# Analog Communications Questions and Answers – Generation of AM Waves

This set of Analog Communications Multiple Choice Questions & Answers (MCQs) focuses on “Generation of AM Waves”.

1. Which of the following devices is used to generate AM waves?
a) Square-law modulator
b) Reactance modulator
c) Transmitter
d) Transducer

Explanation: AM signals are generated by Square-law modulators. The input for generating AM signal should be of type (A + m(t)) instead of m(t). Square-law modulator sums carrier and information signal, then passes them through a non-linear device.

2. What is the disadvantage of FM over AM?
a) high modulating power is needed
b) requires high output power
c) large bandwidth required
d) high noise is produced

Explanation: Advantage of FM over AM is that the amplitude of an FM wave remains constant. In FM, the power of transmitted wave depends on amplitude of unmodulated carrier wave and hence it is constant. FM is less prone to noise compared to AM. However, wide-band FM has a wider bandwidth than AM as it’s BW is given by Carson’s rule which = 2*(β+1)fm, where β = Frequency Modulation Index and fm is frequency of modulating signal. And BW of AM = 2*fm.

3. For low level amplitude modulation, amplifier must be ________
a) Class C amplifier
b) Class B amplifier
c) Class D amplifier
d) class A amplifier

Explanation: In low level AM, modulation is done at low power of carrier and modulating signal therefore output power is low. Therefore, power amplifiers are used to boost the carrier and modulating signal. Thus, Class C amplifier is used.

4. Approximate range of a cordless telephone is ________
a) 900m
b) 100m
c) 1000m
d) 475m

Explanation: Cordless telephone is one in which the handset is portable and communicates with the body of the phone by radio waves instead of being connected by a cord. The range depends on the quality of instrument and is about 90m to 100m.

5. A wave is modulated by two sin waves having modulation indices of 0.3 and 0.5. Find the total modulation index?
a) 0.1
b) 0.7
c) 0.58
d) 0.35

Explanation: Given that m1 = 0.3 and m2 = 0.5. Total modulation index will be equal to . By substituting values we have which is equal to 0.58.
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6. Carrier wave carries information.
a) True
b) False

Explanation: It is a high frequency electro-magnetic wave. A carrier wave does not have any information. One of the properties like amplitude, frequency or phase of the carrier are modulated with respect to an input signal for the purpose of conveying information.

7. What can we do to eliminate distortion in the picture?
a) use a longer transmission line
b) change the antenna orientation
c) use a short transmission line
d) connect a booster

Explanation: According to modulation, length of antenna should be one fourth of wavelength. If distortion occurs whether in television or at any other place, simplest way to eliminate this is to change the orientation of antenna.

8. For 100% amplitude modulation, the power in upper sideband when carrier power is to be 100W?
a) 100W
b) 75W
c) 25W
d) 50W

Explanation: Modulation index, m = 100% = 1. Power in sidebands is (Pcxm24). By substituting the values, we have (100 x 14) which is equal to 25W.
Modulation index, m = 100% = 1.
Power in sidebands = (Pcx m2/4), Pc = Power of Carrier
By substituting the values, we have (100 x 1/4) = 25W.

9. Modem is considered as high speed if data rate is ________
a) 10000
b) 20000
c) 5000
d) 30000

Explanation: For data rate in the range 2000 to 10500 bps, modem is designated as high speed. Modem is a combination of modulator and demodulator. Modulator tends to encode message signal and demodulator decodes the message signal from the modulated waveform.

10. Scanning always takes place from ________
a) right to left
b) left to right
c) left to right for odd fields and right to left for even fields
d) left to right for even fields and right to left for odd fields

Explanation: Scanning is mostly used in TV to obtain continuous frames and to provide motion of picture. Scanning always takes place from left to right.

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