# MCQ on Amplitude Modulation System Posted by

## MCQ on Amplitude Modulation System

MCQ on Amplitude Modulation System, Objective Questions on Amplitude Modulation System, Amplitude Modulation System MCQ, GATE MCQ on Amplitude Modulation System, Multiple-Choice Questions on Amplitude Modulation System, Engineering MCQ, Communication Systems MCQ

### Multiple-Choice Questions

1. The positive RF peaks of an AM voltage rise to a maximum value of 12 V and drop to a minimum value of 4 V. The modulation index assuming single tone modulation is
2. 3
3. 1/3
4. 1/4
5. ½

2. The most suitable method for detecting a modulated signal $(2.5+5\cos \omega _{m}t)\cos \omega _{c}t$ is

• envelope detector
• synchronous detector
• ratio detector
• both envelope detector and synchronous detector

• simple circuit
• better tracking
• improvement in selectivity and sensitivity
• better alignment

Answer: improvement in selectivity and sensitivity

4. The received signal frequency of a superheterodyne receiver having IF = 456 kHz, is 1MHz. The corresponding image signal is

• within its medium band
• outside the medium band
• depends on the modulation index
• depends on modulating frequency

5. The resonant frequency of an RF amplifier is 1 MHz and its bandwidth is 10 kHz. The Q-factor will be

• 10
• 100
• 0.01
• 0.1

6. The plot of modulation index versus carrier amplitude yields a

• horizontal line
• vertical line
• parabola
• hyperbola

7. A carrier is amplitude modulation to a depth of 40%. The increase in power is

• 40%
• 20%
• 16%
• 8%

8. Following is not the purpose of modulation

• multiplexing
• narrow-banding
• increase in signal power

9. An AM wave is given by eAM = 10 (1 + 0.4 cos 103 t + 0.3 cos 104 t) cos 106 t. The modulation index of the envelope is

• 0.4
• 0.5
• 0.3
• 0.9

10. A DSB-SC signal can be demodulated using

• a low pass filter
• asynchronous detector
• a phase shift discriminator
• an envelope detector

11. A product modulator yields

• a full AM signal
• a DSB-SC signal
• a VSB signal
• an SSB signal

12. An FM signal can be detected by using

• an LPF
• a BPF
• a discriminator
• an average detector

13. The image channel rejection in a superheterodyne receiver comes from

• IF stage only
• RF stage only
• detector and RF stage only
• the detector, RF, and IF stages

14. The modulation index of an AM wave is changed from 0 to 1. The transmitted power is (c)

• unchanged
• halved
• increased by 50%

15. A modulated signal is given by, $s(t)=m_{1}(t)\cos (2\pi f_{c}t)+m_{2}(t)\sin (2\pi f_{c}t)$ where the baseband signals ml(t) and m2(t) have bandwidths of 10 kHz and 15 kHz, respectively. The bandwidth of the modulated signal, in kHz, is

• 10
• 15
• 25
• 30

16. The Hilbert transform of $\cos \omega _{1}t+\sin \omega _{2}t$ is

• $\sin \omega _{1}t-\cos \omega _{2}t$
• $\sin \omega _{1}t+\cos \omega _{2}t$
• $\cos \omega _{1}t-\sin \omega _{2}t$
• $\sin \omega _{1}t+\sin \omega _{2}t$

Answer: $\sin \omega _{1}t-\cos \omega _{2}t$

17. A 1 MHz sinusoidal carrier is amplitude modulated by a symmetrical square wave of period 100 µs. Which of the following frequencies will NOT be present in the modulated signal?

• 990 kHz
• 1010 kHz
• 1020 kHz
• 1030 kHz

18. The input to a coherent detector is DSB-SC signal plus noise. The noise at the detector output is

• the in-phase component
• zero
• the envelope

19. The noise at the input to an ideal frequency detector is white. The detector is operating above  threshold. The power spectral density of the noise at the output is

• raised cosine
• flat
• parabolic
• Gaussian

20. An AM signal is detected using an envelope detector. The carrier frequency and modulating signal frequencies are 1 MHz and 2 kHz respectively. An appropriate value for the time constant of the envelope detector is

• 500 µs
• 20 µs
• 0.2 µs
• 1 µs

21. Which of the following analog modulation scheme requires the minimum transmitted power and, minimum channel bandwidth?

• VSB
• DSB-SC
• SSB
• AM

22. The diagonal clipping in amplitude demodulation (using an envelope detector) can be avoided if RC time-constant of the envelope detector satisfies the following condition, (here W is the message bandwidth and ω is the carrier frequency, both in rad/s)

• RC < 1/W
• RC > 1/W
• RC < 1/ω
• RC > 1/ω

23. The Hilbert transform is a

• non-linear system
• non-causal system
• time-varying system
• low-pass system

24. For a message signal $m(t)=\cos (2\pi f_{m}t)$ and carrier of frequency fc, which of the following represents a single sideband (SSB) signal?

• ($\cos (2\pi f_{m}t)\cos (2\pi f_{c}t)$
• $\cos (2\pi f_{c}t)$
• $\cos [2\pi (f_{c}+f_{m})t]$
• $[1+\cos 2\pi f_{m}t]\cos (2\pi f_{c}t)$

Answer: $\cos [2\pi (f_{c}+f_{m})t]$

25. suppose that the modulating signal is latex]m(t)=2\cos (2\pi f_{m}t)[/latex]   and the carrier signal is $x_{c}(t)=A_{c}\cos (2\pi f_{c}t)$. Which one of the following is a conventional AM signal without over-modulation?

• $x(t)=A_{c}m(t)\cos (2\pi f_{c}t)$
• $x(t)=A_{c}[1+m(t)]\cos (2\pi f_{c}t)$
• $x(t)=A_{c}\cos (2\pi f_{c}t )+\frac{A_{c}}{4}m(t)\cos (2\pi f_{c}t)$
• $x(t)=A_{c}\cos (2\pi f_{m}t)\cos (2\pi f_{c}t)+A_{c}\sin (2\pi f_{m}t)\sin (2\pi f_{c}t)$

Answer: $x(t)=A_{c}[1+m(t)]\cos (2\pi f_{c}t)$

26. A 10 kW carrier is sinusoidally modulated by two modulating signals corresponding to a modulation index of 30% and 40% respectively. The total radiated power is

• 11.25 kW
• 12.5 kW
• 15 kW
• 17 kW

27. In an amplitude modulated system, if the total power is 600 W and the power in carrier is 400 W, then the modulation index is

• 0.5
• 0.75
• 0.9
• 1

28. For an AM wave, the maximum voltage was found to be 10 V and the minimum voltage was found to be 5 V. The modulation index of the wave would be

• 0.33
• 0.52
• 0.40
• 0.1

29. In FDM systems used for telephone, which modulation scheme is adopted?

• AM
• DSB-SC
• SSB-SC
• FM