1

EC/IN Hinglish
WorkBook 01
Communication
Amplitude Modulation
1. A 2 kW carrier is to be modulated to a 90% level. The 9. In amplitude modulation, the modulation envelope has
total transmitted power would be ________ kW. a peak value which is double the unmodulated carrier
value. What is the value of the modulation index?
2. An AM broadcast station operates at its maximum
allowed total output of 50 kW with 80% modulation. 10. If the modulation index of an AM wave is changed
What is the power (in kW) in the intelligence part? from 0 to 1, then the transmitted power is increased by
__________%.
3. The aerial current of an AM transmitter is 18 A when
unmodulated but increases to 20 A when modulated. 11. A diode detector has a load of 1 kΩ shunted by a
The modulation index is ________. 10000 pF capacitor. The diode has a forward
resistance of 1 Ω. The maximum permissible depth of
4. A modulating signal is amplified by a 80% efficiency modulation, so as to avoid diagonal clipping, with
amplifier before being combined with a 20 kW carrier modulating signal frequency of 10 kHz will be
to generate an AM signal. The required DC input ________.
power (in kW) to the amplifier, for the system to
operate at 100% modulation, would be ________. 12. An AM voltage signal s(t), with a carrier frequency of
1.15 GHz has a complex envelope e(t) = Ae[1 + m(t)],
5. A 2 MHz carrier is amplitude modulated by a 500 Hz where Ae = 500 V, and the modulating signal is a 1
modulating signal to a depth of 70%. If the kHz sinusoidal test tone described by m(t) = 0.8 sin
unmodulated carrier power is 2 kW, the power of the (2 × 103 t) appears across a 50 Ω resistive load. What
modulated signal is ________ kW. is the actual power (in kW) dissipated in the load?

6. A carrier is simultaneously modulated by two sine 13. For an AM signal, the bandwidth is 10 kHz and the
waves with modulation indices of 0.4 and 0.3. The highest frequency component present is 705 kHz. The
resultant modulation index will be ________. carrier frequency used for this AM signal is ________
kHz.
7. In a DSB-SC system with 100% modulation, the
percentage power saving is ________. 14. A message signal m(t) = sin ct + sin c2(t) modulates
the carrier signal c(t) = A cos 2 fct. The bandwidth of
8. A 10 kW carrier is sinusoidally modulated by two the modulated signal is ________ Hz.
carriers corresponding to a modulation index of 30%
and 40% respectively. The total radiated power is Common Data for Q.15 to 17:
__________ kW. The Fourier transform M(f) of a signal m(t) is shown
in figure below. It is to be transmitted from a source to
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destination. It is known that the signal is normalized,

meaning that –1  m(t)  1.

21. An AM modulator has output x(t) = 40 cos 400 t + 4

cos 360 t + 4 cos 440 t. The modulation efficiency
15. If upper sideband is employed, the bandwidth of the
is ________.
modulated signal is ________ kHz.

22. A super heterodyne receiver uses an IF frequency of

16. If DSB is employed, the bandwidth of the modulated
455 kHz. The receiver is turned to a transmitter having
signal is _________ kHz.
a carrier frequency of 2400 kHz. High-side tuning is
to be used. The image frequency will be ________
17. If an AM modulation scheme with a = 0.8 is used,
kHz.
what will be the bandwidth (in kHz) of the modulated
signal?
23. 12 signals each band-limited to 5 kHz are to be
transmitted over a signal channel by frequency
18. A DSB-SC signal is to be generated with a carrier
division multiplexing. If AM-SSB modulation guard
frequency fc = 1 MHz using a non-linear device with
band of a 1 kHz is used, then the bandwidth of the
the input-output characteristic vo  a0 vi  a1vi3 where multiplexed signal will be ________ kHz.
a0 and a1 are constants. The output of the non-linear
device can be filtered by an appropriate band-pass 24. In an AM signal the received signal power is 10–10 W
filter. Let vi  Ac' cos(2 fc't )  m(t ) where m(t) is the with a maximum modulating signal of 5 kHz. The
noise spectral density at the receiver input is 10–18
message signal. Then, f c' = ________ MHz.
W/Hz. If the noise power is restricted to the message
signal bandwidth only, the signals-to-noise ratio at the
19. A non-linear device with a transfer characteristic
input to the receiver is ________ dB.
given by i  (10  2vi  0.2vi2 ) mA is supplied with a
carrier of 1V amplitude and a sinusoidal signal of 0.5V Common Data for Q.25 to 26:
amplitude in series. If at the output the frequency On an AM signal, the maximum and minimum value
component of AM signal is considered, then the depth read from the graticule on the oscilloscope screen is
of modulation is _________%. respectively.
Amax = 5.9 divisions
20. A message signal is periodic with period T, as shown Amin = 1.2 divisions
in figure. This message signal is applied to an AM 25. What is the modulation index of the signal?
modulator with modulation index a = 0.4. What will
be the percentage modulation efficiency? 26. If the vertical scale is 2 volts per division, then the
amplitude of the carrier signal will be ________.
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27. A standard AM broadcast station is allowed to The positive portion of the envelope of the output of
transmit modulation frequencies utp to 5 kHz. If the an AM modulator is shown in figure below.
AM station is transmitting on a frequency of 980 kHz,
then the total bandwidth occupied by the AM station
will be ________ kHz.

28. An AM transmitter has a carrier power of 30 W. The

percentage of modulation is 85%. What will be the
power (in watt) in lower sideband of AM signal?

29. An antenna has an impedance of 40 Ω. An 33. If the message signal is a waveform having zero dc
unmodulated AM signal produces a current of 4.8 A. value, what will be the modulation index of the signal?
If the modulation is 90%, what will be total power (in
watt) in the modulated signal? 34. What will be the power (in watt) in sideband of the
signal?
30. An antenna experiences a current of a 4.8 A for an
unmodulated signal. If the signal is amplitude Common Data for Q.35 to 36:
modulated such that the antenna current changes to 5.1 An AM modulator operates with the message signal,
A, then the percentage of modulation is ________%. m(t) = 9 cos 2t + 7 cos 60 t

31. An AM transmitter uses high level modulation of the 35. What is the power (in watt) in normalized message
final RF power amplifier, which has a dc supply signal?
voltage of 48 V with a total current of 3.5 A. The
efficiency is 70%. What is the carrier output power (in 36. If the system operates with an index of 1/2, then the
watt)? efficiency of the modulator will be ________%.

32. Consider a periodic message signal with period T, 37. Consider the message signal m(t) shown in figure
shown in figure below. The message signal is applied below.
to an AM modulator.

This signal is periodic with period T, and  is chosen

such that the DC value of m(t) is zero. If the
If the modulation index of AM signal is a = 0.4, what
modulation index of AM signal is a = 1, then the
will be the percentage modulation efficiency of the
efficiency of modulation will be ________%.
modulator?

38. Consider an AM signal with 100% tone modulation

Common Data for Q.33 to 34:
and peak envelope power 32 kW. What will be the
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total transmitted power (in kW) in the modulated 41. Given the amplitude modulated signal,
signal? s(t )  Ac 1  2ka sin mt  ka cos 2mt  cos ct
where c is the carrier frequency, m and 2m are the
39. Consider a periodic message signal with period T,
modulating frequency components. What is the largest
shown in figure below.
value of amplitude sensitivity ka for which envelope
detection can be performed on s(t) to recover the
modulation signal without distortion?

42. Consider an AM signal defined as

s(t )  Ac 1  0.4sin mt  0.2sin 2mt  cos c t
where c is the carrier frequency, m and 2m are two
This message signal is applied to an AM modulator modulating signal frequency components. This signal
having 25% modulation efficiency. What will be the is transmitted through an antenna. What would be the
modulation index of the AM signal? percentage saving in power transmitted, if instead a
suppressed carrier version was sent?
40. For an AM system with modulation index a = 0.6, the
ratio of SSB signal power to the AM signal power is
________.
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Answer Key

1. (2.81) 22. (3310)

2. (12.12) 23. (71)
3. (0.68) 24. (43)
4. (12.5) 25. (0.662)
5. (2.49) 26. (7.1)
6. (0.5) 27. (10)
7. (66) 28. (5.4)
8. (11.25) 29. (1295)
9. (1) 30. (51)
10. (50) 31. (117.6)
11. (0.847) 32. (13.8)
12. (165) 33. (0.6)
13. (700) 34. (37.5)
14. (2) 35. (0.2539)
15. (10) 36. (5.97)
16. (20) 37. (16.7)
17. (20) 38. (6)
18. (0.5) 39. (1)
19. (10) 40. (0.0763)
20. (5.1) 41. (0.3333)
21. (0.2) 42. (90.9)

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