NO SE PUEDE SER Y NO SER AL MISMO TIEMPO Introducción.

167 Also, we saw the merits and demerits of FM as summarized below.

Merits of FM.

i. The amplitude of the frequency modulated wave remains unaffected.

ii. In FM there is a large decrease in noise and hence increase in S/N ratio.

iii. In FM, noise may be further reduced by increasing deviation.

iv. In FM, frequency allocation allows for a guard band. This reduces adjacent channel interference.

v. In the UHF bands where Fm operates, there is less noise than in the HF or MF bands.

vi. FM permits use of several independent transmitters on the same frequency with negligible interference.

Disadvantages of FM

i. A much wider channel, typically 200 kHz, is needed.

ii. Transmitting and receiving equipments are complex and costly.

iii. Reception using conventional methods is limited to line of sight.

Pre-emphasis in FM. In FM transmitter the higher modulation frequencies are boosted up before FM modulation using typically 50 / L-R network.

De-emphasis. When pre-emphasis is used in FM receiver at the output of detector, higher modulation frequencies are relatively attenuated to bring them back to their original relative values. Use of pre-emphasis and de-emphasis result in improved S/N ratio for higher modulation frequencies.

Adjacent Channel Interference in FM. In FM system, use of limiter results in automatic reduction in adjacent channel interference. Adjacent channel interference in FM is also reduced by the guard band provided in FM broadcast channel allocation.

Co-channel interference in FM. Use of amplitude limiter in FM results in interference reduction provided that desired signal channel is reasonably stronger than the undesired co-channel signal.

Wideband FM. It is used for broadcast. Typically the modulating frequencies extent from 30 Hz 15 kHz. Modulation index exceeds unity. Maximum permissible deviation is

=75 kHz.

Narrowband FM- We established that in narrowband FM:

- Modulation index is usually about unity

- The maximum modulating frequency is usually 3 kHz and

- Maximum frequency deviation is usually = 5 kHz. It is used by mobile communication services.

Stereophonic FM Multiplex System- This simply means that itdoes not use two separate channels, rather the sum of the two channels is sent as one signal and the difference as the other signal. The sum signal modulates the FM carrier and is received by monaural receiver and reproduced at output. The difference signal amplitude modulates a sub-carrier at 38 kHz. Which sub-carrier is then suppressed? The sidebands extending from 23 to 53 kHz then frequency modulates the carrier along with the sum signal.

168 5. Explain the effect of noise on carrier in FM system.

6. Why is it necessary to employ pre-emphasis and de-emphasis in FM system?

7. Draw typical pre-emphasis and de-emphasis circuits.

8. Explain how co-channel interference gets reduced in FM. Also, explain how co-channel interference gets reduced in FM system provided that the desired signal is stronger than the co-channel interfering signal.

9. Enumerate the salient features of wideband FM system.

10. Outline the typical applications of narrowband FM system.

11. With a well labeled block diagram, explain the principle of stereophonic FM multiplex system.

7.0 References/Further Readings

1. Sturley, k.R (1971). Frequency Modulated Ratio. George Newness Ltd., London, 1958.

2. Taub, H., and D.L. Schilling: Principle of communication Systems,” McGraw Hill Book company, New York.

3. Mandel M(1973): Principles Electronic communications,” Prentice Hall inc., Eaglewood cliffs, NJ.,.

4. Kennedy G (1977); “Electronic communication Systems,” McGraw Hill Kogakusha Ltd., Tokyo.

5. Termans F.E.: “Electronics and Ratio Engineering,” McGraw Hill Book Company, New York, 1955.

6. Ryder, J.D(1978): “Electronics Fundamentals and Applications,” 5^th edition, prentice Hall of india Private Ltd., New Deihi

7. Stremler, F.G.: Introduction to communication system (1977),” Addison Wesley Publishing Co., Reading, Mass,.

8. Holmes, Thom (2008). "Early Computer Music". Electronic and experimental music:

technology, music, and culture (3rd ed.). Taylor & Francis. pp. 257–8. ISBN 0-415-95781-8. Retrieved 2011-06-04.

9. Stan Gibilisco (2002). Teach yourself electricity and electronics. McGraw-Hill Professional. p. 477. ISBN 978-0-07-137730-0.

10. David B. Rutledge (1999). The Electronics of Radio. Cambridge University Press. p. 310.

ISBN 978-0-521-64645-1.

11. B. Boashash, editor, "Time-Frequency Signal Analysis and Processing – A Comprehensive Reference", Elsevier Science, Oxford, 2003; ISBN0-08-044335-4

12. T.G. Thomas, S. C. Sekhar Communication Theory, Tata-McGraw Hill 2005, ISBN0-07-059091-5 page 136.

13. Der, Lawrence, Ph.D., Frequency Modulation (FM) Tutorial, http://www.silabs.com/Marcom%20Documents/Resources/FMTutorial.pdf, Silicon Laboratories, Inc., accessed 2013 February 24, p. 5

14. B. P. Lathi, Communication Systems, John Wiley and Sons, (1968).ISBN0-471-51832-8, p, 214–217.

15. H. P. Westman, ed. (1970). Reference Data for Radio Engineers (Fifth ed.). Howard W.

Sams & Co. p. 21-11.

16. Alan Bloom (2010). "Chapter 8. Modulation". In H. Ward Silver and Mark J. Wilson (Eds). The ARRL Handbook for Radio Communications. American Radio Relay League.

p. 8.7. ISBN 978-0-87259-146-2.

169 17. Communication Systems (2001). 4th Ed, Simon Haykin.

18. FM Systems of Exceptional Bandwidth" Proc. IEEE vol 112, no. 9, p. 1664, September 1965.

19. A. Michael Noll (2001). Principles of modern communications technology. Artech House. p. 104. ISBN 978-1-58053-284-6.

20. Armstrong, E. H. (May 1936). A Method of Reducing Disturbances in Radio Signaling by a System of Frequency Modulation". Proceedings of the IRE. IRE. 24 (5): 689–740.

doi:10.1109/JRPROC.1936.227383.

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