Avionics Multiple Choice Questions on “Single Sideband Modulation”.
1. The type of signal transmission in which no power is wasted on the carrier is known as ________
a) Amplitude modulation
b) Frequency modulation
c) Sideband suppressed carrier signal
d) Unsuppressed carrier
Answer: c
Clarification: In an SSB or single sideband signal, the carrier is suppressed leaving the upper and lower sidebands. The resulting signal is referred to as the sideband suppressed carrier signal and one of the major benefits of this type of signal is that no power is wasted on the carrier.
2. The spectrum space occupied by a DSB signal is the same as that of a conventional AM signal.
a) True
b) False
Answer: a
Clarification: DSB or double sideband suppressed carrier does not alter the maximum and minimum frequency of the signal. It only suppressed the carrier signal and hence the spectrum space which is dependent on frequency remains unchanged.
3. What is used to generate a DSB signal?
a) Balanced modulator
b) Carrier suppressor
c) Antimodulator
d) Sideband suppressor
Answer: a
Clarification: Balanced modulator is a special circuit which produces DSB or Double sideband suppressed carrier signals. It produces the sum or difference of the carrier and modulating signal but cancel or balance out the carrier frequency.
4. Which of the following is an advantage of SSB over DSB?
a) No change in spectrum space
b) Reduce in spectrum space
c) Carrier is suppressed
d) Power is not wasted on the carrier
Answer: b
Clarification: In a signal, only the sidebands contain the information. Also, there is a duplication of the information in each sideband. Thus one sideband is sufficient form transmission as compared to DSB where both the sidebands are present. Since one of the sideband is also suppressed it reduces the maximum or the minimum frequency of the signal and hence the spectrum space is reduced in an SSB.
5. SSB transmitters are lighter and require less power than their normal AM transmitters.
a) True
b) False
Answer: a
Clarification: In an SSB one of the sidebands is also suppressed with the carrier signal thus reducing wastage of power. Since less power and circuitry are required to run the SSB transmitter than its equivalent AM transmitter they are light and require less power.
6. Where does selective fading occur?
a) Attenuator
b) Amplifier
c) Atmosphere
d) Antenna
Answer: c
Clarification: Since an AM signal is a combination of waves of different frequencies when it is passed through the atmosphere, they get affected by the ionosphere and the upper atmosphere which attenuate signals below the frequency of 50Hz. If one sideband is above 50Hz and the other one is below 50Hz the atmosphere attenuates one sideband alone. This is known as selective fading.
7. Why is no carrier signal transmitted when the amplitude of the modulating signal is 0 in an SSB transmission?
a) Attenuation
b) Undermodulation
c) No sideband is generated
d) All the sideband is suppressed
Answer: c
Clarification: In an SSB the carrier signal is suppressed. Also, only when a modulating signal is present the sideband is produced. When there is no modulating signal there is no sideband produced and the carrier is also suppressed hence nothing is transmitted.
8. Which one of the following is not true with respect to SSB signal?
a) Hard to demodulate
b) Less power
c) Carrier is suppressed
d) Very low range
Answer: a
Clarification: One of the main disadvantages of the sideband suppressed signal transmission is that it is hard to demodulate or recover. Since the carrier signal is not present, the carrier frequency must be fed into the receiver to completely extract the information signal.
9. The low power carrier transmitted with SSB signal is called as?
a) SSB carrier
b) Dummy carrier
c) Fake carrier
d) Pilot carrier
Answer: d
Clarification: Since demodulation is hard in SSB signals a low power carrier wave is transmitted with the SSB. This can be received and amplified at the receiving end to demodulate the SSB signal. This type is used in FM stereo transmissions.
10. What is the peak envelope power if the rms voltage is 127.26V across a 50Ω load?
a) 324W
b) 321W
c) 215W
d) 154W
11. What is the maximum current if the peak envelope power is 500W and the supply to the amplifier is 50V?
a) 10A
b) 10mA
c) 550A
d) 550mA
Answer: a
Clarification: PEP = Vs x Imax
Imax = PEP / Vs = 500/50 = 10A.
12. What is the PEP if the transmitter produces a peak to peak voltage of 178V across a 75Ω load?
a) 75.4W
b) 52.8W
c) 54.2W
d) 45.5W
13. What is the average power of the transmitter if PEP = 500W?
a) 0W to 500W
b) 100W to 300W
c) 125W to 166.66W
d) 0W to 250W
14. Which of the following is true with respect to the bandwidth of a 2kHz audio signal and a 2kHz binary data?
a) Bandwidth is higher for audio signals than binary data
b) Bandwidth is higher for binary data than audio signals
c) Bandwidth remains the same for both
d) Bandwidth keeps increasing with time for the binary data
Answer: b
Clarification: Since binary data is transmitted in square waves or pulses, they are generally made up of numerous fundamental sine waves. This occupies a huge bandwidth as compared to audio signals which are analog in nature and usually comprise of a lesser number of sine waves.