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

Concept:

Modulation: The process of varying some characteristics such as amplitude, frequency, or phase of a carrier signal is varied in accordance with the instantaneous value of the low-frequency audio signal(or modulating signal) is known as Modulation. In other words, in modulation, we use a high-frequency carrier wave to carry audio signals(message) to large distances. Audio signals have low frequencies with a bandwidth of about 20 kHz only. Such low signals cannot be transmitted directly to large distances. That is why we need modulation of these audio signals.

Frequency Modulation: The process of changing the frequency of a message signal in accordance with the amplitude of a modulating signal voltage is known as  Frequency modulation. The frequency of modulated signal increases as the amplitude of modulating signal increases and decreases as the amplitude of modulating signal decreases. Some other features of Frequency modulation are-

• The amplitude of the message signal does not change in FM.
• The amount of frequency deviation is proportional to the amplitude of instantaneous value of modulating signal.
• FM transmission is highly efficient.

Advantages of FM:

1. It is highly efficient.
2. Noise interference is low.
3. High fidelity reception.

Disadvantages of FM:

1. A wider channel is required for FM.
2. Components are costly.
3. It is limited to line of sight.

Explanation:

FM is very efficient, produces less noise, and all the power transmitted is useful. So maximum power transmitted is useful. All these are the advantages of FM. But the receivers and transmitters are very complex and costly.

The correct answer is option (4)Additional Information

Frequency Deviation: The amount by which the frequency of carrier signal shifts above or below the resting frequency fc is called the frequency deviation.

Concept:

According to Carson rule signal, BW is given as:

B.W = (β + 1) 2fm

\(\becauseβ=\frac{Δf}{f_m} \)

B.W. = 2[Δf + fm]          

If multiple frequencies are available in modulating signal then,

B.W. = 2(β + 1) fmax 

Calculation:

Given: 

Δf = 10 kHz

f_m = 2 kHz

B.W. = 2[Δf + fm]   

B.W. = 2(10 + 2)

BW = 24 kHz

Hence option (3) is the correct answer.

Analysis:

The amplitude Ac is constant in a phase-modulated or a frequency modulated signal. RF power does not depend upon the modulation index.

A general expression for a phase or frequency modulated signal is:

\(\phi(t)=A_ccos[ω_ct+g(k_k,m(t))]\)

m(t) = the modulating signal

ωc = Carrier frequency

kk becomes kc for FM and kp for PM.

The average power (Pavg) is given by:

\(P_{avg}=\frac{A_c^2}{2}\) (Always)

We observe that the transmitted power is independent of the modulation index in case of FM.

Transmitted power in AM is given by:

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

P_c = Carrier Power

∴ We conclude that If the modulation index increases, transmitted power increases in case of AM and is unchanged for FM.

Concept:

In FM (Frequency Modulation), the modulation index is defined as the ratio of frequency deviation to the modulating frequency.

Mathematically, this is defined as:

\(m_f=\frac{Δ f}{f_m}\)

m_f = Modulation index

Δf = Frequency deviation

f_m = Modulating frequency

Calculation:

Given Δf = 50 kHz

f_m = 4 kHz

\(m_f=\frac{50~kHz}{4~kHz}=12.5\)

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.

Advantages of FM over AM are:

• Improved signal to noise ratio (about 25dB) w.r.t. to manmade interference.
• Smaller geographical interference between neighboring stations.
• Less radiated power.
• Well-defined service areas for given transmitter power.

Disadvantages of FM:

• Much more Bandwidth (as much as 20 times as much).
• More complicated receiver and transmitter.

Explanation:

Very High Frequency:

It has a range of  30 MHz to 300 MHz and are used in television broadcast.

Some ultra high frequencies are also used in television broadcasting.

Additional Information

The frequency spectrum for the complete range is:

Hence option (3) is the correct answer.

In TV Transmission FM is used for sound transmission and AM for video/picture transmission.

Amplitude Modulation (AM) is preferred for picture transmission in TV because of the following reasons:

• The distortion which arises due to interference between multiple signals is more in FM than AM because the frequency of the FM signal continuously changes. Steady production of the picture is affected because of this.
• If AM were used, the ghost image, if produced is steady.
• Also, the circuit complexity and bandwidth requirements are much less in AM than in FM.

On the other hand, FM is preferred for sound because of the following reasons:

• The bandwidth assigned to the FM sound signal is about 200 kHz, of which not more than 100 kHz is occupied by significant sidebands.
• This is only 1.4 % of the total channel bandwidth of 7 MHz. This results in efficient utilization of the channel.

Important Notes:

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.

• A phase-locked loop is a control system that generates an output signal whose phase is related to the phase of an input signal.
• The circuit can track an input frequency or it can generate a frequency that is a multiple of the input frequency.
• The basic elements of a PLL circuit are phase comparator/detector, a loop filter, voltage controlled oscillator (VCO).

The figure shows a schematic of a phase-locked loop:

Concept:

Modulation Index (β) in FM is given by:

\(\beta = \frac{{Frequency\;deviation}}{{Message\;frequency}} = \frac{{{k_f}{A_m}}}{{{f_m}}}\)

k_f = frequency sensitivity

A_m = modulating voltage

Application:

Given:

f_m = 2 f_m, A_m’ = A_m

So,

\(\beta ' = \frac{{{k_f}A_m'}}{{f_m'}} = \frac{{{k_f}{A_m}}}{{2{f_m}}}\)

∴ We can write:

\(\beta ' = \frac{\beta }{2}\)

Hence, the given modulating system is FM.

Note:

In AM modulation Index is defined as:

\(\mu = \frac{{{A_m}}}{{{A_c}}} = \frac{{{A_{max}} - {A_{min}}}}{{{A_{max}} + {A_{min}}}}\)

Clearly, it is independent of f_m.

∴ it will not fulfill the given criteria.

In PM, the modulation index is defined as:

β = kp A_m

kp = phase sensitivity

It will also not fulfill the given criteria.