Pulse Analog Modulation MCQ Quiz - Objective Question with Answer for Pulse Analog Modulation - Download Free PDF

Explanation:

Wide Band Frequency Modulation (WBFM) is a type of frequency modulation used in FM broadcasting where the frequency of the carrier wave is varied in proportion to the instantaneous amplitude of the modulating signal. WBFM is characterized by a relatively large frequency deviation, which is essential for high-quality audio transmission.

The typical maximum frequency deviation for Wide Band FM used in FM broadcasting is 75 kHz. This deviation is standardized and widely adopted in FM broadcasting systems to ensure high fidelity in audio transmission, making it the correct answer (Option 3).

Technical Details:

• In FM broadcasting, the maximum frequency deviation of 75 kHz is paired with a standard baseband bandwidth of 15 kHz (the range of frequencies in the audio signal).
• The Carson's Rule, which estimates the bandwidth required for FM signals, is given as:
  B = 2 × (Δf + fm)
  Where:
  • B: Total bandwidth
  • Δf: Maximum frequency deviation (75 kHz)
  • fm: Maximum modulating frequency (15 kHz)
  Substituting the values:
  B = 2 × (75 kHz + 15 kHz) = 180 kHz
  Thus, the FM signal occupies a bandwidth of approximately 180 kHz.
• The 75 kHz deviation ensures sufficient dynamic range and minimizes distortion in audio signals.
• FM broadcasting systems are designed to operate within the 88 MHz to 108 MHz frequency band, with each station allocated a bandwidth of 200 kHz. The 75 kHz deviation fits well within this allocation while leaving room for guard bands to prevent interference between adjacent channels.
   

Correct Option Analysis:

The correct option is:

Option 3: 75 kHz

This is the standard maximum frequency deviation used in Wide Band FM for FM broadcasting. It provides an optimal balance between audio quality and efficient use of the frequency spectrum, making it the correct answer

The correct statements regarding Frequency Modulation (FM) and Amplitude Modulation (AM) are a, b, and c.

Explanation:

a) FM provides better noise immunity.

• True. FM encodes information in frequency variations, making it less susceptible to amplitude noise (e.g., interference, static). AM, which relies on amplitude changes, is highly vulnerable to noise.

b) AM requires lower bandwidth than FM.

• True. AM bandwidth is twice the maximum modulating frequency (e.g., 10 kHz for a 5 kHz signal). 

c) FM utilizes transmitted power more efficiently.

• True. FM maintains a constant envelope (amplitude), allowing all transmitted power to carry information. In AM, up to ⅔ of power is wasted in the carrier (non-information-bearing), reducing efficiency.

Additional Information

Calculation:

The total modulation index is the square root of the sum of the squares of the individual modulation indices. If the modulation indices for the two waves are 0.3 and 0.4, then the total modulation index is:

m_total = √(0.3² + 0.4²)

m_total = √(0.09 + 0.16) = √0.25 = 0.5

Therefore, the total modulation index is 0.5.

Concept

The total transmitted power for an AM system is given by:

\(P=P_c({1+{μ\over 2}^2})\)

where, P_c = Carrier Power

μ = Modulation index

Calculation

Given, Pc = 10 kW

μ = 0.6

\(10=P_c({1+{0.6\over 2}^2})\)

P_c = 8.47 kW

Concept:

PAM (Pulse Amplitude Modulation):

• It is a technique in which the amplitude of each pulse is controlled by the instantaneous amplitude of the modulation signal.
•  It is a modulation system in which the signal is sampled at regular intervals and each sample is made proportional to the amplitude of the signal at the instant of sampling. 

• The demodulation of the PAM signals is done by using a Low-pass filter as shown:

In TV transmission:

• A video is Vestigial sideband modulated, which is a type of amplitude modulated waveform
• The Video signals are thus encoded in amplitude variations of the carriers
• The Audio signal is encoded in FM waveform
• Thus, the audio signals are encoded as frequency variations of the carrier

Noise is the signal that affects amplitude majorly. Thus, the video signal is distorted from amplitude variations.

Advantages of FM over AM are:

• Improved signal to noise ratio.
• Smaller geographical interference between neighbouring stations.
• Less radiated power.
• Well defined service areas for given transmitter power.

Disadvantages of FM:

• Much more Bandwidth
• More complicated receiver and transmitter.

The AM, FM and PM output waveforms are as shown:

In Frequency modulation, the frequency of the carrier is varied according to the amplitude of the message signal and the amplitude of the carrier remains constant.

• The Noise affects the amplitude of the modulated signal
• In the Amplitude Modulated system, the message is contained in the amplitude modulations of the carrier; hence the introduction of noise distorts the amplitude and hence the message contained in the signal
• In a frequency modulated system the message is contained in the frequency variations of the carrier, therefore, the introduction of noise does not affect the message contained in the message signal
• Since the amplitude of FM remains constant, the noise can be eliminated using an amplitude limiter at the demodulator.

• Analog modulation is a process in which analog low-frequency baseband signals (like an audio or TV signal) are transmitted over a larger distance without getting faded away, by superimposing over a higher frequency carrier signal such as a radio frequency band.
• Different modulation techniques are explained with the help of the following block diagram:

• If the amplitude of a pulse or duration of a pulse is varied according to the instantaneous values of the baseband modulating signal, then such a technique is called as Pulse Amplitude Modulation (PAM).

Concept:

The total transmitted power for an AM system is given by:

\({P_t} = {P_c}\left( {1 + \frac{{{μ^2}}}{2}} \right)\)

Pc = Carrier Power

μ = Modulation Index

Calculation:

Given P_t = 600 W, P_c = 400 W

Putting these values in the total power of AM wave expression, we get:

\(600 = 400\left( {1 + \frac{{{μ^2}}}{2}} \right)\)

\(\frac{6}{4}=\left( {1 + \frac{{{μ^2}}}{2}} \right)\)

\(\frac{6}{4}=\left( {1 + \frac{{{μ^2}}}{2}} \right)\)

\(\frac{μ^2}{2}=\frac{1}{2}\)

μ = 1.00

• T.V. picture transmission uses vestigial sideband modulation which is a type of amplitude modulation
• The vestigial sideband has the advantage of reducing the bandwidth
• The T.V. audio signal uses FM modulation

Concept:

• Frequency Demodulators are often sensitive to amplitude variations. Therefore a limiter amplifier stage must be used before the detector, to remove amplitude variations in the signal which would be detected as noise.
• The limiter acts as a Class-A amplifier at lower amplitudes; at higher amplitudes, it becomes a saturated amplifier that clips off the peaks and limits the amplitude.
• The FM ratio detector was the more common because it offered a better level of amplitude modulation rejection of amplitude modulation.
• This enabled the circuit to provide a greater level of noise immunity as most noise is amplitude noise.
• Additional limiter circuit is not required since the output voltage is not affected by amplitude variations in the incoming signals. 

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It also enables the FM detector to operate more effectively even with lower levels of limiting in the preceding IF stages of the receiver.

Effective modulation index will be:

\({\rm{\mu }} = \sqrt {{\rm{\mu }}_1^2 + {\rm{\mu }}_2^2}\)

Given \({{\rm{\mu }}_1} = 0.3\) and \({{\rm{\mu }}_2} = 0.4\)

The overall modulation index will be:

\({\rm{\mu }} = \sqrt {0.09 + 0.16} = \sqrt {0.25} = 0.5\)

• Fidelity in communications refers to the ability of the receiver to produce an exact replica of transmitted signal. It is provided at output stage of receiver i.e. audio stage.
• Fidelity of a receiver is its ability to reproduce the exact replica of the transmitted signals at the receiver output.
• In other words, the fidelity of a receiver is its ability to accurately reproduce, in its output, the signal that appears at its input. 
• For better fidelity, the amplifier must pass high bandwidth signals to amplify the frequencies of the outermost sidebands, while for better selectivity the signal should have narrow bandwidth.
• Low frequency response of IF amplifier determines fidelity at the lower modulating frequencies while high frequency response of the IF amplifier determines fidelity at the higher modulating frequencies.