# Multiple Choice Questions and Answers on Sampling Theory and Pulse Modulation

1)   A periodic signal is

a. May be represented by g(t) = g(t + T0)
b. Value may be determined at any point
c. Repeats itself at regular intervals
d. All of the above

ANSWER: (d) All of the above

2)   Sine wave is a

a. Periodic signal
b. Aperiodic signal
c. Deterministic signal
d. Both a and c

3)   An even function f(x) for all values of x and x holds

a. f(x) = f(-x)
b. f(x) = -f(x)
c. f(x) = f(x)f(-x)
d. None of the above

4)   Random signals is

a. May be specified in time
b. Occurrence is random
c. Repeat over a period
d. None of the above

5)   Unit step function is

a. Exists only for positive side
b. Is zero for negative side
c. Discontinuous at time t=0
d. All of the above

ANSWER:  (d) All of the above

6)   In Unit impulse function

a. Pulse width is zero
b. Area of pulse curve is unity
c. Height of pulse goes to infinity
d. All of the above

ANSWER: (d) All of the above

7)   For a Unit ramp function area of pulse curve is unity

a. Discontinuous at time t=0
b. Starts at time t=0 and linearly increases with t
c. Both a and b
d. None of the above

ANSWER:  (b) Starts at time t=0 and linearly increases with t

8)   The spectrum of the sampled signal may be obtained without overlapping only if

a. fs ≥ 2fm
b. fs < 2fm
c. fs > fm
d. fs < fm

9)   The desired signal of maximum frequency wm centered at frequency w=0 may be recovered if

a. The sampled signal is passed through low pass filter
b. Filter has the cut off frequency wm
c. Both a and b
d. None of the above

ANSWER: (c)  Both a and b

10)   A distorted signal of frequency fm is recovered from a sampled signal if the sampling frequency fs is

a. fs > 2fm
b. fs < 2fm
c. fs = 2fm
d. fs ≥ 2fm

11)   Calculate the minimum sampling rate to avoid aliasing when a continuous time signal is given by x(t) = 5 cos 400πt

a. 100 Hz
b. 200 Hz
c. 400 Hz
d. 250 Hz

Explanation:
In the given signal, the highest frequency is given by f = 400 π/ 2π
= 200 Hz

The minimum sampling rate required to avoid aliasing is given by Nyquist rate. The nyquist rate is = 2 * f
= 2 * 200
= 400 Hz.

12)   Calculate the Nyquist rate for sampling when a continuous time signal is given by
x(t) = 5 cos 100πt +10 cos 200πt – 15 cos 300πt

a. 300Hz
b. 600Hz
c. 150Hz
d. 200Hz

Explanation:
For the given signal,
f1 = 100π/2π = 50Hz
f2 = 200π/2π = 100Hz
f3= 300π/2π = 150Hz

The highest frequency is 150Hz. Therefore fmax = 150Hz
Nyquist rate = 2 fmax
= 2 * 150
= 300Hz.

13)   A low pass filter is

a. Passes the frequencies lower than the specified cut off frequency
b. Rejects higher frequencies
c. Is used to recover signal from sampled signal
d. All of the above

ANSWER:  (d) All of the above

14)   The techniques used for sampling are

a. Instantaneous sampling
b. Natural sampling
c. Flat top sampling
d. All of the above

ANSWER: (d) All of the above

15)   The instantaneous sampling

a. Has a train of impulses
b. Has the pulse width approaching zero value
c. Has the negligible power content
d. All of the above

ANSWER: (d) All of the above

16)   The sampling technique having the minimum noise interference is

a. Instantaneous sampling
b. Natural sampling
c. Flat top sampling
d. All of the above

17)   Types of analog pulse modulation systems are

a. Pulse amplitude modulation
b. Pulse time modulation
c. Frequency modulation
d. Both a and b

ANSWER: (d) Both a and b

18)   In pulse amplitude modulation,

a. Amplitude of the pulse train is varied
b. Width of the pulse train is varied
c. Frequency of the pulse train is varied
d. None of the above

ANSWER:  (a) Amplitude of the pulse train is varied

19)   Pulse time modulation (PTM) includes

a. Pulse width modulation
b. Pulse position modulation
c. Pulse amplitude modulation
d. Both a and b

ANSWER: (d)  Both a and b

20)   Drawback of using PAM method is

a. Bandwidth is very large as compared to modulating signal
b. Varying amplitude of carrier varies the peak power required for transmission
c. Due to varying amplitude of carrier, it is difficult to remove noise at receiver
d. All of the above

ANSWER: (d)  All of the above

21)   In Pulse time modulation (PTM),

a. Amplitude of the carrier is constant
b. Position or width of the carrier varies with modulating signal
c. Pulse width modulation and pulse position modulation are the types of PTM
d. All of the above

ANSWER: (d)  All of the above

22)   In different types of Pulse Width Modulation,

a. Leading edge of the pulse is kept constant
b. Tail edge of the pulse is kept constant
c. Centre of the pulse is kept constant
d. All of the above

ANSWER: (d) All of the above

23)   In pulse width modulation,

a. Synchronization is not required between transmitter and receiver
b. Amplitude of the carrier pulse is varied
c. Instantaneous power at the transmitter is constant
d. None of the above

24)   In PWM signal reception, the Schmitt trigger circuit is used

a. To remove noise
b. To produce ramp signal
c. For synchronization
d. None of the above