Analog Communications Questions and Answers – Frequency Domain Description

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This set of Analog Communications Multiple Choice Questions & Answers (MCQs) focuses on “Frequency Domain Description”.

1. What is Amplitude Modulation?
a) Change in amplitude of carrier according to modulating signal amplitude
b) Change in frequency of carrier according to modulating signal amplitude
c) Change in amplitude of carrier according to modulating signal frequency
d) Change in amplitude of modulating signal according to carrier signal amplitude
View Answer

Answer: a
Explanation: Amplitude modulation is a modulation process in which amplitude of carrier wave is varied with respect to amplitude of the message signal to be transmitted. Whereas, Frequency modulation is a modulation process in which frequency of carrier wave is varied with respect to amplitude of the message signal to be transmitted.

2. Frequency components of an AM wave are?
a) Carrier frequency (ωc) with amplitude A
b) Lower side band (ωc + ωm) having amplitude mA2
c) Upper side band (ωc – ωm) having amplitude mA2
d) Carrier frequency (ωc/2) with amplitude A
View Answer

Answer: a
Explanation: The frequency components of AM waves are: Carrier frequency (ωc) with amplitude A, Lower sideband (ωc – ωm) having amplitude mA/2 and Upper side band (ωc + ωm) having amplitude mA2 (where m represents modulation index, A is the amplitude of the carrier signal, ωm is the amplitude of the message signal).

3. In amplitude modulation frequency and phase of carrier ________
a) varies simultaneously
b) varies alternately
c) initially varies but become same after sometime
d) remains constant
View Answer

Answer: d
Explanation: In AM, amplitude of carrier signal varies according to instantaneous amplitude of baseband signal. The frequency and phase of carrier signal remains constant.
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4. Envelope of AM wave has the same shape as the message of baseband signal.
a) True
b) False
View Answer

Answer: a
Explanation: Modulation Envelope of an AM wave is said to follow the outline of the message signal when carrier, upper sideband and lower sideband are combined in a single impedance and observed on time versus amplitude plot. Thus, AM wave has same shape as the message signal.

5. When aliasing takes place?
a) Sampling signals less than Nyquist Rate
b) Sampling signals more than Nyquist Rate
c) Sampling signals equal to Nyquist Rate
d) Sampling signals at a rate which is twice of Nyquist Rate
View Answer

Answer: a
Explanation: Aliasing causes different signals to become indistinguishable during sampling. It happens because sampling rate is less than Nyquist rate. In order to avoid aliasing, signals should be sampled at a rate twice of Nyquist rate.

6. Why the Synchronous detection of AM signals is considered as a disadvantage?
a) Needs additional system for synchronization of carrier
b) Receiver is available at cheap prices
c) Needs less number of system as estimated for generation of carrier
d) Receiver is not complex
View Answer

Answer: a
Explanation: Disadvantage of synchronous detection of AM signal are that it needs an additional system for generation of carrier. It also needs additional system for synchronization of carrier. Moreover receiver is complex and costly.

7. Which devices did we use for AM Demodulation?
a) Envelope detector and Square law demodulator
b) PLL detector and Foster-Seeley discriminator
c) Ratio detector and Slope detector
d) Only quadrature detector
View Answer

Answer: a
Explanation: AM signals can be demodulated using Square law demodulator or Envelope detector. Demodulator extracts information from received AM signal by decoding it. Other options are the demodulators for FM signal.
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8. Square Law modulators ________
a) used for amplitude modulation
b) have non linear current-voltage characteristics
c) have non linear current-voltage characteristics as well as used for Amplitude Modulation
d) used for frequency modulation
View Answer

Answer: c
Explanation: Square law modulators are used for amplitude modulation and have non-linear current-voltage characteristics. The output of Square law demodulator is said to vary with respect to square of the input. They are highly linear in low voltage region.

9. What do you understand by low level AM?
a) Output power is low
b) Modulation is done at high power of carrier and modulating signal
c) Collector Modulation Method is low level AM
d) Output power is high
View Answer

Answer: a
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. Collector Modulation Method is high level AM. At high-level AM, output power is high.
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10. What do you understand by high level AM?
a) Output power is low
b) Modulation is done at high power of carrier and modulating signal
c) No need to boost the carrier and modulating signal
d) Modulation is done at low power of carrier and modulating signal
View Answer

Answer: b
Explanation: For high level AM, modulation is done at high power of carrier and modulating signal, so power amplifiers are not used to boost the carrier and modulating signal. Therefore, output power is high. At low-level AM, output power is low.

Sanfoundry Global Education & Learning Series – Analog Communications.

To practice all areas of Analog Communications, here is complete set of 1000+ Multiple Choice Questions and Answers.

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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