Electronics and Communication Engineering - Communication System
Exam Duration: 45 Mins Total Questions : 30
An AM broadcast station operates at its maximum allowed total output of 60 kW with 90% modulation. The power in the intelligence part is
- (a)
17.3 kW
- (b)
6.42 kW
- (c)
31.12 kW
- (d)
12.12 kW
The signal \(m(t)=sin\quad c\left( 2\times { 10 }^{ 4 }t \right) \) is frequency modulated with k=103 Hz/V. The maximum instantaneous frequency of the modulated signal is
- (a)
0.999 MHz
- (b)
0.998 MHz
- (c)
1.002 MHz
- (d)
1.001 MHz
An FM signal with a deviation \(\Delta f\) is passed through a mixer, and has its frequency reduced three folds. The deviation in the output of the mixer is
- (a)
\(3\Delta f\)
- (b)
\(\Delta f/3\)
- (c)
\(\Delta f\)
- (d)
intermediate
If m is the mean, \(\sigma \) is the variance of a normally distributed variable, then its PDF is given by
- (a)
\(\frac { 1 }{ \sqrt { \sigma 2\pi } } { e }^{ -\frac { { \left( x-m \right) }^{ 2 } }{ 2{ \sigma }^{ 2 } } }\)
- (b)
\(\frac { 1 }{ 2\pi \sqrt { \sigma } } { e }^{ -\frac { 1 }{ 2 } { \left( \frac { x-m }{ \sigma } \right) }^{ 2 } }\)
- (c)
\(\frac { 1 }{ \sigma \sqrt { 2\pi } } { e }^{ -\frac { { \left( x-m \right) }^{ 2 } }{ { 2\sigma }^{ 2 } } }\)
- (d)
\(\frac { 1 }{ { \sigma }^{ 2 }\sqrt { 2\pi } } { e }^{ -\frac { 1 }{ 2 } { \left( \frac { x-m }{ \sigma } \right) }^{ 2 } }\)
Two carriers 40 MHz and 80 MHz respectively are frequency modulated by a signal of frequency 4 kHz, such that the bandwidth of the FM signal in the two cases are the same. The peak deviation in the two cases are the same. The peak deviation in the two cases are in the ratio of
- (a)
1:4
- (b)
1:2
- (c)
1:1
- (d)
2:1
In a PCM system each quantization level is encoded into 8 bit. The signal to quantization noise is equal to
- (a)
\(\frac { 1 }{ 12 } { \left( \frac { 1 }{ 256 } \right) }^{ 2 }\)dB
- (b)
48 dB
- (c)
64 dB
- (d)
256 dB
A PCM system uses a uniform quantizer followed by an 8 bit encoder. The bit rate of the system is equal to 108 bit/s. The maximum message bandwidth for which the system operates satisfactorily is
- (a)
25 MHz
- (b)
6.25 MHz
- (c)
12.5 MHz
- (d)
50 MHz
An analog signal is sampled at 36 kHz and quantized into 256 levels. The time duration of a bit of the binary coded signal is
- (a)
7.86 ms
- (b)
6.43 ms
- (c)
3.47 \(\mu s\)
- (d)
5.78 \(\mu s\)
A stationary zero mean random process X(t) is ergodic has average power of 24 W and has no periodic component. The valid auto-correlation function is
- (a)
16+18cos\((3\tau )\)
- (b)
\(24{ \delta a }^{ 2 }(2\pi )\)
- (c)
\(\frac { { e }^{ (-6\tau ) } }{ (1+3{ \tau }^{ 2 }) } \)
- (d)
\(24\delta (t-\tau )\)
For a random process \({ R }_{ XX }(\tau )=P{ cos }^{ 4 }({ \omega }_{ 0 }\tau )\) , where P and \({ \omega }_{ 0 }\) are constants. The power in process is
- (a)
P
- (b)
2P
- (c)
3P
- (d)
4P
The sample space for an experiment is S={0,1,2,5,6}. the value of random variable X=5s2-1 is
- (a)
-1
- (b)
30.25
- (c)
179
- (d)
All of these
Four signals each band limited to 5 kHz are sampled at twice the Nyquist rate. The resulting PAM samples are transmitted over a single channel after time division multiplexing. The theoretical minimum transmission bandwidth of the channel should be equal to
- (a)
5 kHz
- (b)
20kHz
- (c)
40kHz
- (d)
80kHz
A band-pass signal has significant frequency components in the range of 1.5MHz to 2MHz. If the signal is to be reconstructed from its samples, the minimum sampling frequency will be
- (a)
1MHz
- (b)
2MHz
- (c)
3.5MHz
- (d)
4MHz
An amplifier has an average standard noise figure of 3 dB and an average operating noise figure of 6dB when used with a source of average effective source temperature \(\overline { { T }_{ s } } \) . The \(\overline { { T }_{ s } } \) is
- (a)
97.31K
- (b)
156.32K
- (c)
100.81K
- (d)
None of these
A channel having bandwidth of 10kHz carries a 1\(\mu W\) signal. If the channel is affected by noise having PSD of 10-13 W/Hz, then the capacity of the channel is
- (a)
232.5kbit/s
- (b)
100 kbit/s
- (c)
50 kbit/s
- (d)
116.25 kbit/s
A telephone channel has bandwidth B of kHz and SNR of 30dB. It is connected to a teletype machine having 32 different symbols. The symbol rate required for errorless transmission is nearly
- (a)
1800 symbol/s
- (b)
3000 symbol/s
- (c)
5000 symbol/s
- (d)
6000 symbol/s
Consider an angle modulated signal \(x(t)=6cos\left[ 2\pi \times { 10 }^{ 6 }t+2sin(8000\pi t) \right] \)V. The average power of x(t) is
- (a)
10W
- (b)
18W
- (c)
20W
- (d)
28W
For a message signal \(m(t)=cos\left( 2\pi { f }_{ m }t \right) \)and carrier of frequency fc , which of the following represents a Single Side Band (SSB)signal?
- (a)
\(cos\left( 2\pi { f }_{ m }t \right) cos(2\pi { f }_{ c }t)\)
- (b)
\(cos(2\pi { f }_{ c }t)\)
- (c)
\(cos\left[ 2\pi ({ f }_{ c }+{ f }_{ m })t \right] \)
- (d)
\(\left[ 1+cos\left( 2\pi { f }_{ m }t \right) \right] cos(2\pi { f }_{ c }t)\)
Spectral density of flicker noise varies as
- (a)
f
- (b)
f2
- (c)
\(\frac { 1 }{ f } \)
- (d)
\(\frac { 1 }{ { f }^{ 2 } } \)
Match List I with List II and select the correct answer using the codes given below the lists.
| List I | List II |
|---|---|
| A. IF amplifier | 1. Demodulator |
| B. Mixer | 2. Narrow band tuned amplifier |
| C. Detector | 3. Variable gain amplifier |
| D. Automatic gain control | 4. Frequency conversion |
- (a)
A B C D 1 4 2 3 - (b)
A B C D 2 3 1 4 - (c)
A B C D 1 3 2 4 - (d)
A B C D 2 4 1 3
Match List I with List II and select the correct answer using the codes given below the lists.
| List I | List II |
|---|---|
| A. Entropy coding | 1. McMillan's rule |
| B. Channel capacity | 2. Redundancy |
| C. Minimum Length code | 3. Shannon-Fano |
| D. Equivocation | 4. Shannon law |
- (a)
A B C D 1 2 3 4 - (b)
A B C D 3 4 1 2 - (c)
A B C D 1 4 3 2 - (d)
A B C D 3 2 1 4
At a given probability of error, binary coherent FSK is inferior to binary coherent PSK by
- (a)
6 dB
- (b)
3 dB
- (c)
2 dB
- (d)
zero
If g(t) has Hilbert transform g(t) then g(t) has Hilbert transform
- (a)
g(t)/2
- (b)
-g(t)/2
- (c)
g(t)
- (d)
-g(t)
The image channel rejection in a superheterodyne receiver comes from
- (a)
IF stage only
- (b)
RF stages only
- (c)
detector and RF stages
- (d)
detector, RF and IF stages
In the angle modulated signal X(t)=10cos \(\left[ { \omega }_{ c }+3cos{ \omega }_{ n }t \right] \) with fm=1000Hz, if f m is increased by a factor 4, the resultant bandwidth will be
- (a)
8 kHz
- (b)
4 kHz
- (c)
30 kHz
- (d)
3 kHz
The auto-correlation of a function X(t) is given as below
\({ R }_{ XX }(t)=\left[ ...+7,p,q,-4,5,-6,+7,10,.. \right] \)
The correct values of p and q are respectively
- (a)
-6,5
- (b)
6,-5
- (c)
-6,-5
- (d)
6,5
An analog signal has significant spectral components from 1kHz to 5kHz. What is the Nyquist sampling rate for this signal?
- (a)
5 k sample/s
- (b)
4 k sample/s
- (c)
8 k sample/s
- (d)
10 k sample/s
A' frequency modulated voltage wave is given as \(V=10\quad sin\left( 2\pi \times { 10 }^{ 8 }t+5\quad sin400\pi t \right) \)The carrier frequency and maximum deviation of the signal are respectively
- (a)
10 MHz and 10kHz
- (b)
100 MHz and 1 kHz
- (c)
10 MHz and 1 kHz
- (d)
100 MHz and 10kHz
An examination consists of two papers, Paper 1 and Paper 2. The probability of failing in Paper 1 is 0.3 and that in Paper 2 is 0.2. Given that a student has failed in Paper 2, the probability of failing in Paper 1 is 0.6. The probability of a student failing in both the papers is
- (a)
0.5
- (b)
0.18
- (c)
0.12
- (d)
0.06
If S represents the carrier synchronization at the receiver and p represents the bandwidth efficiency, then the correct statement for the coherent binary PSK is
- (a)
\(\rho \)=0.5,S is required
- (b)
\(\rho \)=1.0, S is required
- (c)
\(\rho \) =0.5,S is not required
- (d)
\(\rho \)=1.0, S is not required
1654060945.png)
1654060942.png)
1653987551.png)
1653987086.png)
1582095229.jpg)
