4fc and so on. ◊ Occupy frequency intervals of width 2W centered at 0, ±3fc, ±5fc and so on, where W is the message bandwidth. Be removed by a band filter with mid frequency f and ◊ using band-pass mid-band fc bandwidth 2W, provide that fc >2W Generation of AM waves There are two methods to generate AM waves Square-law modulator Switching modulator EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE Page 24 Square-law modulator: - A Square-law modulator requires three features: a means of summing the carrier and modulating waves, a nonlinear element, and a band pass filter for extracting the desired modulation products. Semi-conductor diodes and transistors are the most common nonlinear devices used for implementing square law modulators. The filtering requirement is usually satisfied by using a single or double tuned filters. When a nonlinear element such as a diode is suitably biased and operated in a restricted portion of its characteristic curve, that is ,the signal applied to the diode is relatively weak, we find that transfer characteristic of diode-load resistor combination can be represented closely by a square law : V0 (t) = a1Vi (t) + a2 Vi2(t) ……………….(i) Where a1, a2 are constants Now, the input voltage Vi(t) is the sum of both carrier and message signals i.e., Vi(t) =Accos 2 fct+m (t) ……………. (ii) Substitute equation (ii) in equation (i) we get V0 (t) =a1Ac [1+kam (t)] cos2 fct +a1m (t) +a2Ac 2cos22 fct+a2m2 (t) ………..( iii) Where ka=2a2/a1 Now design the tuned filter /Band pass filter with center frequency fc and pass band frequency width 2W.We can remove the unwanted terms by passing this output voltage V0(t) through the band pass filter and finally we will get required AM signal. V0 (t) =a1Ac [1+2a2/a1 m (t)] cos2 fct EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE Page 25 Assume the message signal m (t) is band limited to the interval –W f W Switching Modulator: - Assume that carrier wave C (t) applied to the diode is large in amplitude, so that it swings right across the characteristic curve of the diode .we assume that the diode acts as an ideal switch, that is, it presents zero impedance when it is forward-biased and infinite impedance when it is reverse-biased. We may thus approximate the transfer characteristic of the diode-load resistor combination by a piecewise-linear characteristic. The input voltage applied Vi(t) applied to the diode is the sum of both carrier and message signals. Vi (t) =Accos 2 fct+m (t) …………….(i) During the positive half cycle of the carrier signal i.e. if C (t)>0, the diode is forward biased, and then the diode acts as a closed switch. Now the output voltage Vo (t) is same as the input voltage Vi (t) . During the negative half cycle of the carrier signal i.e. if C (t) <0, the diode is reverse biased, and then the diode acts as a open switch. Now the output voltage VO (t) is zero i.e. the output voltage varies periodically between the values input voltage Vi(t) and zero at a rate equal to the carrier frequency f c. i.e., Vo (t) = [Accos 2 fct+m (t)] gP(t)……….(ii) Where gp(t) is the periodic pulse train with duty cycle one-half and period Tc=1/fc and which is given by gP(t)= ½+2/ [(-1)n-1/(2n-1)]cos [2 fct(2n-1)]…………(iii) EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE Page 26 Now design the tuned filter /Band pass filter with center frequency fc and pass band frequency width 2W.We can remove the unwanted terms by passing this output voltage V0(t) through the band pass filter and finally we will get required AM signal. V0 (t) =Ac/2[1+kam (t)] cos2 fct The AM spectrum consists of two impulse functions which are located at fc & -fc and weighted by Aca1/2 & a2Ac/2, two USBs, band of frequencies from fc to fc +W and band of frequencies from -fc-W to –fc, and two LSBs, band of frequencies from fc-W to fc & -fc to -fc+W. 10. (i)Compare the various amplitude modulation schemes.(6)Compare various Amplitude modulation systems(i)Compare the characteristics of DSBFC, DSBSC, SSBFC, SSBSC, VSB schemes. Description B.W Power saving for sinusoidal Power saving for non sinusoidal (AUC MAY 2012) DSB-FC DSB-SC SSB-SC VSB fm<2f1 75% 2fm 2fm fm 33.33% 66.66% 83% 33.3% 50% 75% 75% Generation methods Not difficult Not difficult Difficult Difficult Application AM broad cast application Carrier telephony Police wireless mobile etc. TV high speed tx 11. How SSB can be generating using Weaver’s method? Illustrate with a neat block diagram (AUC MAY 2012) EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE Page 27 12. (i)What is frequency division multiplexing? Explain.With neat block diagram, explain the transmitter and receiver section of Frequency Division Multiplexing. (10) (ii)Explain the concept of FDM with a suitable block diagram. Discuss Frequency Division Multiplexing (i) Discuss in detail about frequency translation and frequency division multiplexing technique with diagrams. (10) (AUC NOV 2007)(AUC DEC 2011) EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE Page 28 13. Explain in detail about demodulation of AM waves. (AUC MAY 2013) EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE Page 29 EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE Page 30 EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE Page 31