Analog Communication Test Questions - Set 3

ANSWER: Both a and b

Explanation:
TRF receivers and super heterodyne receivers are used for detection of modulating signal and removal of unwanted signal. The receivers receive the modulated signal and detect the information signal from the received modulated signal.

ANSWER: Both a and b are correct

Explanation:
Disadvantage of using a DSB or SSB signal modulation is that it is difficult to recover information at the receiver. Demodulation depends upon the carrier present in the received signal at the receiver. If the carrier is not present, carrier has to be regenerated at the receiver so a complex circuitry is required.

ANSWER: 51.63%

Explanation:
The total power in an AM is given by
P_t = P_c ( 1 + m²/2)
17 = 15(1 + m²/2)
m²/2 = 0.134
m = 0.5163
= 51.63%

ANSWER: 83.14%

Explanation:
Modulation index m is given by
m= √ (2{I_t/I_c}²-1)
= √ (2 (5.8/5)² -1)
= √ (2 (5.8/5)² -1)
= 0.8314
= 83.14%

ANSWER: 108 W

Explanation:
The total power in an AM is given by
P_t = Pc (1 + m²/2)
Given: m = 0.6
Therefore DSB power = (m²/2)P_c
= 600* (0.6)²/2
= 108 W

ANSWER: All of the above

Explanation:
Analog communication means that the information is transmitted in the form of a continuous signal, such as a sound wave. The signal has varying amplitude or phase or frequency.

Coding is not applied to the signal as in case of digital data. Amplitude modulated signal and frequency modulated signals are the examples of analog communication.

ANSWER: All of the above

Explanation:
Modulation is a process by which some characteristics of signal such as amplitude or phase or frequency are varied in accordance with the instantaneous value of the information signal .

The signal whose characteristics are varied is known as carrier signal, as it carries the information. The signal that carries the information is known as modulating signal as according to it, the characteristics of carrier signal are varied.

Methods of analog modulations are

1) Amplitude modulation
2) Frequency modulation
3) Phase modulation

ANSWER: 0.1364

Explanation:
The total power in an AM is given by
P_t = P_c (1 + m²/2)
Where P_c is the carrier power and m is the modulation index.

Therefore,

P_t1 = P_c (1 + 0.8²/2) = 1.32 P_c
P_t2 = P_c ( 1 + 1²/2) = 1.5 P_c
Increase in power = (1.5 P_c - 1.32 P_c)/ 1.32 P_c
= 0.1364

ANSWER: Both a and b

Explanation:
Synchronous detection means demodulation of received signal and extracting information from it. It requires a reference signal at the receivers with predetermined frequency and phase which is generated using extra circuitry. Synchronous detection helps in extracting weak signals from noise.

ANSWER: Sampling

Explanation:
Conversion of analog signal into digital signal converts a continuous time signal in the form of digits. The conversion is done with the help of sampling technique. A sample is a set of values at a point in time space. The average number of samples obtained in one second is known as sampling frequency.

ANSWER: λ/4

Explanation:
For effective transmission of signal by the transmitter, the antenna height should be at least quarter length of the signal wavelength i.e., λ/4

L= λ/4
= c/4f

Required antenna height decreases with increase in frequency of the signal so modulation is done. With modulation, the low frequency signals are shifted to high frequency signals.

ANSWER: All of the above

Explanation:
Due to analog to digital conversion, the data rate becomes high in digital communication. So the transmission bandwidth is also increased. Digital transmission also needs synchronization in certain applications which is not needed in case of analog communication systems.

ANSWER: Electromagnetic waves

Explanation:
Radio communication or wireless communication takes place through electromagnetic waves.

The message is transmitted through open space via electromagnetic waves or radio waves. The waves are transmitted in the open space through antenna after processing of signal.

ANSWER: Envelope of the AM wave

Explanation:
An AM wave may be represented by
s(t) = [A+ x(t)] cos ω_ct
= E(t) cos ω_ct

Where x(t) is the modulating signal

A is the amplitude of carrier wave
E(t) is the Envelope of the AM wave

The envelope consists of the modulating signal that may be recovered from AM signal using demodulation techniques.

ANSWER: Both a and c are correct

Explanation:


USB (Upper Side Band) is the band of frequency above the carrier frequency that lies in AM spectrum. It is a part of the shifted spectrum obtained after amplitude modulation. It is denoted by ω_c+ ω_m where ω_c is the carrier frequency and ω_m is the modulating frequency.

ANSWER: Both a and c are correct

Explanation:


LSB (Lower Side Band) is the band of frequency below the carrier frequency that lies in AM spectrum. It is a part of the shifted spectrum obtained after amplitude modulation. It is denoted by ω_c- ω_m where ω_c is the carrier frequency and ω_m is the modulating frequency.

ANSWER: Both a and b are correct

Explanation:
Bandwidth of the AM wave is the difference in the two extreme frequencies of the AM signal. It is given by

B = (ω_c + ω_m)- (ω_c - ω_m)
= ω_m
i.e., the bandwidth of the AM wave is twice the highest frequency present in the modulating signal.

ANSWER: 149 KHz to 169 KHz

Explanation:
Carrier frequency f_c = 1/2Π√LC
= 1/ 2Π√100 * 10 - 6 * 10 * 10^-9
= 1/2Π * 10^-6
= 1.59 * 10⁵ Hz
= 159 KHz
The modulating frequency f_m is 10KHz
Therefore the range of AM spectrum is given by (f_c f_m ) to (f_c + f_m )
= (159 - 10) to (159 + 10)
= 149 KHz to 169 KHz

ANSWER: All of the above

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.

ANSWER: All of the above

Explanation:
In High level Amplitude Modulation system, Modulation is done at high power of carrier and modulating signals so Power amplifiers are used to boost the carrier and modulating signals before modulation. Collector modulation method is the example of High level Amplitude Modulation.