UNIT 1 QUESTIONS WITH ANSWERS 1. Define modulation? Modulation is a process by which some characteristics of high frequency carrier signal is varied in accordance with the instantaneous value of the modulating signal. 2. What are the types of analog modulation? Amplitude modulation. Angle Modulation 1. Frequency modulation 2. Phase modulation 3. Define depth of modulation. It is defined as the ratio between message amplitude to that of carrier amplitude m=vm/vc 4. What are the degrees of modulation? Under modulation. (m<1) Critical modulation (m=1) Over modulation (m>1) 5. Define Amplitude Modulation It is the process by which the amplitude of the carrier wave is changed in accordance with the instantaneous value of the message signal. 6. Define modulation index for AM It is defined as the ratio of the maximum modulating voltage to the maximum carrier voltage. It is also called as Depth of modulation. m = Vm / Vc 7. What is the relation between total power and carrier power? 2 m Pt 1 2 P c
Pt : Total power Pc : Carrier power m : Modulation index 8. What is the need for modulation? Ease of transmission Multiplexing Reduced noise Narrow bandwidth Frequency assignment Reduce the equipments limitations. 9. A 400W carrier is modulated to a depth of 75 %. Calculate the total power in the modulated wave Solution: Pc = 400 W m = 0.75 Pt=? Pt = Pc ( 1 + m 2 /2 ) Pt=512.5 W Pt = 400 (1 + 0.75 2 /2) 10. What is the relation between total power and carrier power? I t =I c (1+m 2 /2) ½ 11. What is the bandwidth of AM? Bandwidth =2fm 12. Define demodulation. Demodulation or detection is the process by which modulating voltage is recovered from the modulated signal. It is the reverse process of modulation. 13. A transmitter supplies 8 kw to the antenna when modulated. Determine the total power radiated when modulated to 30%. Solution: m =0.3; Pc =8 kw Pt =Pc (1+m 2 /2) =8.36 kw 14. Define frequency modulation. Frequency modulation is defined as the process by which the frequency of the carrier wave is varied in accordance with the modulating signal
15. Define modulation index of frequency modulation. It is defined as the ratio of maximum frequency deviation to the modulating frequency m = f / f m 16. Define phase modulation. Phase modulation is defined as the process of changing the phase of the carrier signal in accordance with the instantaneous amplitude of the message signal 17. Define phase deviation. The maximum phase deviation of the total angle from the carrier angle is called phase deviation. 18. Define frequency Deviation. The maximum departure of the instantaneous frequency from the carrier frequency is called frequency deviation. 19. Differentiate between narrow band and wide band FM signal. WBFM NBFM 1. Modulation index is greater than one. 2. Frequency deviation=75 KHz 3. Modulating frequency range from 30 Hz to 15 KHz. 4. Bandwidth 15 times NBFM. 5. Noise is more suppressed. 6. Use: Entertainment and broadcasting. 1. Modulation index is less than one 2. Frequency deviation=5khz 3. Modulating frequency=3khz. 4. Bandwidth = 2 FM. 5. Less suppressing of noise. 6. Use: Mobile communication. 20. What are the disadvantages of FM system? 1. A much wider channel is required by FM. 2. FM transmitting and receiving equipments tend to be more complex and hence it is expensive 21. What are the disadvantages of conventional (or) double side band full carrier system? In conventional AM,carrier power constitutes two thirds or more of the total transmotted power.this is a major drawback because the carrier contains no information ;the sidebands contain the information. 22. Define Single sideband suppressed carrier AM.
AM Single sideband suppressed carrier is a form of amplitude modulation in which the carrier is totally suppressed and one of the sidebands removed. 23. Define AM Vestigial sideband. AM vestigial sideband is a form of amplitude modulation in which the carrier and one complete sideband are transmitted,but only part of the second sideband is transmitted. 24. What are the advantages of single sideband transmission? The advantages of SSBSC are 1.Power conservation: Normally,with single side band transmission,only one sideband is transmitted and the carrier is suppressed. So less power is required to produce essentially the same quality signal. 2.Bandwidth conservation: Single sideband transmission requires half as much bandwidth as conventional AM double side band transmission. 3.Noise reduction: Because a single side band system utilizes half as much bandwidth as conventional AM,the thermal noise power is reduced to half that of a double side band system. 25. What are the disadvantages of single side band transmission? 1. Complex receivers: Single side band systems require more complex and expensive receivers than conventional AM transmission. 2. Tuning Difficulties: Single side band receivers require more complex and precise tuning than conventional AM receivers. 2 marks Questions 1. How FM wave can be converted to PM wave? 2. How PM wave can be converted to FM wave? 3. What are the types of Frequency Modulation? 4. List the properties of the Bessel function. 5. State the Carson s rule. 6. Why is FM superior to AM in performance? Detailed questions 1. Expression for AM & its Power and Efficiency calculation AM Definition Power relation in AM Current relation in AM Efficiency 2.Explain the AM generation with the waveforms AM definition Generation wave forms AM equations 3. Explain the power distribution for an AM wave with power spectrum diagram. AM power relation Power spectrum Power efficiency
4. Explain the AM current calculation. AM current relation Spectrum 5. Explain the voltage distribution for an Am wave AM voltage equations Modulation index Voltage spectrum 6. Explain the frequency analysis of angle modulated waves using Bessel identity. Modulation by a single frequency sinusoid Bessel functions Derivation Graph Frequency analysis 7. Explain the bandwidth requirements of angle modulated waves in detail. Bandwidth relation to modulation indices Carson s rule Efficiency