# Analog Communications Questions and Answers – Single Tone Modulation

This set of Analog Communications Multiple Choice Questions & Answers (MCQs) focuses on “Single Tone Modulation”.

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

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

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) consists of only one frequency component
b) contains a large number of frequency components
c) contains no frequency components
d) contains infinite number of frequency components

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. AM spectrum consists of ________
a) Carrier frequency
b) Upper sideband
c) Lower sideband
d) Carrier frequency with both upper and lower sideband

Explanation: Spectrum of Am wave consists of a carrier with upper sideband and lower sideband. If carrier frequency is Wc, then the two sidebands produced by it are (Wc+Wm) and (Wc-Wm), where Wm is the frequency of the message signal. The amplitude of the carrier is A and that of the two sidebands are mA/2, where m is the modulation index.

5. The minimum channel Bandwidth is used by which modulation technique?
a) VSB
b) SSB-SC
c) DSB-SC
d) AM

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

Explanation: Neper is a logarithmic unit used for finding ratios of power quantities. Like decibel it is also a dimensionless unit. 1Np = 20/ln⁡10 = 8.686 db.

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

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) 868KHz
d) 888KHz

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

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.

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