Pulse Analog Modulation MCQ Quiz in తెలుగు - Objective Question with Answer for Pulse Analog Modulation - ముఫ్త్ [PDF] డౌన్‌లోడ్ కరెన్

• The main purpose of modulation is to transmitting low-frequency information over long distance efficiently
• A low-frequency signal requires a large antenna for efficient transmission
• A low-frequency signal is easily affected by Noise
• Modulation helps to reduce the size of the antenna and transmit the message signal efficiently

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 μ = 1 and P_t = P

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

\({P} = {P_c}\left( {\frac{{{3}}}{2}} \right)\)

\({P_c} = \frac{2}{3}P\)

Concept:

The transmission efficiency of an AM signal is given by the expression:

\(\eta = \frac{{{μ ^2}}}{{2 + {μ ^2}}} \times 100\)

μ = Modulation index

Calculation:

With μ = 0.75, the transmission efficiency will be:

\(\eta = \frac{{{{\left( {0.75} \right)}^2} \times 100}}{{2 + {{\left( {0.75} \right)}^2}}} \)

\(\mu= 21.9\% \approx 22\%\)

Derivation:

The transmission efficiency of an AM wave is defined as the percentage of total power contributed by the sidebands.

Mathematically it is expressed as:

\(\eta = \frac{{{P_\;}_{SB}}}{{{P_t}}} \times 100\%\)

For sinusoidal input

P_SB = Sideband power given by:

\({P_{SB}} = \frac{{{P_c}\;{μ ^2}}}{2}\)

P_t = Total power given by:

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

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

\(\eta = \frac{{{μ ^2}}}{{2 + {μ ^2}}} \times 100\)

PWM

The amplitude and position of pulses are constant in modulated signal (PWM), but the width (or duration) of pulses varies proportionally with the amplitude of an analog useful signal.

The carrier signal is produced by a clock.

PPM

The amplitude and width of pulses are constant in a modulated signal (PPM), but the direction of pulses varies proportionally with the amplitude of analogical useful signal.

PAM

The width and location of pulses in a modulated signal (PAM) are constant, while the amplitude of pulses varies proportionally with the amplitude of an analogical useful signal.

Hence from the above option (4) is correct

• Pulse Repetition Frequency (PRF) of the radar system is defined as the number of pulses that are transmitted per second.
• It is normally measured in pulses per second.
• Radar systems radiate each pulse at the carrier frequency during transmit time (or Pulse Width PW), wait for returning echoes during listening or rest time, and then radiate the next pulse.
• The time between the beginning of one pulse and the start of the next pulse is called pulse-repetition time (PRT) and is equal to the reciprocal of PRF.

• High PRF  pulse Doppler radar has no ambiguity in Doppler frequency but suffers from range ambiguities.
• A high PRF pulse Doppler radar provides an accurate measurement of target's radial velocity.
• In high PRF, It becomes increasingly difficult to take multiple samples between transmit pulses at these pulse frequencies, so range measurements are limited to short distances.

In FM broadcasting, pre-emphasis improvement is the improvement in the signal-to-noise ratio of the high-frequency portion of the baseband.

In the process of detecting a frequency modulated signal, the receiver produces a noise spectrum that rises in frequency (a so-called triangular spectrum). This is the reason why a preemphasis is required.

Without pre-emphasis, the received audio would sound unacceptably noisy at high frequencies, especially under conditions of low carrier-to-noise ratio, i.e., during fringe reception conditions.

Preemphasis increases the magnitude of the higher signal frequencies, thereby improving the signal-to-noise ratio.

De-emphasis:

At the output of the discriminator in the FM receiver, a deemphasis network restores the original signal power distribution.

De-emphasis means attenuating those frequencies by the same amount by which they are boosted by the pre-emphasis.

Concept:

For an amplitude modulated system, the modulation index is defined as:

\(\mu=\frac{A_m}{A_c}\)

A_m = Message signal amplitude

A_c = Carrier Amplitude

Observation: Since the modulation index is related to the carrier amplitude through an inverse relationship, we conclude that the plot of modulation index versus carrier amplitude will result in a hyperbola.

A wave has 3 parameters Amplitude, Phase, and Frequency. Thus there are 3 types of modulation techniques.

Amplitude Modulation: The amplitude of the carrier is varied according to the amplitude of the message signal.

Frequency Modulation: The frequency of the carrier is varied according to the amplitude of the message signal.

Phase Modulation: The Phase of the carrier is varied according to the amplitude of the message signal.

The transmitted power is given by:

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

μ = 0.5

\(\begin{array}{l} {P_t} = {P_c}\left( {1 + \frac{{{\mu^2}}}{2}} \right)\\ {P_t} = {P_c}\left( {1 + \frac{{0.25}}{2}} \right) \end{array}\)

\( {P_t} = {P_c}\left( {1 + \frac{{0.25}}{2}} \right) \)

\(P_t= {P_c}\left( {\frac{2.25}{2}} \right) \)

\(P_c=\frac{2}{2.25}P_t\)

For both frequency and phase modulation, an increase in modulation index results in an increase in transmission bandwidth.

For AM

the Modulation index is given by 

\(\mu = \frac{{{A_m}}}{{{A_c}}}\)

Bandwidth = 2 f_m which is independent of modulation index

For PM

the Modulation index is given by 

\(\beta = {k_p}{A_m}\)

Bandwidth = 2 (1 +β ) fm 

bandwidth depends on modulation index β 

For FM,

modulation index

 \({\rm{\beta }} = \frac{{{{\rm{k}}_{\rm{f}}}{{\rm{A}}_{\rm{m}}}}}{{{{\rm{f}}_{\rm{m}}}}}\).

Bandwidth = 2 (1 +β ) fm 

bandwidth depends on modulation index β 

• Double spotting in a superheterodyne receiver means that the wanted station is tuned in at two spots on the dial.
• This results in an undesired input frequency to be demodulated by the superheterodyne receivers along with the desired incoming signal. This results in two stations being received at the same time, thus producing interference.
• This is mainly because of poor front-end selectivity of the RF stage, i.e. due to insufficient adjacent channel rejection by the front-end RF stage.
• The concept is understood with the help of the following diagram:

Image frequency is given by fsi = fs + 2If

Where fsi = image frequency

fs = incoming signal

If = intermediate frequency