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Chapter: Communication Theory : Amplitude Modulation

Important Short Questions and Answers: Amplitude Modulation

Communication Theory - Amplitude Modulation - Important Short Questions and Answers: Amplitude Modulation



1. Define modulation?


Modulation is a process by which some characteristics of high frequency carrier signal is varied in accordance with the instantaneous value of the modulating signal.


2. What are the types of analog modulation?

ü   Amplitude modulation.


ü   Angle Modulation

§       Frequency modulation

§       Phase modulation.


3. Define depth of modulation.


It is defined as the ratio between message amplitude to that of carrier amplitude. m=Em/Ec


4. What are the degrees of modulation?


·        Under modulation. m<1

·        Critical modulation m=1

·        Over modulation m>1


5. What is the need for modulation?

Ø Needs for modulation:

Ø Ease of transmission

Ø Multiplexing

Ø Reduced noise

Ø Narrow bandwidth

Ø Frequency assignment

Ø Reduce the equipments limitations


6. What are the types of AM modulators?


There are two types of AM modulators. They are

ü   Linear modulators


ü   Non-linear modulators


Linear modulators are classified as follows

·        Transistor modulator

There are three types of transistor modulator.


·        Collector modulator

·        Emitter modulator

·        Base modulator

·        Switching modulators

Non-linear modulators are classified as follows


·        Square law modulator

·        Product modulator

·        Balanced modulator


7.     What is the difference between high level and low level modulation?


In high level modulation, the modulator amplifier operates at high power levels and delivers power directly to the antenna. In low level modulation, the modulator amplifier performs modulation at relatively low power levels. The modulated signal is then amplified to high power level by class B power amplifier. The amplifier feeds power to antenna.

 8. Define Detection (or) Demodulation.


Detection is the process of extracting modulating signal from the modulated carrier. Different types of detectors are used for different types of modulations.


9. Define Amplitude Modulation.


In amplitude modulation, the amplitude of a carrier signal is varied according to variations in amplitude of modulating signal.


The AM signal can be represented mathematically as, eAM = (Ec + Em sinωmt ) sinωct and the modulation index is given as,m = Em /EC (or) Vm/Vc


10. What is Super Heterodyne Receiver?


The super heterodyne receiver converts all incoming RF frequencies to a fixed lower frequency, called intermediate frequency (IF). This IF is then amplitude and detected to get the original signal.


11. What is single tone and multi tone modulation?


If modulation is performed for a message signal with more than one frequency component then the modulation is called multi tone modulation.


If modulation is performed for a message signal with one frequency component then the modulation is called single tone modulation.


12. Compare AM with DSB-SC and SSB-SC.


13.   What are the advantages of VSB-AM?

1. It has bandwidth greater than SSB but less than DSB system.

2. Power transmission greater than DSB but less than SSB system.

3. No low frequency component lost. Hence it avoids phase distortion.

14. How will you generating DSBSC-AM?


There are two ways of generating DSBSC-AM such as


b).Ring modulators.


15. What are advantages of ring modulator? a).Its output is stable.


b). It requires no external power source to activate the diodes. c).Virtually no maintenance.

d). Long life.


16. Define Demodulation.


Demodulation or detection is the process by which modulating voltage is recovered from the modulated signal. It is the reverse process of modulation. The devices used for demodulation or detection are called demodulators or detectors. For amplitude modulation, detectors or demodulators are categorized as,


a)     Square-law detectors


b)    Envelope detectors


17. Define Multiplexing.


Multiplexing is defined as the process of transmitting several message signals Simultaneously over a single channel.

18. Define Frequency Division Multiplexing.


Frequency division multiplexing is defined as many signals are transmitted simultaneously with each signal occupying a different frequency slot within a common bandwidth.

19. Define Guard Band.


Guard Bands are introduced in the spectrum of FDM in order to avoid any interference between the adjacent channels. Wider the guard bands, Smaller the interference.

20.   Define SSB-SC.


(i)    SSB-SC stands for Single Side Band Suppressed Carrier


(ii) When only one sideband is transmitted, the modulation is referred to as Single side band modulation. It is also called as SSB or SSB-SC.


21.   Define DSB-SC.


After modulation, the process of transmitting the sidebands (USB, LSB) alone and suppressing the carrier is called as Double Side Band-Suppressed Carrier.


22.   What are the disadvantages of DSB-FC?


(i)    Power wastage takes place in DSB-FC


(ii) DSB-FC is bandwidth inefficient system.



23.   Define Coherent Detection.


During Demodulation carrier is exactly coherent or synchronized in both the frequency and phase, with the original carrier wave used to generate the DSB-SC wave.


This method of detection is called as coherent detection or synchronous detection.


24. What is Vestigial Side Band Modulation?


Vestigial Sideband Modulation is defined as a modulation in which one of the sideband is partially suppressed and the vestige of the other sideband is transmitted to compensate for that suppression.


25.   What are the advantages of signal sideband transmission?


a)     Power consumption

b)    Bandwidth conservation

c) Noise reduction


26.   What are the disadvantages of single side band transmission?


a)     Complex receivers: Single side band systems require more complex and expensive receivers thn conventiaonal AM transmission.

b)    Tuning difficulties: Single side band receivers require more complex and precise tunig than conventional AM receivers.


27. Compare linear and non-linear modulators?

Linear Modulators

1. Heavy filtering is not required.

2. These modulators are used in high level modulation.

3. The carrier voltage is very much greater than modulating signal voltage.


Non Linear Modulators

1. Heavy filtering is required.

2. These modulators are used in low level modulation.

3. The modulating signal voltage is very much greater than the carrier signal voltage.


28. What is frequency translation?


Suppose that a signal is band limited to the frequency range extending from a frequency f1 to a frequency f2. The process of frequency translation is one in which the original signal is replaced with a new signal whose spectral range extends from f1‘ and f2‘ and which new signal bears, in recoverable form the same information as was borne by the original signal.


29.   What are the two situations identified in frequency translations?


a)     Up Conversion: In this case the translated carrier frequency is greater than the incoming carrier


b)    Down Conversion: In this case the translated carrier frequency is smaller than the increasing carrier frequency.


Thus, a narrowband FM signal requires essentially the same transmission bandwidth as  the AM signal.


30. What is BW for AM wave?

 The difference between these two extreme frequencies is equal to the bandwidth of the AM wave.

 Therefore, Bandwidth, B = (fc + fm) - (fc - fm) B = 2fm


31. What is the BW of DSB-SC signal?

 Bandwidth, B = (fc + fm) - (fc - fm) B = 2f

 It is obvious that the bandwidth of DSB-SC modulation is same as that of general AM waves.


32. What are the demodulation methods for DSB-SC signals?

The DSB-SC signal may be demodulated by following two methods:

(i) Synchronous detection method.

(ii)Using envelope detector after carrier reinsertion.


33.   Write the applications of Hilbert transform?


(i)    For generation of SSB signals,

(ii) For designing of minimum phase type filters,

(iii) For representation of band pass signals.


34.   What are the methods for generating SSB-SC signal?


SSB-SC signals may be generated by two methods as under:

(i)Frequency discrimination method or filter method.

(ii)Phase discrimination method or phase-shift method.




1. Amplitude modulation: The modulation of a wave by varying its amplitude, used especially as a means of broadcasting an audio signal by combining it with a radio carrier wave.


2. The modulation index: (modulation depth) of a modulation scheme describes by how much the modulated variable of the carrier signal varies around its unmodulated level.


3. NarrowbandFM: If the modulation index of FM is kept under 1, then the FM produced is regarded as narrow band FM.


4. Frequency modulation (FM): the encoding of information in a carrier wave by varying the instantaneous frequency of the wave.


5. Amplication: The level is carefully chosen so that it does not overload the mixer when strong signals are present, but enables the signals to be amplified sufficiently to ensure a good signal to noise ratio is achieved.


6. Modulation: The process by which some of the characteristics of carrier wave is varied in accordance with the message signal.

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