SIDDHARTH GROUP OF INSTITUTIONS :: PUTTUR

Siddharth Nagar, Narayanavanam Road – 517583

QUESTION BANK (DESCRIPTIVE)

Subject with Code :ACS(15A04402) Course & Branch: B.Tech - ECE

Year & Sem: II-B.Tech & II-Sem Regulation: R15
UNIT-I
AMPLITUDE MODULATION AND DEMODULATION
10-MARKS

1. (a) With necessary expressions, waveforms and spectrums, Explain AM for an arbitrary baseband signal x(t).
(b) The output power of an AM transmitter is 1KW when sinusoidal modulated to a depth of 100%.
Calculate the power in each side band when the modulation depth is reduced to 50%. (L4) CO1
2. (a) Draw the circuit diagram for balanced ring modulator and explain its operation indicating all the waveforms and
spectrums. (L6) CO1
(b) In an AM-SC system, modulating signal is a single tone sinusoid 4 cos 2_ 103t,which modulates a carrier
signal 6 cos 2_106t. Write the equation of modulated wave. Plot the two sided spectrum of the modulated wave.
Calculate the amount of power transmitted. (L1) CO1
3. (a) Describe the time domain band-pass representation of SSB with necessary sketches. (L6) CO1
(b) Find the percentage of power saved in SSB when compared with AM system. (L6) CO1
4. (a) Explain the operation of the balanced slope detector using a circuit diagram and draw its response characteristics.
Discuss in particular the method of combining the outputs of the individual diodes. In what way is this circuit an
improvement on the slope detector and in turn what are the advantages?
(L1) CO1
(b)Draw the block diagram of an SSB - SC transmitter employing sideband suppression filter and explain.
(L1) CO1
5. (a) What are the main objectives of a communication system design? What are the primary resources of any
communication system? (L3) CO1
(b) The RC load for a diode envelope detector consists of a 1000 pF capacitor in parallel with a 10-K resistor.
Calculate the maximum modulation depth that can be handled for sinusoidal modulation at a frequency of 10 KHz if
diagonal peak clipping is to be avoided.
(L6) CO1
(c) A broadcast AM transmitter radiates 50 KW of carrier power. What will be the radiated power at 85 % of
modulation and what is the side band power? (L6) CO1
6. (a) Why VSB system is widely used for TV broadcasting - Explain? (L3) CO1
(b) An AM transmitter of 1KW power is fully modulated. Calculate the power transmitted if it is transmitted as SSB.

(L6) CO1
(c) Calculate the filter requirement to convert DSB signal to SSB Signal, given that the two side bands are separated
by 200HZ. The suppressed carrier is 29 MHZ. (L6) CO1
7. Explain (a) generation AM signals (b). Detection of Standard AM signal (L1) CO1
8. Explain the generation of DSB-SC (b). Detection of Standard DSB-SC signal (L1) CO1
9. Explain the generation of SSB-SC (b). Detection of Standard SSB-SC signal (L1) CO1
10. Explain super heterodyne AM receiver. (L1) CO1

2- MARKS

1. Define modulation. Why is modulation required? (L1) CO1

 2. Define is modulation index? (L1) (L4) CO1
3. Describe the DSB-SC wave modulation with spectrum?
4. Compare Square law detector with envelope detector? (L4) (L1) CO1
5. List the various types of modulations? (L1) CO1
6. What are the Advantages of SSB systems? (L1) CO1
7. List Application of different AM systems? (L1) (L4) CO1
8. What is Hilbert Transform? (L1) (L4) CO1
9. Draw the spectrum of SSB and VSB modulated signal? (L4) (L1) CO1
10. List Application of SSB? (L4) (L1) CO1
11. Define TDM. (L1) CO2
12. Define image frequency. (L1) CO2
13. Define Image frequency rejection ratio. (L1) CO2
14. Define Costas loop. (L1) CO2
15. Consider sinusoidal modulation in an AM systems. Assuming no over modulation, the maximum and minimum
values of the envelope, respectively, are 3V and 1V calculate modulation index. (L6) CO2
16. Define amplitude modulation? (L1) (L4) CO1
17. List Application of modulation? (L4) (L1) CO1
18. Define phase modulation? (L6) CO2
19. What are the frequency components in AM wave? (L1) CO1
20. Write the expression for AM wave? (L1) CO1

UNIT-II

ANGLE MODULATION AND DEMODULATION

10-MARKS

1. a) Describe generation of FM carrier by Transistor reactance modulator with necessary diagrams.

(L1) CO2
b) Compare the phasor diagram of narrow band FM signal and AM signal and discuss about the similarities and
differences of the two signals. (L1) CO2
2. ( a) FM Give the procedure to determine the effective bandwidth of an signal. (L3) CO1
(b) Which method of FM signal generation is the preferred choice, when the stability of the carrier frequency is of
major concern? Discuss about the method in detail. (L3) CO1
3. (a) An angle modulated signal has the form v(t) = 100 cos (2πfct+4 sin 2000 πt) when fc =10 MHz.
(L6) CO1
i. Determine average transmitted power. ii. Determine peak phase deviation.
iii. Determine the peak frequency deviation. iv. Is this an FM or a PM signal? Explain.

(b) Compute the bandwidth requirement for the transmission of FM signal having a frequency deviation 75 KHz and
an audio bandwidth of 10KHz. (L3) CO1
(c) An FM radio link has a frequency deviation of 30 kHz. The modulating frequency is 3 kHz. Calculate the
bandwidth needed for the link. What will be the bandwidth if the deviation is reduced to 15 kHz?
(L3) CO1
4. Explain generation of FM signal using direct methods. (L4) CO2
5. Explain generation of FM signal using indirect method. (L4) CO2
6. Explain (a) pre-emphasis (b) De-emphasis (c) FM capture effect (L1) CO2
7. (a) An FM radio link has a frequency deviation of 30 kHz. The modulating frequency is 3 kHz. Calculate the
bandwidth needed for the link. What will be the bandwidth if the deviation is reduced to 15 kHz?
(L6) CO1
(b) Draw the black diagram of FM stereo broadcast transmitters and explain its operation (L4) CO2
8. Explain FM and PM with neat expressions and wave forms (L1) CO2
9. compare AM , FM and PM (L1) CO2
10. Explain in detail about the concept of Angle Modulation (L4) CO2

2- MARKS

1. What is Angle modulation? What are different types of Angle modulation? (L1) CO2
2. Define PM & FM? (L1) (L4) CO1
3. What is frequency deviation & phase deviation? (L1) CO2
4. Compare AM and FM? (L1) CO1
 5. What are Advantages & Applications of FM? (L1) (L4) CO1
6. What is wideband FM & Narrowband FM? (L1) (L4) CO1
7. State Carson’s Rule? (L1) CO1
8. Define is deviation ratio? (L1) (L4) CO1
9. Define pre-emphasis &de-emphasis? (L1) CO2
10. Compare PM and FM? (L1) CO1
11. (A modulation
Δ , Δ signal is given by �(�)=�−��cos[(( �+ΔΔ )�]�(�
), where,, � ���Δ
Δ are positive constants, and
Δ� ≫ΔΔΔ�.Derive The complex envelope of s(t). (L1) CO2
12. State the applications of NBFM? (L1) (L4) CO1
13. Write the differences between VSB, FM, and PM? (L1) (L4) CO1
14. Write the modulation index for FM&PM? (L1) (L4) CO1
15. Define FDM. (L1) CO2
16. Write the advantages of super heterodyning? (L6) CO2
17. Define phase locked loop? (L1) CO2
18. Define VCO? (L1) (L4) CO1
19. Write the different forms of AM? (L6) CO2
20. What is the frequency deviation and carrier swing frequency to provide 75% modulation
in the FM broadcast band? (L6) CO2

UNIT-III
NOISE IN COMMUNICATION SYSTEMS
10-MARKS

1. (a) Find the output SNR in a PM system for tone modulation.

(b) A phase modulation (PM) system, with the modulated wave defined by S (t)= Ac Cos [2π fct + kpm(t)]
where kp is a constant and m(t) is the message signal. The additive noise n(t) at the phase detector input is
n(t) =nI(t) cos (2πfct)−nQ (t) sin 2πfc assuming that the carrier-to-noise ratio at the detector input is high
compared with unity, determine i. the output signal-to-noise ratio and ii. The figure of merit of the
system [L1, L4&CO.3] 2*5M
2. Explain how S/N ratio is a figure of merit in case of performance of a communication
Channel [L2 &CO.3] 10M
3. Derive the expression for figure of merit of AM system for large case[L4 &CO.3] 10M
4. Explain the noise performance of SSB - SC receiver and prove its S/N Ratio is
Unity. [L2 &CO.3] 10M
5. Compare noise performance of PM and FM system. [L3 &CO.3] 10M
6. (a) Explain the equivalent model of a generalized communication system for noise calculation.
(b) Explain the noise performance of DSB -SC scheme with the help of block
Diagram [L2 &CO.3] 2*5M
7. (a) Compare the noise performance of the DSB-SC and SSB-SC. [DEC-2016]
(b) Define noise equivalent Bandwidth and noise temperature [L2,L1&CO.3] 2*5M
8. (a) Derive the expression for the S/N ratio of AM system.
(b) What is capture effect? Explain FM Threshold effect. [L4 &CO.3] 2*5M
9. Prove that narrow band FM offers no improvement in SNR over AM. [L4 &CO.3] 10M
10. Explain the quadrature representation of narrow band noise along with the envelope of narrow band noise.
[DEC-2016] [L2 &CO.3] 10M

2 Marks Questions

1. Define figure of merit? ( Understand)

2. Define White noise and Shot noise. ( Remember)
3. List out various noise sources. ( Remember)
4. Explain pre-emphasis &de-emphasis ( Understand)
5. Explain noise equivalent bandwidth. ( Understand)
6. Define noise temperature? ( Understand)
7. Define noise figure? ( Understand)
8. What sampling rate would be appropriate for each of the following? (i). A telephone channel limited to 3.3
KHz band width (ii). A music channel with a maximum signal frequency of 20Khz
 9. Define thermal noise and noise power
10. Define noise and its types.
11. What is meant by thermal noise? [DEC-2016]
12. What is the noise performances of AM signal?
13. Define signal to noise ratio?
14. What is the noise performances of FM signal?
15. What is the noise performances of SSB signal?
16. What are the types of external noises?
17. What are the types of internal noises?
18. Define flicker noise?
19. What is narrowband noise?
20. State the reasons for higher noise in mixers

UNIT-IV
ANALOG PULSE MODULATION SCHEMES
10-MARKS

1. Explain Super heterodyne working principle

(a) Compare Continuous wave and pulse modulation technique.
(b) state and prove sampling theorem [L2 &CO.4] 2*5M
2. (a) Compare PAM,PPM,PWM [DEC-2016]
(b) List out the applications of pulse modulation techniques. [L2 &CO.4] 2*5M
3. Explain Flat top PAM with neat diagram and expressions [L2 &CO.4] 10M
4. Explain PAM modulator and Demodulator [L2 &CO.4] 10M
5. Explain PWM modulator and Demodulator [L2 &CO.4] 2*5M
6. Explain PPM modulator and Demodulator [L2 &CO.4] 2*5M
7. a) Explain of the block diagram TRF receiver. Also explain the basic super heterodyne principle.
b) List out the advantages and disadvantages of TRF receiver [L2 &CO.4] 2*5M
8. Explain the followings (a) Ideal sampling (b) Selectivity (c) Natural Sampling (d) Sensitivity (e) Pulse
Width Modulation (f).Fidelity (g) Flat top sampling (h) Pulse Amplitude Modulation
9. (a) Sketch the interleaving scheme for TDM, the following PAM channels: five 4Khz telephone channels
and one 20Khz music channel.
(b) What sampling rate would be appropriate for a telephone channel limited to 3.3 Khz bandwidth.
[L2 &CO.4] 2*5M
10. How to demodulate the PPM signal? What are its advantages and disadvantages?
[DEC-2016] [L2 &CO.4] 2*5M

2 Marks Questions

1. State Sampling Theorem.( Remember) [DEC-2016]

2. Compare Continuous wave and pulse modulation technique.( Understand)
3. Write Merits and Demerits of PAM. ( Understand)
4. Compare PAM, PPM, PWM. ( Understand)
5. List out the applications of pulse modulation techniques. .( Remember)
6. Define Sensitivity and Selectivity( Understand)
7. List the Classification of receivers. ( Understand)
8. Explain flat top sampling ( Understand)
9. List the draw backs of PAM. ( Understand)
10. Compare ideal and natural sampling( Understand)
11. Compare the sampling techniques of PAM?
12. Define Nyquist rate?
 13. Define quantization?
14. What is the purpose of sample and hold circuit?
15. Define PAM?
16. What is the aliasing? What is the effect of aliasing?
17. Define instantaneous sampling?
18. Define fidelity?
19. Draw the block diagram of PAM?
20. What are the types of sampling? Explain the operation of the sample and hold circuit.
21.

UNIT-V
INFORMATION AND CHANNEL CAPACITY

10-MARKS

1. Explain the followings (a). Entropy (b). Information rate (c) Channel capacity (d) Conditional entropy (e).
Memory channel (f). Memory less channels (g). Find the entropy the source that emits one of the three
symbols A,B,C in a statistically independent sequence with probabilities ½,1/4 and 1/4,.
2. Explain about the Shanon Encoding algorithm which can convert a message into the binary sequence and
its properties with example? [DEC-2016]
3. Discuss about the discrete stationary Mark off process with state diagram?
4. Two resistors of 20K Ohms and 50K Ohms, are at room temperature. Calculate for a bandwidth of 100
kHz, the thermal noise voltage (i) For each resistor (ii) When two resistors are in series (iii) When two
resistors are in parallel. Assume kT=4*10-21 W/Hz at room temperature.
5. Explain entropy and Shannon Hartley theorem
6. One of the four possible symbols having probabilities 1/8,13/8,3/8,1/8 is transmitted calculate average
information and information rate if source is generating 1000 symbols/sec
7. Explain Information Rate and Entropy over discrete memory less channel?
8. A Discrete source emits one of 5 symbols once every millisecond. The symbol probabilities are ½,
1/4,1/8,1/16 and 1/16. Find entropy and information rate?
9. Explain Capacity of discrete memory less channels
10. a).Explain Entropy and information rate of mark off sources?
b).Calculate the Capacity of a low pass channel with a usable bandwidth of 3000Hz and S/N= 103 at the
channel output. Assume the channel noise to Gaussian and white. [DEC-2016]

2 Marks Questions
1. Define entropy
2. Explain about information source.
3. Define information, signal and message.
4. Define conditional entropy.
5. Define memory and memory less channels.
6. Find the entropy the source that emits one of the three symbols A,B,C in a statistically independent
sequence with probabilities ½,1/4 and 1/4.
7. State the shanon Hartely theorem
8. Define information rate.
9. What is channel capacity and channel efficiency? [DEC-2016]
10. One of the four possible symbols having probabilities 1/8,13/8,3/8,1/8 is transmitted calculate average
information and information rate if source is generating 2000 symbols/sec. [DEC-16]
11. Define mutual information.
12. Define rate of information across the channel.
 13. Define channel capacity of the discrete memory less channels.
14. What is mean by discrete channel?
15. Give two properties of the entropy.
16. Define Bit rate?
17. Give advantages of Shano fano encoding algorithm?
18. What are the source encoding algorithms?
19. A source is emitting symbols X1, X2 and X3 with probabilities respectively 0.6, 0.3 and 0.1. What is the
entropy of the source?
20. What are the types of the channel?

1. In commercial TV transmission in India, picture and speech signals are modulated respectively [ ]
GATE -1990
(A). VSB & VSB (B). ) VSB & SSB (C). VSB & FM (D). FM & VSB

2. 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 _____ GATE-2014
(A). 1 (B). 2 (C). 4 (D). 8
3. The maximum power efficiency of an AM modulator is GATE -1992 [ ]

(A). 25% (B). 33% (C). 50% (D).100%

4. Consider sinusoidal modulation in an AM systems. Assuming no over modulation , the [ ]
maximum and minimum values of the envelope, respectively, are 3V and 1V calculate
modulation index ___________ _ GATE-2014
(A). 0.50 (B). 0.75 (C). 1.0 (D). 1.5

5. Which of the following Analog modulation scheme requires the minimum transmitted power and [ ]
minimum channel band-width? GATE-2005
 (A). AM (B). DSB-SC (C). SSB-SC (D). VSB

6. Suppose that the modulating signal is x(�)=2cos(2𝜋𝑓𝑚�) and the carrier signal is [ ]
𝑥�(�)=𝐴�cos(2𝜋𝑓𝑐�).Which one of the following is a conventional AM signal without over-
modulation? GATE-2010
(A).S(�)=𝐴�𝑚(�)cos(2𝜋𝑓𝑐) (B).S(�)=𝐴�[1+𝑚(�)]cos(2𝜋𝑓𝑐) (C). S(�)=𝐴�cos(2𝜋𝑓𝑐�)+ (D).
𝐴�4𝑚(�)cos(2𝜋𝑓𝑐�) None
7. For a message signal (�)=cos(2𝜋𝑓𝑐�) and carrier of frequency𝑓𝑐. Which of the following [ ]
represents a single side-band (SSB) signal? GATE-2009
(A).cos(2𝜋𝑓𝑚�)cos(2𝜋𝑓𝑐�) (B). cos(2𝜋𝑓𝑐�) (C).cos[2𝜋(𝑓𝑐+𝑓𝑚) (D)[1+cos(2𝜋𝑓𝑚�)].cos(2𝜋𝑓𝑐�)

8. A DSB-SC signal is generated using the carrier cos(𝜔��+�) and modulating signal x(t). The [ ]
envelop of the DSB-SC signal is GATE-2014
(A). 𝑥(�) (B). |𝑥(�)| (C). Only positive portion (D).None
of x(t)
9. A 1 MHz sinusoidal carrier is amplitude modulated by a symmetrical square wave of period [ ]
100 µsec. Which of the following frequencies will not be present in the modulated signal?
GATE-2002
(A). 1000KHZ (B). 1020KHZ (C). 990KHZ (D).875KHZ

10 A 4 GHz carrier is DSB-SC modulated by a low-pass message signal with maximum frequency [ ]
. of 2 MHz. The resultant signal is to be ideally sampled. The minimum frequency of the sampling
impulse train should be GATE-1990
(A). 5MHZ (B).8GHZ (C).8MHZ (D).8.006MHZ

11. Consider the amplitude modulated (AM) signal𝐴�cos𝜔𝑐�+2cos𝜔𝑚�cos𝜔𝑐�. For demodulating the [ ]
signal using envelope detector, the minimum value of AC should be GATE-2008
(A). 2 (B). 1 (C). 0.5 (D). 0

12 A super heterodyne radio receiver with an intermediate frequency of 455 KHz is tuned to a station [ ]
. operating at 1200 KHz. The associated image frequency is -----------KHz GATE-1993
(A). 1120HZ (B).2110HZ (C). 2200HZ (D). 2000HZ

13 The image channel selectivity of super heterodyne receiver depends upon GATE-1998 [ ]
.
(A). IF amplifiers only (B). RF and IF (C). Pre selector, RF (D). Pre selector
amplifiers only and IF amplifiers and RF amplifiers

14 Which of the following demodulator (s) can be used for demodulating the signal GATE-1993 [ ]
. 𝑥(�)=5(1+2cos200 𝜋�)𝑐𝑜�20000𝜋�
(A)Envelope demodulator (B)Square-law demodulator (C) Synchronous demodulator (D).None

15 There analog signals, having bandwidth 1200 Hz, 600 Hz and 600 Hz, are sampled at their [ ]
. respective Nyquist rates, encoded with 12 bit words, and time division multiplexed. The bit rate
for the multiplexed signal is
(A). 100kbps (B). 57.6kbps (C). 35.6kbps (D). 40kbps

16 Bandwidth of VSB is [ ]
.
(a) Same as SSB (d) Slightly higher than SSB (c) Same as AM (d) Same as DSB
 17 In VSB [ ]
.
(A). ) A gradual cut-off of (B). Rejecting one side (C). Rejecting both side (D).
one side band is allowed band completely bands completely None
18 If µ=1, Pc=_________ [ ]
.
(A) 33.33% of Pt (B) 80% of Pt (C) 0% of Pt (D) 66.66% of Pt

.19The process of retrieval of information is called [ ]

.
(A). Modulation (B). Transducer (C). Receiver (D) Demodulation

20 The higher modulating frequency used in AM broadcasting system is [ ]

.
(A) 10KHz (C).5KHZ (B) 15KHz (D).2MHZ

21.Most common used information source in broadcasting [ ]

(A)Voice (B) Audio (C) Video (D) Both A & B

22. Which is caused by imperfect response of the system [ ]

(A)Interference (B) Distortion (C) Noise (D) All

23. Which is contamination by extraneous signals [ ]

(A)Interference (B) Distortion (C) Noise (D) All

24. The process of retrieval of information is called [ ]

(A)Modulation (B) Transducer (C) Receiver (D) Demodulation

25. Interference most often occurs in [ ]

(A)T.V (B) Voice (C)Broadcasting (D)None

26. What is/are the primary resource in communication system: [ ]

(A)Transmitted power (B) Channel bandwidth (C) Both (D) None

27. In ever communication system most appear noise is [ ]

(A)Gaussian noise (B) White noise (C) Pink noise (D) Thermal noise

28. Modulation is the process of [ ]

(A)High Frequency to Low Frequency (B) Wide band to Narrow band

(C)Pass band to band pass (C) None

29. Coding is a [ ]

(A)Signal processing (B) Symbol processing (C) both (D) None

30. Two key barriers to human communication are [ ]

 (A)Distance (B) Cost (C) Ignorance (D) Language

31. Communication medium causes the signal to be [ ]

(A)Amplified (B) Modulation (C) Attenuated (D) Interfered with

32. The high frequency range extends from [ ]

(A) 300-3000KHz (B)3-30 KHz (C)30-300 MHz (D)300-3000 MHz

33. The Fourier transform of a voltage signal x(t) if X(f). The unit of mod{X(f)} [ ]

(A)Volts (B) Volts-sec (C) Volt/Sec (D) Volts2

34. If a signal f(t) has energy E, the energy of the signal f(2t) is equal to [ ]

(A)E (B) E/2 (C)2E (D)4E

35. Transform a digital message into a new sequence of symbols [ ]

(A)Decoding (B) Sampling (C)Quantizer (D)Encoder

36. Which technique is used to improve the performance reliability in noisy channel [ ]

(A)Channel coding (B) Error Control coding (C) Source coding (D) Hamming coding

37. Which technique is used to improve the wide band noise reduction [ ]

(A)Channel coding (B) Error Control coding (C) Source coding (D) Hamming coding

38. What is/are the fundamental limitation of information transmission by electrical means [ ]

(A)Noise (B) Bandwidth (C) Both (C) None

39. Which is/are continuous wave modulation? [ ]

(A)AM (B) FM (C) PM (D) All the above

40. The message signal contains three frequencies 5 kHz, 10 kHz, 20 kHz respectively. The BW of the AM signal is

[ ]

(A) 40 kHz (B) 10 kHz (C) 20 kHz (D) 30 kHz

UNIT-II: ANGLE MODULATION AND DEMODULATION

1. Consider an angle modulation signal (�)=6𝑐𝑜�[2𝜋×103+2sin(8000𝜋�)+4cos(8000𝜋�)]�. The average [ ]
power of 𝑥(�) is GATE-2014
(A). ) 10W (B). ) 18W (C). 20W (D). 28W
2. A modulation signal is (�)=𝑚(�)cos(40000𝜋�), where the baseband signal m(t) has frequency [ ]
components less than 5 kHz only. The minimum required rate (in kHz) at which y(t) should be sampled
to recover m(t) is GATE-2014
(A). 15 K samples/sec (B). 20 K samples/sec (C). 10K samples/sec (D). 8
3. A modulation signal is given by (�)=�−𝑎�cos[(𝜔𝑐+Δ𝜔)�]�(�), where,𝜔𝑐 𝑎𝑛� Δ𝜔 are positive constants, [ ]
and 𝜔𝑐≫Δ𝜔. The complex envelope of s(t) is given by GATE-1999
(A).exp(−𝑎�)�𝑥�[�(𝜔𝑐+Δ𝜔)]�(�) (B). (C). �𝑥�(�Δ𝜔�)�(�) (D).none
exp(−𝑎�)exp(�Δ𝜔�)�(�)
4. A 10 MHz carrier is frequency modulated by a sinusoidal signal of 500 Hz, the maximum frequency [ ]
deviation being 50 KHz. The bandwidth required. as given by the Carson’s rule is GATE-1994
(A). 101khz (B). 125khz (C). 102khz (D). 150khz
5. Consider the frequency modulated signal 10[cos2𝜋×105�+5sin(2𝜋×1500�)+7.5sin(2𝜋×1000�)] with [ ]
carrier frequency of 105 Hz. The modulation index is GATE-2008
(A). 15 (B). 12.5 (C). 10 (D). 11
6. A device with input (�) and output (�) is characterized by: (�)=𝑥2(�). [ ]
An FM signal with frequency deviation of 90 KHz and modulating signal bandwidth of 5 KHz is
applied to this device. The bandwidth of the output signal is GATE -2005
(A).370 KHz (B).190 KHz (C).380 KHz (D). 95
KHz
7. An angle-modulation signal is given by �(�)=cos(2𝜋×2×106�+2𝜋×30sin150�+2𝜋×40cos150�) [ ]
The maximum frequency deviations of s(t) is is GATE -2002
(A).10.5KHz, 140π rad (B). 6 KHz,100π rad (C). 7.5 KHz, 100π rad (D). 9 KHz,100π rad

8. An angle-modulation signal is given by �(�)=cos(2𝜋×2×106�+2𝜋×30sin150�+2𝜋×40cos150�) [ ]

The maximum phase deviations of s(t) is
(A). 100π rad (B). |80π rad (C). 140π rad (D).None
9. An FM signal with a modulation index 9 is applied to a frequency tripler. The modulation index in the [ ]
output signal will be GATE-1996
(A). 3 (B). 27 (C). 0 (D). 9
10 Consider an FM wave 𝑓(�)=cos[2𝜋𝑓𝑐�+�1sin2𝜋𝑓1�+�22𝜋𝑓2�] [ ]
. The maximum deviation of the instantaneous frequency from the carrier frequency fc is GATE-2014

(A). �1𝑓1+�2𝑓2 (B). �1𝑓2+�2𝑓1 (C). �1+�2 (D). 𝑓1+𝑓2

11 �(�)=5[cos(106𝜋�)−sin(103𝜋�)×sin(106𝜋�)] represents GATE-1994 [ ]

.
(A). DSB-SC signal (B). AM signal (C). SSB upper sideband signal (D). NB FM signal

12 An AM signal and a narrow-band FM signal with identical carriers, modulating signals and modulation [ ]
. indices of 0.1 are added together. The resultant signal can be closely approximated by

(A). Broadband FM (B). SSB with carrier (C). DSB-SC (D).SSB without carrier

13 The input to a coherent detector is DSB-SC signal plus noise. The noise at the detector output is GATE [ ]
. -2003

(A). in-phase component (B).quadrature-component (C). zero (D the envelope

14 A message signal with bandwidth 10 KHz is Lower-Side Band SSB modulated with carrier frequency [ ]
. 𝑓𝑐1=106𝐻�. The resulting signal is then passed through a narrow-band frequency Modulator with carrier
frequency 𝑓𝑐2=109𝐻�. The bandwidth of the output would be GATE -2006
 UNIT-3

1. The capacity of an analog communication channel with 4 kHz bandwidth and 15dB SNR is

approximately [A ]
(a) 20000bps (b) 16000bps (c) 10000bps (d) 8000bps
2. The Signal to Noise of Fm at modulation index = 5 is _ _ _ _ _ _ _ _ _ _ factor of S/N of AM [ C ]
(a) 57 (b) 70 (c) 75 (d) 81
3. S/N ratio improvement is proportional to the square of the bandwidth of transmission in [A ]
(a) FM (b) Conventional AM (c) AM-DSB-SC (d) AM-SSB-SC
4. The Signal to Noise ratio of DSB-SC Scheme is [B ]
a) 3 (b) 2 (c) 1 (d) Zero
5. Which of the following demodulation methods exhibit threshold effect [B ]
(a) Synchronous demodulation of AM (b) Envelope detection of AM
(c) Square law detector (d) Balanced demodulator
6. Noise performance of conventional AM, when compared with that of suppressed carrier systems is
[B ]
(a) Superior (b) Inferior (c) Same (d) Can’t be compared
7. TDM can be implemented using [B ]
(a) AM (b) PAM (c) FM (d) VSB-AM
8. fs >> 2fm results in [C ]
(a) Aliasing (b) Distortion (c) Inefficient Channel utilization (d) Attenuation
9. Which functional block readily identifies it is an FM receiver [C ]
(a) RF amp (b) IF amp (c) detector (d) mixer
10. The width (τ ) of a sample of a base band signal of band limited frequency fm can have a value[A ]
(a) τ < 1/ fm (b) τ >1/ fm (c) τ =1/ fm (d) τ > 1/ fm
11. The Signal to Noise ratio of SSB-SC scheme is (a)3 (b) 2 (c) 1 (d) 0 [C ]
12. Noise voltage varies in resistor is [A ]

(a) 4KTRb (b) sqrt(4KTRb) (c) 2KTRb (d) sqrt(2KTRb)

13. Which one of the following types of noise gain importance at high frequency? [A ]
(a) Shot noise (b) Random noise (c) Impulse Noise (d) Transit-time noise
14. The capacity of an analog communication channel with 4kHz bandwidth and 15dB SNR is
approximately [B ]
(a) 20000bps (b) 16000bps (c) 10000bps (d) 8000bps
15. Anti-aliasing filter is a _______ filter [A ]
(a) LPF (B) HPF (C) BPF (d) All

16. The Signal to Noise ratio of VSB-SC scheme is [C ]

(a) 3 (b) 2 (c) 1 (d) zero

17. Noise performance of AM-DSB-SC signal is same as that of AM-SSB-SC system due to [B ]

(a) More i/p signal power (b) More bandwidth (c) Less i/p signal power (d) Less Noise power

18. The Figure of Merit of FM is [A ]

(a) (3/2)* ß2 (b) (2/3)* ß2 (c) (3/2)*ß (d) 1/3

19. The S /N ratio of FM to S/N ratio of AM is [A ]

(a) v3 modulation index of FM (b) modulation index of FM / v3 (c) v3 modulation index of AM (d) 1

20. The ratio of S/N of PM to S/N of AM is given by [B ]

(a) fm /∆f (b) ∆f / fm (c) fm*∆f (d) ∆f+fm
21. Noise performance of AM-DSB-SC signal is same as that of AM-SSB-SC system due to [B ]

(a) More i/p signal power (b) More bandwidth (c) Less i/p signal power (d) Less Noise power

22. The Signal to Noise ratio of FM demodulator is [A ]

(a) 3 Kf2 (b) Kf (c) 2 Kf 2 (d) Kf 2

23. The S /N ratio of FM to S/N ratio of AM is [ A ]
(a)v3 modulation index of FM (b) modulation index of FM / v3 (c)v3 modulation index of AM (d) 1

24. The ratio of S/N of PM to S/N of AM is given by [ B ]

(a) fm /∆ f (b)∆ f / fm (c)fm x ∆ f (d)∆ f+fm

25. In PM, the power spectral density of the O/P noise, varies with frequency as [ A ]

a. Parabola b. Hyperbola c. Uniform d. circle

26. For a modulation index β > 0.5 the noise performance of FM when compared to AM is [ A ]

a. Better b. Same c. worst d. Can't be compared

27. In PM, the power spectral density of the O/P noise, varies with frequency as [ C ]

(a) Parabola (b) Hyperbola (c) Uniform (d) circle

28. For a modulation index β > 0.5 the noise performance of FM when compared to AM is [ A ]

(a) Better (b) Same (c) worst (d) Can't be compared

29. Threshold improvement in FM is due to [ B ]

(a) Pre-emphasis at the Tx end (b) De-emphasis at the Rx end

(c) Pre-emphasis at the Rx end (d) De-emphasis at the Tx end

30. In FM, the power spectral density of the O/P noise, varies with frequency as [ A ]

(a) Parabola (b) Hyperbola (c) Uniform (d) circle

31. The SNR of PM over AM is _ _ _ _ _ _ _ _ _ _ _ _ _ _ of BW of Transmission [ C ]

(a) Same (b) double (c) square (d) quadruple

32. Broadcast transmitters are designed for transmitting [ C ]

(a) Coded signal (b) Telephone signal (c) Speech or music (d) Continuous carrier
33. The Signal to Noise of FM at modulation index = 5 is _ _ _ _ _ _ _ _ _ _ factor of S/N of AM [ C ]
(a). 57 (b) 70 (c) 75 (d) 81
34. S/N ratio improvement is proportional to the square of the bandwidth of transmission in [ A ]
(a) FM (b) Conventional AM (c) AM-DSB-SC (d) AM-SSB-SC
35. The Signal to Noise ratio of DSB-SC Scheme is [ B ]
(a). 3 (b) 2 (c) 1 (d) Zero
36. Which of the following demodulation methods exhibit threshold effect [ B ]
(a) Synchronous demodulation of AM (b) Envelope detection of AM
(c) Square law detector (d) Balanced demodulator
37. Noise performance of conventional AM, when compared with that of suppressed carrier systems is [ B ]
 (a) Superior (b) Inferior (c) Same (d) Can’t be compared
38. The Signal to Noise ratio of SSB-SC scheme is [ C ]
(a) 3 (b)2 (c)1 (d) zero
39. The Signal to Noise ratio of VSB-SC scheme is [ C ]
(a)3 (b) 2 (c) 1 (d) zero
40. To undo the distortion caused by the channel, which of the following can be used [ B ]

(a) Amplifier (b) Equalizer (c) Attenuator (d) Clamper

41. Quantising noise occurs in [ C ]

(a) Time-division multiplexing (b) Frequency-division multiplexing

(c) Pulse-code modulation (d) Pulse position modulation

UNIT-IV

1. A PAM signal may be generated using [B ]

(a) Impulse sampling (b) A sample and hold circuit (c) Natural sampling (d) A clipper circuit
2. The impulse response function, h(t), of a zero-order-hold circuit is [B ]
(a) an impulse (b) A rectangular pulse (c) A triangular pulse (d) None

3. A band limited low pass signal is sampled at twice its nyqyist rate with fs=2000sps. The signal is band

limited to [B ]
(a) 250Hz (b) 1000Hz (c) 500Hz (d) 2000Hz
4. A PAM signal may be demodulated using [B ]

(a) A LPF (b) A differentiator followed by a LPF (c) An integrator (d) A LPF followed by an equalizer

5. Aliasing occurs due to which of the following [A ]

(a) fs = 2 fm (b) Ts = 2 fm (c) fs = 1 / 2 fm (d) Ts = fm
6. A Pulse width modulated signal can be generated by [A ]
(a) An astable multivibrator (b) A monostable multivibrator
(c) Integrating the signal (d) Differentiating the PPM signal
7. PWM can be demodulated using [A ]
(a) Integrator and LPF (b) Differentiator and LPF (c) Integrator and HPF (d) Differentiator and HPF
8. PPM can be generated from [C ]
(a) AM (b) FM (c) PWM (d) PM
9. Quantizing noise in a PCM system can be reduced by [D ]
(a) Decreasing the number of standard levels (b) Having more no.of samples per second
(c) Using low noise circuitry in PCM receiver (d) Increasing the number of standard levels
10. In an FM receiver the stage between IF amplifier and detector is called [D ]
(a) Mixer (b) Limiter (c) local Oscillator (d) AF amplifier
11. FM capture effect is due to the following circuit [A ]
(a) Limiter (b) AGC (c) Discriminator (d) Deemphasis
12. Which of the following is used to convert PPM into PWM [B ]
(a) Clipping Circuit (b) Bi-stable Multi-vibrator(c) Astable Multi-vibrator (d) Clamping
13. Harmonic generator is operated as a _ _ _ _ _ _ _ _ _ _ _ _ _ amplifier [A ]
(a) C (b) A (c) AB (d) B
14. Pulse communication system that is inherently highly immune to noise is [D ]
(a) PWM (b) PAM (c) PPM (d) PCM
15. A PWM signal can be generated by [B ]
(a) mono-stable Multi-vibrator (b) Astable multi-vibrator
(c) Integrating the PPM signal (c) Differentiating the PPM signal

16. Pulse communication system that is high inherently noise is [D ]

(a) PAM (b) PWM (c) PPM (d) PCM
17. Quantization noise can be reduced by increasing the no.of samples per seconds. It is true [D ]
(a) Yes, it is (b) No, it is (c) Not necessarily (d) None of these
18. In pulse modulation, the no.of samples required to ensure no loss of information is given by [ A ]
(a) Nyquist theorem (b) Parseval’s theorem (c) Fourier Transform (d) All of the
19. Aliasing occurs if ……… [ ]
A) fs>2fm B) fs=2fm C) fs<2fm D)none

20) PWM and PPM are the forms of …….. [ ]

A) PAM B) PCM C) PTM D) QAM

21)……….. is known as Nyquist rate. [ ]

A) fs=2fm B) fs>2fm C) fs<2fm D) none

22) Under sampling is used in ……….. Pictures [ ]

A) slow-motion B) western C) fast-motion D) both B and C

23) Bandwidth gets …………… with over-sampling. [ ]

A) Increased B) decreased C) remains constant D) none

24)…………. Modulation is mostly effected by noise. [ ]

A) PTM B) PAM C) PCM D) PPM

25) To generate PWM signal ……… is used. [ ]

A) AMV B) BMV C) MMV D) NONE

26) Transmitter power remains constant in ………. Pulse modulation. [ ]

A) PAM B) PWM C) PPM D) PTM

27) Band-width remains constant in …….. Pulse modulation. [ ]

A) PAM B) PPM C) PTM D) NONET

28) To demodulate PWM ……… is used. [ ]

A) Integrator B) differentiator C) Schmitt-trigger D) none

29) At receiver to avoid aperture effect ………. Is used after LPF. [ ]

A) Equalizer B) integrator C) Schmitt-trigger D) none

30) Find the sampling frequency fs for the given message signal m(t)=Sinc2 500t. [ ]

A) 50 B) 500 C) 1000 D) none

31) Find the sampling frequency fs for the given message signal m(t)= 5 Cos 2π 3000t. [ ]

A) 5000 B) 6000 C) 1500 D) none

32) Find the sampling frequency fs for the given message signal m(t)= 10Cos2 2π 4000t. [ ]

A) 5K B) 8K C) 16KD) 4K

33) A PPM signal can be converted into PWM signal by using ……….. [ ]

A) AMV B) BMV C) MMV D) NONE

34) To generate PPM signal, first PWM signal is generated then differentiator followed by ……is used

A) Clamper B) clipper C) flip-flop D) NONE. [ ]

35) Find the sampling frequency fs for the given message signal m(t)=Sinc 500t. [ ]

A) 50 B) 500 C) 1000 D) none

36) Find the sampling frequency fs for the given message signal m(t)=Sinc3 500t. [ ]
 A) 50 B) 1500 C) 500 D) none

37) Find the sampling frequency fs for the given message signal m(t)=5 Cos 2π 3000t + 5 Cos2π 4000t.

A) 2000 B) 6000 C) 8000 D) 15000. [ ]

38) Find the sampling frequency fs for the given message signal m(t)= 5 Cos 2π 3000t * Cos2π 4000t.

A) 14K B) 6K C) 12K D) 15K. [ ]

UNIT-V

1. What is information source used in radio broadcasting [ D ]

(a) Audio (b) Music (c) Video (d) A & B
2. When the sources are analog signal then the source are called [ A ]
(a) Analog source (b) digital source (c) Both (d) None
3. When the sources are producing discrete output, then they are called [ B ]
(a) Analog source (b) digital source (c) Both (d) None
4. An analog source can transform to discrete source by [ C ]
(a) Sampling (b) Quantization (c) Both (d) None
5. Set of source symbols is called [ A ]
(a) Source alphabet (b) Symbols (c) letters (d) All
6. The elements of set of source symbols called [ B/C ]
(a) Source alphabet (b) Symbols (c) letters (d) All
7. DMS stands [ B ]
(a) Dynamic Memory System (b) Discrete Memory less Source
(c) Discrete Memory Source (d) Dynamic Memory less System

8. In I(xi) = -logbP(xi), if b=2, the units for I(xi) [ A ]

(a) Bits (b) Nats (c) Hartley (d) No units
9. Mutual information between pain of events can be either [ D ]
(a) +ive (b) –ive (c) Zero (d) All
10. Entropy satisfies. [ C ]
(a) 0<H(x)<log2m (b)0>H(x)>log2m (c) 0<H(x)<log2m (d) 0>H(x)>log2m
11. A pure AC signal is called a [ A ]
(a) sine wave (b) square (c) rectangular (d) Triangular
12. One of the following system is analog [ D ]
(a) PCM (b) Delta (c) DPCM (d) PAM

13. In I(xi) = -logbP(xi), if b=e, the units for I(xi) [ B ]

(a) Bits (b) Nats (c) Hartley (d) No units
14. Average information can be represents (units) [ B ]
(a) Bits-symbols (b) Bits per symbols (c) Bits-sec (d) Bits per Sec
15. Mutual information between pair of event can be either [ D ]
(a) +ive (b) –ive (c) Zero (d) Either of the above

16. In I(xi) = -logbP(xi), if b=10, the units for I(xi) [ C ]

(a) Bits (b) Nats (c) Hartley (d) No units
17. Information rate denotes [ B ]
(a) R = r/H(s) (b) R = r*H(s) (c) R = H(s) (d) R = 1/H(s)
18. The units for information rate [ D ]
(a) Symbols per Sec (b) Symbols per Bits (c) Bits-sec (d) Bits per Sec
19. The time rate at which source X emits symbols is ‘r’, the units for ‘r’ [ A ]
(a) Symbols per Sec (b) Symbols per Bits (c) Bits-sec (d) Bits per Sec
20. What is information source used in radio broadcasting [ D ]
(a) Audio (b) Music (c) Video (d) A & B
21. An analog source can transform to discrete source by [ C ]
(a) Sampling (b) Quantization (c) Both (d) None