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Provide very Short Answers
1. Name the circuit that causes one signal to modulate another, and give the names of the two
signals applied to this circuit
2. In AM, how does the carrier vary in accordance with the information signal?
3. True or false? The carrier frequency is usually lower than the modulating frequency
4. True or false? The carrier frequency remains constant during AM.
5. What mathematical operation does an amplitude modulator perform?
6. What is the ideal relationship between the modulating signal voltage Vm and the carrier voltage
Vc?
7. What is the modulation index called when it is expressed as a percentage?
8. Explain the effects of a modulation percentage greater than 100.
9. What is the name given to the new signals generated by the modulation process?
10. Does the carrier of an AM signal contain any information? Explain.
11. What is the name of a signal that has both sidebands but no carrier?
12. What is the name of the circuit used to eliminate the carrier in DSB/SSB transmissions?
13. What is the minimum bandwidth AM signal that can be transmitted and still convey all the
necessary intelligence?
14. State the four main benefits of SSB over conventional AM.
15. Name the type of AM used in TV picture transmission. Why is it used?
16. Define depth of modulation (or) modulation index.
17. Define Modulation
18. What are the degrees of modulation?
19. If a 10KW amplitude modulated transmitter is modulated sinusoidally by 50%, what is the
total RF power delivered?
20. What is the power saving in DSB-SC-AM and SSB-SC AM?
21. What are the types of AM detectors?
22. Compare linear and non-linear modulators.
23. What are the advantages of DSB-SC and SSB-SC?
25. Compare AM with DSB-SC and SSB-SC. Bandwidth power bands
AM signal
DSB-SC
SSB-SC
26. A transmitter supplies 8 Kw to the antenna when modulated. Determine the total power
radiated when modulated to 30%.
27. The antenna current of an AM transmitter is 8A when only carrier is sent. It increases to 8.93A
when the carrier is modulated by a single sine wave. Find the percentage modulation.
24.
28. A 1MHz carrier is amplitude modulated by 400Hz modulating signal to a depth of 50%.The
modulated carrier power is 1KW.Calculate the power of the unmodulated signal. Solution:-
An AM wave displayed on an oscilloscope has values of Vmax = 4.8 and Vmin = 2.5 as read from the
graticule. What is the percentage of modulation?
To achieve 75 percent modulation of a carrier of Vc = 50 V, what amplitude of the modulating signal Vm
is needed?
The maximum peak-to-peak value of an AM wave is 45 V. The peak-to-peak value of the modulating
signal is 20 V. What is the percentage of modulation?
An AM radio transmitter operating on 3.9 MHz is modulated by frequencies up to 4 kHz. What are the
maximum upper and lower side frequencies? What is the total bandwidth of the AM signal?
How much power appears in one sideband of an AM signal of a 5-kW transmitter modulated by 80
percent?
What is the total power supplied by an AM transmitter with a carrier power of 2500 W and modulation
of 77 percent?
An AM signal has a 12-W carrier and 1.5 W in each sideband. What is the percentage of modulation?
An AM transmitter puts a carrier of 6 A into an antenna whose resistance is 52 ohm . The transmitter is
modulated by 60 percent. What is the total output power
The antenna current produced by an unmodulated carrier is 2.4 A into an antenna with a resistance of
75 V. When amplitude-modulated, the antenna current rises to 2.7 A. What is the percentage of
modulation?
An SSB transmitter has a power supply voltage of 250 V. On voice peaks, the final amplifier draws a
current of 3.3 A. What is the input PEP?
The peak-to-peak output voltage of 675 V appears across a 52-Ohm antenna on voice peaks in an SSB
transmitter. What is the output PEP?
What is the average output power of an SSB transmitter rated at 100-W PEP?
An SSB transmitter with a carrier of 2.3 MHz is modulated by an intelligence signal in the 150-Hz to 4.2kHz range. Calculate the frequency range of the lower sideband.
1. An AM signal is represented by x(t) = (20+4sin500πt)cos(2πx105t)V. The modulation index is
(A) 20
(B) 4
(C) 0.2
(D) 10
2. An AM signal is represented by x(t) = (20+4sin500πt)cos(2πx105t)V. The total signal power is
(A) 208 W
(B) 204 W
(C) 408 W
(D) 416 W
3. An AM signal is represented by x(t) = (20+4sin500πt)cos(2πx105t)V. The total sideband power is
(A) 4 W
(B) 8 W
(C) 16 W
(D) 2 W
4. A 2 kW carrier is to be modulated to a 90% level. The total transmitted power would be
(A) 3.62 kW
(B) 2.81 kW
(C) 1.4 kW
(D) None of the above
5. An AM broadcast station operates at its maximum allowed total output of 50 kW with
80% modulation. The power in the intelligence part is
(A) 12.12 kW
(B) 31.12 kW
(C) 6.42 kW
(D) None of the above
6. A 2 MHz carrier is amplitude modulated by a 500Hz modulating signal to a depth of
70%. If the unmodulated carrier power is 2 kW, the power of the modulated signal is
(A) 2.23 kW
(B) 2.36 kW
(C) 1.18 kW
(D) 1.26 kW
7. In a DSB-SC system with 100% modulation, the power saving is
(A) 50%
(B) 66%
(C) 75%
(D) 100%
8. In amplitude modulation, the modulation envelope has a peak value which is double the
unmodulated carrier value. What is the value of the modulation index?
(A) 25%
(B) 50%
(C) 75%
(D) 100%
9. If the modulation index of an AM wave is changed from 0 to 1, the transmitted power
(A) increases by 50%
(B) increases by 75%
(C) increases by 100%
(D) remains unaffected
10. A diode detector has a load of 1kΩ shunted by a 10000 pF capacitor. The diode has a forward
resistance of 1Ω. The maximum permissible depth of modulation, so as to avoid diagonal clipping,
with modulating signal frequency of 10 kHz will be
(A) 0.847
(B) 0.628
(C) 0.734
(D) None of the above
11. An AM signal is detected using an envelope detector. The carrier frequency and modulating signal
frequency are 1 MHz and 2 kHz respectively. An appropriate value for the time constant of the
envelope detector is.
(A) 500 μ sec
(B) 20 μ sec
(C) 0.2 μ sec
(D) 1 μ sec
12. For an AM signal, the bandwidth is 10 kHz and the highest frequency component
present is 705 kHz. The carrier frequency used for this AM signal is
(A) 695 kHz
(B) 700 kHz
(C) 705 kHz
(D) 710 kHz
13. In a broadcast transmitter, the RF output is represented as e(t) =
50[1+0.89cos5000t+0.30sin9000t]cos(6x106t)V. What are the sidebands of the signals in radians?
(A) 5x103 and 9x103
(B)
5.991x106,
5.995x106,
6.005x106
&
6.009x106
(C) 4x103, 1.4x104
(D) 1x106, 1.1x107, 3x106 and 1.5x107
14. An AM modulator has output x(t) = 40cos400πt + 4cos360πt + 4cos440πt. The modulation
efficiency is
(A) 0.01
(B) 0.02
(C) 0.03
(D) 0.04
15. An AM modulator has output x(t) = Acos400πt + Bcos380πt + Bcos420πt. The carrier power is 100
W and the efficiency is 40%. The value of A and B are
(A) 14.14, 8.16
(B) 50, 10
(C) 22.36, 13.46
(D) None of the above
16. A super heterodyne receiver is designed to receive transmitted signals between 5 and 10 MHz.
High-side tuning is to be used. The tuning range of the local oscillator for IF frequency 500 kHz would
be
(A) 4.5 MHz - 9.5 MHz
(B) 5.5 MHz - 10.5 MHz
(C) 4.5 MHz - 10.5 MHz
(D) None of the above