Analog Communications Questions and Answers Part-6

1. For Amplitude Modulation, Emitter modulator ________
a) Operates in class C mode
b) Has a low efficiency
c) Output power is high
d) Operates in class B mode

Answer: b
Explanation: Emitter Modulator operates in class A region. It has very low efficiency. The output power is low so for modulation at high level, it is not suitable.

2. Why AM is used for broadcasting?
a) More immune to noise
b) Less transmitting power is required
c) It has high fidelity
d) Avoids Receivers Complexity

Answer: d
Explanation: AM detectors are generally, square law demodulators or envelope detectors at the receiver. As AM detectors at the receiver end are simple circuits and avoid any kind of complex structure, therefore, AM used for broadcasting.

3. Singletone amplitude modulation ________
a) contains no frequency components
b) contains a large number of frequency components
c) consists of only one frequency component
d) contains infinite number of frequency components

Answer: c
Explanation: Single tone modulation consists of only one frequency component in the baseband or message signal. Thus, modulation of carrier wave is done by a single frequency component only.

4. The minimum channel Bandwidth is used by which modulation technique?
a) VSB
d) AM

Answer: b
Explanation: A signal has two sidebands which are exactly the mirror images of each other. So we can remove one side band which further reduces its bandwidth. In SSB-SC modulation technique, the carrier is suppressed and only either of the sidebands is transmitted. Thus, SSB-SC has minimum channel Bandwidth.

5. Neper is ________ decibel.
a) 20/(ln10)
b) 20ln10
c) Same as
d) Exactly twice of

Answer: a
Explanation: Neper is a logarithmic unit used for finding ratios of power quantities. Like decibel it is also a dimensionless unit. 1Np = 20/ln10 = 8.686 db.

6. Overmodulation results in ________________
a) Distortion
b) Weakens signal
c) Strengthens the signal
d) provides immunity to noise

Answer: a
Explanation: When instantaneous level of modulating signal exceeds the value necessary to provide 100% modulation, the signal is said to over-modulated. In other words, when modulation index is greater than 1, it results in Overmodulation. Thus, Overmodulation results in distortion of the modulating signal.

7. AM broadcast station transmits modulating frequency upto 6KHz. If transmitting frequency is 810KHz, then maximum and lower sidebands are ________
a) 816KHz and 804KHz
b) 826KHz and 804KHz
c) 916KHz and 904KHz
d) 822KHz and 816KHz

Answer: a
Explanation: Maximum frequency = 810 + 6 = 816KHz and Minimum frequency = 810 – 6 = 804KHz. Moreover it has a bandwidth of (816 – 804) = 12KHz.
( Formula: Fmax = fc + fm,
Fmin = fc-fm,
Bandwidth = 2*fm = Fmax = Fmin,
fc = Carrier frequency, fm = message signal frequency).

8. Find lower frequency component in AM wave, given that highest frequency component is 900KHz and bandwidth is 12KHz?
a) 832KHz
b) 600KHz
c) 888KHz
d) 868KHz

Answer: c
Explanation: Highest frequency component is 900KHz and bandwidth is 12KHz. So lower frequency component is 900 – 12 = 888KHz.
(Formula: Fmin = Fmax-2*fm = Fmax-Bandwidth, where fm = Message Signal Frequency).

9. Amplitude Modulated wave is ________
a) Sum of carrier and modulating wave
b) Product of carrier and modulating wave
c) Difference of carrier and modulating wave
d) Sum of carrier and its product with modulating wave

Answer: d
Explanation: The modulation of a carrier wave by varying its amplitude with respect to amplitude of baseband signal is known as amplitude modulation. It is represented as,
s(t) = [1 + mx(t)] c(t),
where, x(t) = Modulating Wave, m=Modulating Index
c(t) = Carrier Wave = Ac (Cos ωc) t
Thus, Amplitude Modulated wave is the Sum of carrier and its product with modulating wave.

10. Noise performance of a square law demodulator of AM signal is?
a) Better than that of synchronous detector
b) Weaker than that of synchronous detector
c) Better than that of envelope detector
d) Weaker than that of envelope detector

Answer: a
Explanation: Process of recovering message signal from received modulated signal is called demodulation. It is exactly opposite to modulation. There are two most used AM demodulators: Square Law Demodulator and Envelope Demodulator. Noise performance of Square Law Demodulator is far better than that of Synchronous Detector.