CHAPTER 4
Frequency Modulation
1. In FM, the information signal varies the frequency of the carrier.
2. The amount of frequency change from the carrier center frequency is called the frequency deviation.
3. In PM, the deviation is proportional to the amplitude of the modulating signal.
4. During FM, the carrier amplitude remains constant.
5. Both FM and PM are types of angle modulations.
6. In PM, the phase shift of the carrier is varied by the amplitude of the modulating signal.
7. Phase modulation produces frequency modulation.
8. The FM produced by PM is called indirect FM.
9. Maximum frequency deviation in a PM signal occurs where the rate of change of the modulating signal amplitude is greatest, which is at its zero-crossing points.
10. Frequency deviation does not occur at the output of a phase modulator unless the modulating signal amplitude varies.
11. The amount of frequency deviation produced by a phase modulator increases with the modulating frequency.
12. To produce true FM from a PM signal, the amplitude of the modulating signal must be decreased with frequency so that frequency deviation does not change with modulating frequency.
13. In PM, a low-pass filter on the modulating signal compensates for increased frequency deviation at the higher modulating frequencies.
14. Frequency modulation produces pairs of sidebands spaced from the carrier in multiples of the modulating frequency.
15. The modulation index m of an FM signal is the ratio of the frequency deviation fd to the modulating frequency fm (m = fd / fm).
16. The deviation ratio is the maximum frequency deviation divided by the maximum modulating frequency.
17. The modulation index determines the number of significant pairs of sidebands in an FM signal.
18. The amplitudes of the carrier and sidebands vary with the modulation index and can be calculated with a mathematical procedure known as the Bessel functions.
19. The carrier or sideband amplitudes are zero at some modulation indexes.
20. The bandwidth of an FM signal is proportional to the modulation index.
21. There are two ways to calculate the bandwidth of an FM signal.
a. BW = 2Nfm max
b. BW = 2(fd max + fm max)
22. For FM, the percentage of modulation is the ratio of the actual frequency deviation and the maximum allowed frequency deviation multiplied by 100.
23. The primary advantage of FM over AM is its immunity to noise.
24. Noise is short-duration amplitude variations caused by lightning, motors, auto ignitions, power transients, and other sources.
25. Limiter circuits in FM receivers clip off noise signals.
26. Another benefit of FM over AM is the capture effect that allows the strongest signal on a frequency to dominate without interference from the other signal.
27. A third benefit of FM over AM is greater transmitter efficiency since class C amplifiers may be used.
28. A major disadvantage of FM is that its bandwidth is wider than the bandwidth of AM.
29. The spectrum space taken up by an FM signal may be limited by carefully controlling the deviation ratio.
30. Another disadvantage of FM is that the circuits to produce and demodulate it are usually more complex and expensive than AM circuits.
31. Noise occurs primarily at high frequencies; therefore, noise interferes more with high modulating frequencies.
32. Interference from high-frequency noise can be minimized by boosting the amplitude of high-frequency modulating signals prior to modulation. This is called pre-emphasis.
33. Pre-emphasis is accomplished by passing the modulating signal through an RC network that linearly boosts the amplitude of frequencies above 2122 Hz in proportion to frequency. This increases the signal-to-noise ratio at the higher frequencies.
34. The effect of pre-emphasis is corrected for, in an FM receiver by de-emphasizing the higher frequencies by passing them through an RC low-pass filter.
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