Analog Communications - Communications - Electronics & Communication Engineering Previous Year Questions

1

2008 · Electronics & Communication Engineering · Communications · Analog Communications

Electronics & Communication Engineering (EC) 2008

Consider the amplitude modulated (AM) signal \(A_c \cos \omega_c t + 2\cos \omega_m t \cos \omega_c t\). For demodulating the signal using envelope detector, the minimum value of \(A_c\) should be

A

2

B

1

C

0.5

D

0

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2

2008 · Electronics & Communication Engineering · Communications · Analog Communications

Electronics & Communication Engineering (EC) 2008

Consider the frequency modulated signal 10 cos[2π × 10⁵t + 5 sin(2π × 1500)t + 7.5 sin(2π × 1000)t] with carrier frequency of 10⁵ Hz. The modulation index is

A

12.5

B

10

C

7.5

D

5

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3

2008 · Electronics & Communication Engineering · Communications · Analog Communications

Electronics & Communication Engineering (EC) 2008

The signal cos ω_ct + 0.5 cos ω_mt sin ω_ct is

A

FM only

B

AM only

C

both AM and FM

D

neither AM nor FM

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4

2010 · Electronics & Communication Engineering · Communications · Analog Communications

Electronics & Communication Engineering (EC) 2010

Suppose that the modulating signal is \(m(t) = 2\cos(2π f_m t)\) and the carrier signal is \(x_c(t) = A_c \cos(2π f_c t)\). Which one of the following is a conventional AM signal without over-modulation?

A

\(x(t) = A_c m(t) \cos(2π f_c t)\)

B

\(x(t) = A_c [1 + m(t)] \cos(2π f_c t)\)

C

\(x(t) = A_c \cos(2π f_c t) + \frac{A_c}{4} m(t) \cos(2π f_c t)\)

D

\(x(t) = A_c \cos(2π f_m t) \cos(2π f_c t) + A_c \sin(2π f_m t) \sin(2π f_c t)\)

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5

2010 · Electronics & Communication Engineering · Communications · Analog Communications

Electronics & Communication Engineering (EC) 2010

Consider an angle modulated signal \(x(t) = 6\cos[2π×10^6 t + 2\sin(8000π t) + 4\cos(8000π t)]\) V. The average power of x(t) is

A

10 W

B

18 W

C

20 W

D

28 W

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6

2011 · Electronics & Communication Engineering · Communications · Analog Communications

Electronics & Communication Engineering (EC) 2011

The Column-1 lists the attributes and the Column-2 lists the modulation systems. Match the attribute to the modulation system that best meets it.

Column-1	Column-2
P. Power efficient transmission of signals	I. Conventional AM
Q. Most bandwidth efficient transmission of voice signals	II. FM
R. Simplest receiver structure	III. VSB
S. Bandwidth efficient transmission of signals with significant dc component	IV. SSB-SC

A

P-IV, Q-II, R-I, S-III

B

P-II, Q-IV, R-I, S-III

C

P-III, Q-II, R-I, S-IV

D

P-II, Q-IV, R-III, S-I

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7

2011 · Electronics & Communication Engineering · Communications · Analog Communications

Electronics & Communication Engineering (EC) 2011

A message signal \(m(t) = \cos 2000\pi t + 4\cos 4000\pi t\) modulates the carrier \(c(t) = \cos 2\pi f_c t\) where \(f_c = 1\) MHz to produce an AM signal. For demodulating the generated AM signal using an envelope detector, the time constant RC of the detector circuit should satisfy

A

\(0.5 ms < RC < 1 ms\)

B

\(1 \mu s < RC < 0.5 ms\)

C

\(RC << 1\ \mu\text{s}\)

D

\(RC >> 0.5 ms\)

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8

2012 · Electronics & Communication Engineering · Communications · Analog Communications

Electronics & Communication Engineering (EC) 2012

The signal \( m(t) \) as shown is applied both to a phase modulator (with \( k_p \) as the phase constant) and a frequency modulator (with \( k_f \) as the frequency constant) having the same carrier frequency. The ratio \( k_p / k_f \) (in rad/Hz) for the same maximum phase deviation is

A

\(8\pi \)

B

\(4\pi \)

C

\(2\pi \)

D

\(\pi \)

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9

2014 · Electronics & Communication Engineering · Communications · Analog Communications

Electronics & Communication Engineering (EC) 2014 [Session 3]

The phase response of a passband waveform at the receiver is given by
\( \phi(f) = -2\pi\alpha(f - f_c) - 2\pi\beta f_c \)
where \( f_c \) is the centre frequency, and \( \alpha \) and \( \beta \) are positive constants. The actual signal propagation delay from the transmitter to receiver is

A

\(\frac{\alpha - \beta}{\alpha + \beta} \)

B

\(\frac{\alpha \beta}{\alpha + \beta} \)

C

\(\alpha \)

D

\(\beta \)

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10

2014 · Electronics & Communication Engineering · Communications · Analog Communications

Electronics & Communication Engineering (EC) 2014 [Session 4]

In a double side-band (DSB) full carrier AM transmission system, if the modulation index is doubled, then the ratio of total sideband power to the carrier power increases by a factor of ________.

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11

2016 · Electronics & Communication Engineering · Communications · Analog Communications

Electronics & Communication Engineering (EC) 2016 [Session 1]

The block diagram of a frequency synthesizer consisting of a Phase Locked Loop (PLL) and a divide-by-\(N\) counter (comprising \( = 2, = 4, = 8, = 16 \) outputs) is sketched below. The synthesizer is excited with a 5 kHz signal (Input 1). The free-running frequency of the PLL is set to 20 kHz. Assume that the commutator switch makes contacts repeatedly in the order 1-2-3-4.
The corresponding frequencies synthesized are:

A

10 kHz, 20 kHz, 40 kHz, 80 kHz

B

20 kHz, 40 kHz, 80 kHz, 160 kHz

C

80 kHz, 40 kHz, 20 kHz, 10 kHz

D

160 kHz, 80 kHz, 40 kHz, 20 kHz

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12

2016 · Electronics & Communication Engineering · Communications · Analog Communications

Electronics & Communication Engineering (EC) 2016 [Session 1]

A superheterodyne receiver operates in the frequency range of 5 – 60 MHz. The intermediate frequency f_IF and local oscillator frequency f_LO are chosen such that f_IF ≤ f_LO. It is required that the image frequencies fall outside the 5 – 60 MHz band. The minimum required f_IF (in MHz) is ______

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13

2016 · Electronics & Communication Engineering · Communications · Analog Communications

Electronics & Communication Engineering (EC) 2016 [Session 1]

The amplitude of a sinusoidal carrier is modulated by a single sinusoid to obtain the amplitude modulated signal s(t) = 5 cos 1600πt + 20 cos 1800πt + 5 cos 2000πt. The value of the modulation index is ______

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14

2016 · Electronics & Communication Engineering · Communications · Analog Communications

Electronics & Communication Engineering (EC) 2016 [Session 1]

An analog pulse s(t) is transmitted over an additive white Gaussian noise (AWGN) channel. The received signal is r(t) = s(t) + n(t), where n(t) is additive white Gaussian noise with power spectral density \( \frac{N_0}{2} \). The received signal is passed through a filter with impulse response h(t). Let \( E_s \) and \( E_h \) denote the energies of the pulse s(t) and the filter h(t), respectively. When the signal-to-noise ratio (SNR) is maximized at the output of the filter (SNR_max), which of the following holds?

A

\(E_s = E_h \) ; SNR_max = \(\frac{2E_s}{N_0} \)

B

\(E_s = E_h \) ; SNR_max = \(\frac{E_s}{2N_0} \)

C

\(E_s > E_h \) ; SNR_max = \(\frac{2E_s}{N_0} \)

D

\(E_s < E_h \) ; SNR_max = \(\frac{2E_h}{N_0} \)

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15

2016 · Electronics & Communication Engineering · Communications · Analog Communications

Electronics & Communication Engineering (EC) 2016 [Session 1]

An antenna pointing in a certain direction has a noise temperature of 50 K. The ambient temperature is 290 K. The antenna is connected to a pre-amplifier that has a noise figure of 2 dB and an available gain of 40 dB over an effective bandwidth of 12 MHz. The effective input noise temperature \( T_e \) for the amplifier and the noise power \( P_{ao} \) at the output of the preamplifier, respectively, are

A

\(T_e = 169.36 \text{ K and } P_{ao} = 3.73 \times 10^{-10} \text{ W} \)

B

\(T_e = 170.8 \text{ K and } P_{ao} = 4.56 \times 10^{-10} \text{ W} \)

C

\(T_e = 182.5 \text{ K and } P_{ao} = 3.85 \times 10^{-10} \text{ W} \)

D

\(T_e = 160.62 \text{ K and } P_{ao} = 4.6 \times 10^{-10} \text{ W} \)

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16

2016 · Electronics & Communication Engineering · Communications · Analog Communications

Electronics & Communication Engineering (EC) 2016 [Session 3]

For a superheterodyne receiver, the intermediate frequency is 15 MHz and the local oscillator frequency is 3.5 GHz. If the frequency of the received signal is greater than the local oscillator frequency, then the image frequency (in MHz) is _____________

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17

2017 · Electronics & Communication Engineering · Communications · Analog Communications

Electronics & Communication Engineering (EC) 2017

The unmodulated carrier power in an AM transmitter is 5 kW. This carrier is modulated by a sinusoidal modulating signal. The maximum percentage of modulation is 50%. If it is reduced to 40%, then the maximum unmodulated carrier power (in kW) that can be used without overloading the transmitter is ________

A

2

B

7/4

C

3/2

D

4/3

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18

2017 · Electronics & Communication Engineering · Communications · Analog Communications

Electronics & Communication Engineering (EC) 2017

A modulating signal given by x(t) = 5 sin(4π103t − 10π cos 2π103t) V is fed to a phase modulator with phase deviation constant kp = 5 rad/V. If the carrier frequency is 20 kHz, the instantaneous frequency (in kHz) at t = 0.5 ms is ________

A

1.5

B

3

C

4.5

D

6

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19

2018 · Electronics & Communication Engineering · Communications · Analog Communications

Electronics & Communication Engineering (EC) 2018

Consider the following amplitude modulated signal: \( s(t) = \cos(2000 \pi t) + 4 \cos(2400 \pi t) + \cos(2800 \pi t) \). The ratio (accurate to three decimal places) of the power of the message signal to the power of the carrier signal is __________.

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20

2018 · Electronics & Communication Engineering · Communications · Analog Communications

Electronics & Communication Engineering (EC) 2018

A binary source generates symbols \( X \in \{-1, 1\} \) which are transmitted over a noisy channel. The probability of transmitting \( X = 1 \) is 0.5. Input to the threshold detector is \( R = X + N \). The probability density function \( f_N(n) \) of the noise \( N \) is shown below. If the detection threshold is zero, then the probability of error (correct to two decimal places) is __________.

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