Analog Communication Test Questions - Set 4

1)   Square law modulators

a. Have non linear current-voltage characteristics
b. Are used for Amplitude Modulation
c. Are used for frequency modulation
d. Both a and b are correct
Answer  Explanation  Related Ques

ANSWER: Both a and b are correct

Explanation:
Square law modulator for AM

Square law modulators are used for Amplitude Modulation. They have non linear current-voltage characteristics. Square law modulators are highly nonlinear in low voltage region.


2)   AM demodulation techniques are

a. Square law demodulator
b. Envelope detector
c. PLL detector
d. Both a and b are correct
Answer  Explanation  Related Ques

ANSWER: Both a and b are correct

Explanation:
AM signals are detected or demodulated using Square law demodulator or envelope detector. The demodulators extract the information from the received AM signal.

Square law demodulators are highly nonlinear in low voltage region so they are used for low level modulated signals. Envelope detectors extract the envelope of the AM wave.


3)   The process of recovering information signal from received carrier is known as

a. Detection
b. Modulation
c. Demultiplexing
d. Sampling
Answer  Explanation  Related Ques

ANSWER: Detection

Explanation:
Detection or demodulation of the received signal is the recovery of information or the original message that was transmitted by the transmitter after modulation. The process is also called demodulation it recovers the original signal from the modulated signal received.


4)   Ring modulator is

a. Is used for DSB SC generation
b. Consists of four diodes connected in the form of ring
c. Is a product modulator
d. All of the above
Answer  Explanation  Related Ques

ANSWER: All of the above

Explanation:
Ring modulator is a product modulator used for DSB SC generation. It consists of four diodes connected in the form of ring. In AM , the ring modulator acts as a product modulator for a square wave carrier and modulating signal and generated a Double Side Band-Suppressed Carrier signal.


5)   What is the maximum transmission efficiency of an AM signal?

a. 64.44%
b. 33.33%
c. 56.66%
d. 75.55%
Answer  Explanation  Related Ques

ANSWER: 33.33%

Explanation:
The transmission efficiency (η) of AM wave is defined as the percentage of total power contributed by side bands of the AM signal. The maximum transmission efficiency of an AM signal is 33.33%, i.e., only one third of the total transmitted power is carried by the side bands in an AM wave. The remaining two third of the total transmitted power gets wasted.


6)   In synchronous detection of AM signal

a. Carrier is locally generated
b. Passed through a low pass filter
c. The original signal is recovered
d. All of the above
Answer  Explanation  Related Ques

ANSWER: All of the above

Explanation:
In synchronous detection of AM signal, the carrier signal is locally generated and then passed through a low pass filter. At the output of the low pass filter, the original modulating signal is recovered. The synchronous detection requires the generation of carrier at the receiver also. So additional circuitry is required at the receiver.


7)   Requirements of synchronous detection of AM signal are:

a. Local generation of carrier
b. The frequency of the locally generated carrier must be identical to that of transmitted carrier
c. The phase of the locally generated carrier must be synchronized to that of transmitted carrier
d. All of the above
Answer  Explanation  Related Ques

ANSWER: All of the above

Explanation:
Synchronous detection of AM signal requires local generation of the carrier signal at the receiver. The frequency of the locally generated carrier must be identical to that of transmitted carrier. And the phase of the locally generated carrier must be synchronized to that of transmitted carrier otherwise the detected signal would get distorted.


8)   Disadvantages of using synchronous detection of AM signal are:

a. Needs additional system for generation of carrier

b. Needs additional system for synchronization of carrier
c. Receiver is complex and costly
d. All of the above
Answer  Explanation  Related Ques

ANSWER: All of the above

Explanation:
Disadvantages of using synchronous detection of AM signal are that it needs an additional system for generation of carrier. The frequency of the locally generated carrier must be identical to that of transmitted carrier. And the phase of the locally generated carrier must be synchronized to that of transmitted carrier otherwise the detected signal would get distorted so it needs an additional system for synchronization of carrier. This makes the receiver complex and costlier.


9)   Quadrature Amplitude Modulation (QAM) is

a. Have same bandwidth used for two DSB-SC signals
b. Is also known as Bandwidth Conservation scheme
c. Is used in color television
d. All of the above
Answer  Explanation  Related Ques

ANSWER: All of the above

Explanation:
The modulation scheme Quadrature Amplitude Modulation (QAM) enables same bandwidth used for transmission of two DSB-SC signals. It is also known as Bandwidth Conservation scheme. However it needs synchronous detection so it needs an additional system for generation of carrier. Also known as Quadrature Carrier Multiplexing, the scheme is used in color television.


10)   Pilot carrier is

a. Used with DSB-SC signal
b. A small carrier transmitted with modulated signal
c. Used for synchronization with local carrier at the receiver
d. All of the above
Answer  Explanation  Related Ques

ANSWER: All of the above

Explanation:
Pilot carrier is a small carrier transmitted with modulated signal from the transmitter. It is separated at the receiver and used to phase lock the locally generated carrier signal generated at the receiver. It provides synchronization at the receiver.

As a weak carrier is transmitted with the DSB signal, it is also referred to as partially suppressed carrier system, as the carrier is not totally suppressed. So it is a case of over modulation.


11)   Generation of SSB SC signal is done by

a. Phase discrimination method
b. Frequency discrimination method
c. Product modulator
d. Both a and b
Answer  Explanation  Related Ques

ANSWER: Both a and b

Explanation:
SSB-SC signals are the signals where only one of the sidebands is transmitted in an AM wave. The carrier is also suppressed while transmitting the AM signal. SSBSC signal may be generated by two methods:

i) Frequency discrimination method or filter method
ii) Phase discrimination method or phase shift method


12)   Limitations of Frequency discrimination method are:

a. Cannot be used for video signals
b. Designing of band pass filter is difficult
c. Both a and b
d. None of the above
Answer  Explanation  Related Ques

ANSWER: Both a and b

Explanation:
Limitations of Frequency discrimination method are that as the base band signal must be appropriately related to the carrier signal frequency, the designing of band pass filter is difficult if the carrier frequency is quite higher than the bandwidth of the baseband signal. The system is not useful for video communication purpose.


13)   Phase shift method is

a. Includes two balanced modulators
b. Two phase shifting networks
c. Avoids the use of filters
d. All of the above
Answer  Explanation  Related Ques

ANSWER: All of the above

Explanation:
Phase shift method is one of the methods used for the generation of SSB-SC signals. This method includes two balanced modulators and two phase shifting networks and avoids the use of filters. Both the balanced modulators produce side band as an output.

The SSB-SC signals are generated using multiple tone modulating signals as if done with single tone modulating signal, they do not carry any useful information.


14)   Vestigial side band signals are detected by

a. Filters
b. Synchronous detection
c. Balanced modulator
d. None of the above
Answer  Explanation  Related Ques

ANSWER: Synchronous detection

Explanation:
Vestigial side band signals are detected by Synchronous detection or coherent detection technique. The distortions in a VSB signal may be reduced by reducing the modulation index and increasing the width of VSB.


15)   Automatic gain control is

a. Provides controlled signal amplitude at the output
b. Adjusts the input to output gain to a suitable value
c. Is used in AM radio receiver
d. All of the above
Answer  Explanation  Related Ques

ANSWER: All of the above

Explanation:
Automatic gain control provides controlled signal amplitude at the output even if there are amplitude variations at the input. This controlled output adjusts the input to output gain to a suitable value so that the system may work properly even for wide range of variations at the input.

The AGC is used in AM radio receivers for controlling the weak and strong sound signals.


16)   In an Amplitude Modulation

a. Amplitude of the carrier varies
b. Frequency of the carrier remains constant
c. Phase of the carrier remains constant
d. All of the above
Answer  Explanation  Related Ques

ANSWER: All of the above

Explanation:
In an Amplitude Modulation, the amplitude of the carrier varies according to the instantaneous values of the baseband signal. The frequency and the phase of the carrier remain constant.

The envelope of the AM wave has the same shape as the message of the base band signal.


17)   If modulation index is greater than 1

a. The baseband signal is not preserved in the envelope of the AM signal
b. The recovered signal is distorted
c. It is called over modulation
d. All of the above
Answer  Explanation  Related Ques

ANSWER: All of the above

Explanation:
Modulation index is the measure of extent to which the parameter of the carrier is varied in accordance with the modulating signal. If modulation index is greater than 1, or the modulation percentage is more than 100%, it is called over modulation.

The baseband signal is not preserved in the envelope of the AM signal and therefore, the recovered signal is distorted at the output of the receiver.


18)   Examples of low level modulation are

a. Square law diode modulation
b. Switching modulation
c. Frequency discrimination method
d. Both a and b
Answer  Explanation  Related Ques

ANSWER: Both a and b

Explanation:
In a Low level Amplitude Modulation system, modulation is done at lower power of carrier and modulating signal. Therefore the output power of modulation is low. So power amplifiers are required to boost the signal up to the desired power level. Examples of low level modulation are

i) Square law diode modulation
ii) Switching modulation


19)   Frequency components of an AM wave (m = modulation index) are

a. Carrier frequency (ωc ) with amplitude A
b. Upper side band (ωc + ωm) having amplitude mA/2
c. Lower side band (ωc - ωm) having amplitude mA/2
d. All of the above
Answer  Explanation  Related Ques

ANSWER: All of the above

Explanation:
Frequency components of an AM wave having m modulation index are

- carrier frequency (ωc ) with amplitude A

- upper side band (ωc + ωm) having amplitude mA/2

- lower side band (ωc - ωm) having amplitude mA/2

with these components of AM wave, the frequency spectrum may be plotted as
frequency spectrum of AM signal


20)   Squelch circuit is

a. Suppresses output audio
b. Works when there is insufficient desired input signal
c. Is used to suppress the unwanted channel noise when there is no reception by the receiver
d. All of the above
Answer  Explanation  Related Ques

ANSWER: All of the above

Explanation:
Squelch circuit suppresses the output audio when there is insufficient desired input signal at the receiver. The circuit is used to suppress the unwanted channel noise when there is no reception by the receiver. Squelch circuits may be used in wireless microphones stay away from replicating the noise when enough signal is not received. Regulating squelch is provided in some systems for the receiver.