Tough Avionics Questions and Answers on “Frequency, Phase Modulation, Modulation Index and Sidebands”.
1. What are FM and AM collectively referred together as?
b) Angle modulation
c) Fast band modulation
d) Hi-fi Modulation
Clarification: Both FM and PM are collectively referred to as angle modulation. It includes varying the phase or the frequency of the carrier wave. Frequency and phase of the signal is dependent on the instantaneous angle and hence the name angle modulation.
2. The value of a change in frequency of the carrier wave by the modulating signal is called as?
a) Maximum modulation frequency
b) Maximum carrier frequency
c) Frequency deviation
d) Modulating frequency deviation
Clarification: The amount of carrier frequency change by the modulating signal is referred to as the frequency deviation. The maximum frequency deviation occurs when the amplitude of the modulating signal is maximum.
3. The maximum shift in frequency is 3kHz and the minimum and maximum deviation in frequency of the actual signal are 149.97MHz and 150.03MHz. What is the carrier frequency?
Clarification: Maximum deviation in frequency = carrier frequency + maximum shift in frequency
Minimum deviation in frequency = carrier frequency – maximum shift in frequency
Carrier frequency = Maximum deviation in frequency – maximum shift in frequency
Carrier frequency = 150.03MHZ – 0.03MHz = 150MHz.
4. The frequency of modulating signal has no effect on the amount of frequency deviation of a carrier signal in FM.
Clarification: In frequency modulation, the instantaneous amplitude of the modulating signal changes the frequency of the carrier. The frequency of the modulating signal has no effect Frequency modulation.
5. What type of modulation uses discrete values of carrier frequencies to transmit binary data?
a) Frequency modulation
b) Amplitude shift keying
c) Frequency shift keying
d) Phase modulation
Clarification: Frequency shift keying assigns different values of carrier frequencies for binary digits and transmits data by varying the frequency of the carrier in accordance with the binary data. This type of modulation is widely used in the transmission of binary data in digital cell phones and in some types of low speed modems.
6. FM and PM are more susceptible to noise than AM signals.
Clarification: Noise only affects the amplitude and not the frequency of the signal. In Amplitude modulation, the effect of noise is loss in information whereas in FM it does not affect as much as in amplitude modulation since the information is in the frequency and not in the amplitude.
7. Which of the following is false with respect to Pulse modulation?
a) Depends only on the frequency of the modulating signal
b) Phase of the carrier wave changes with respect to the modulating signal
c) Less affected by noise
d) Used to digitally transmit analog data
Clarification: Phase modulation depends on both the amplitude and frequency of the modulating signal. Higher amplitude causes a greater phase shift and higher frequency causes a greater rate of change of phase in the carrier signal.
8. What is the ratio of frequency deviation to the modulating frequency is known as?
a) Frequency index
b) Modulating index
c) Modulation index
d) Modulation ratio
Clarification: The modulation ratio is defined as the ratio of the frequency deviation to that of the modulating frequency. In communication systems using FM techniques, there are limits put on the maximum frequency and the modulation index.
9. What is the modulation index of a system with modulating frequency 75kHz and maximum frequency deviation 15kHz?
Clarification: Modulation index= mf = fd⁄fm = 75⁄15 = 5.
10. What is the deviation ratio if the maximum deviation is 25kHz and the maximum modulating frequency is 15kHz?
Clarification: mf = fd⁄fm = 25⁄15 = 1.667.
11. What is it called when an FM signal occupies no more spectrum space than AM signal?
a) Amplitude modulated frequency wave
b) Narrowband FM
c) Shortband FM
d) Equalband FM
Clarification: A narrowband FM is in which the FM signal occupies the same spectrum space as of its equivalent AM signal. Common FM radios use the narrowband transmission to reduce spectrum space.
12. What is the modulation index for the FM to behave as a narrowband?
Clarification: When the modulation index is around 0.25 the modulation produces only single pairs of significant sidebands like that of AM. This occupies comparatively less spectrum space.
13. What is the number of significant sidebands produced when the bandwidth of the signal is 24kHz and the modulating frequency is 3kHz?
14. According to Carson’s rule, what is the bandwidth of the signal with a deviation of 30kHz and a maximum modulating signal of frequency 5kHz?
Clarification: According to Carson’s rule, BW [fd(max) + fm(max)] = 2(30 + 5) = 70Khz.