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

The correct answer is: 4) Both S1 and S2

Explanation:
S1: At the error amplifier output, we get a demodulated FM output.
True. In a Phase-Locked Loop (PLL) used for FM detection:

The error amplifier (or phase detector output) produces a voltage proportional to the phase difference between the input FM signal and the VCO signal.

Since frequency modulation (FM) is the derivative of phase modulation, this voltage directly represents the demodulated message signal.

S2: The FM signal is applied to the input of the PLL.
True. The FM signal is fed into the PLL's input (phase detector). The PLL locks onto the carrier frequency and tracks the frequency variations (modulation), allowing demodulation.

Explanation:

Carson's Rule:

Carson's Rule is a mathematical formula used to estimate the bandwidth requirements for a frequency-modulated (FM) signal. According to Carson's Rule, the total bandwidth (BT) required for an FM signal can be approximated using the following formula:

BT = 2(Δf + f_m)

Where:

• Δf = Peak frequency deviation of the carrier signal (in this case, 5 kHz)
• f_m = Maximum modulating frequency (in this case, 3 kHz)

Using the given values:

• Δf = 5 kHz
• f_m = 3 kHz

Applying these values to Carson's Rule:

BT = 2(5 kHz + 3 kHz) = 2(8 kHz) = 16 kHz

Therefore, the approximate bandwidth required for a VHF/UHF two-way radio signal using FM with a 5 kHz maximum frequency deviation and a maximum audio frequency of 3 kHz is 16 kHz.

The correct answer is: 4) Two-way mobile radio communication

Explanation:
Narrowband FM (NBFM) is characterized by:

Small frequency deviation (typically ±5 kHz or less).

Limited bandwidth (just wide enough for voice communication).

Primary Applications:

• Two-way mobile radios (e.g., police, ambulance, taxi communications).
• Aviation and marine VHF radios.
• Amateur radio (ham) transmissions.
   

Why Not the Other Options?

• Television transmission → Uses wideband FM for audio (higher fidelity) or AM for video.
• High-fidelity audio systems → Requires wideband FM (e.g., ±75 kHz deviation for FM radio).
• FM radio broadcasting → Uses wideband FM (88–108 MHz band with large deviation).

In a Phase-Locked Loop (PLL)-based FM demodulator, the phase detector is a key component that:

• Compares the input FM signal with the output of a Voltage-Controlled Oscillator (VCO).

• Produces an output voltage proportional to the phase difference between the two signals.

How it Works in FM Demodulation:

1. FM signal → fed into the phase detector.

2. Phase detector compares it with VCO output.

3. The output of the phase detector reflects the instantaneous phase difference.

4. This varying voltage corresponds to the modulating signal (since frequency variations in FM lead to phase changes).

5. After low-pass filtering, it gives the original message signal.

 Incorrect Options:

In Frequency Modulation (FM), the carrier wave's frequency varies according to the message signal. 

Explanation:

Frequency Modulation (FM):

Frequency modulation is a method of encoding information in a carrier wave by varying the instantaneous frequency of the wave. It is widely used in radio broadcasting, telecommunications, and signal processing due to its resilience to signal degradation and noise.

Applications: FM is widely used in various applications, including:

• FM radio broadcasting: FM radio stations use frequency modulation to transmit high-fidelity audio signals over the airwaves.
• Two-way radio communication: FM is commonly used in walkie-talkies, police radios, and other communication devices due to its noise immunity.
• Television audio transmission: FM is used to transmit audio signals in analog television broadcasts.
• Data transmission: FM is used in some data communication systems to achieve reliable and efficient data transfer.

Frequency Modulation:

In frequency modulation, the carrier amplitude remains constant, but its frequency is changed in accordance with the modulating signal. 

The bandwidth of a frequency modulated signal is:

BW = \(2(\Delta f+f_m)\)

Calculation:

Given, (f_m)₁ = 10 kHz

(BW)₁ = \(2\Delta f+20\)

(f_m)₂ = 20 kHz

(BW)₂ = \(2(\Delta f+20)\)

(BW)2 = \(2\Delta f+40\)

(BW)2 = (BW)1 + 20

The bandwidth is increased by 20 kHz.

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:

\(m_f=\frac{Δ f}{f_m}\)

mf = Modulation index

Δf = Frequency deviation

fm = Modulating frequency

Calculation:

Given Δf = 50 kHz

fm = 100 Hz

\(m_f=\frac{50~kHz}{100~Hz}\)

m_f = 500 radians

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 correct answer is Asymptotic to the x-axis.

Key Points

• A Frequency curve is a graph of frequency distribution where the line is smooth.
• It is just like a frequency polygon.
• In the polygon is line is straight, but in the curve the line is smooth.
• It is an area diagram.
• It is the graphical representation of frequency distribution.
• The X-axis is marked with class intervals.
• The Y-axis is marked with frequencies.
• The beginning and end of the curve should touch the last class interval at the mad posts of the first and last interval.
• The area of the curve is equal to that of a histogram.
• The frequency curve is divided into 3 types based on the shape of the curve.
  • They are Normal distribution curves.
    • Positively skewed distribution curve.
    • Negatively skewed distribution curve.

Frequency Modulation:

• Frequency Modulation is a modulation in which the frequency of the carrier wave is altered according to the instantaneous amplitude of the modulating signal, keeping phase and amplitude constant.
• So, the variation in carrier amplitude and carrier phase does not affect the signal in the receiving end.
• Line of sight (LoS) is a type of propagation that can transmit and receive data only where transmit and receive stations are in view of each other without any sort of an obstacle between them. Eg: FM radio, microwave, and satellite transmission.
• Frequency Modulation works on the Line of sight propagation.

Types of FM detection:

• Slope detection
• Phase-locked loop detection
• Foster Seeley detection
• Ratio detector
• Quadrature detectors.

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 the of FM.

Calculation:

A = 1

Hence, P = 1/2 = 0.5 W. 

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:

\(m_f=\frac{Δ f}{f_m}\)

mf = Modulation index

Δf = Frequency deviation

fm = Modulating frequency

Calculation:

Given Δf = 1000 Hz = 1 kHz

fm = 1 kHz

\(m_f=\frac{1~kHz}{1~kHz}=1\)

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.

Frequency modulated (FM) signal can generally be demodulated by the following methods:

Frequency Discrimination:

Phase Discrimination:

Phase-locked loop (PLL):

• The FM sidebands are dependent on both the level of frequency deviation and the frequency of the modulation.
• The total FM spectrum consists of the carrier, plus an infinite number of sidebands spreading out on either side of the carrier at integral multiples of the modulating frequency as shown:

• The parameters for the FM sidebands are determined by a formula using Bessel functions of the first kind.
• In FM, modulation index for wide Band FM is greater than 1 and for Narrow Band FM, it is less than 1. (option 2 is incorrect)

• B.W. required in AM is = 2fm. Hence, less bandwidth is required in case of AM and the bandwidth required in case of FM is 2(β + 1)f_m. Therefore, the bandwidth required is more in FM as compared to AM.

AGC (Automatic Gain Control):

•  Automatic gain control (AGC) works in FM radio transmitter/receiver that maintains Automatic controlling of weak and strong signals which is received by the radio receiver.
• The automatic frequency control voltage of the FM transmitter VCO is DC voltage.
• AGC maintains a constant level of the output signal based on the received signal nature, i.e. it maintains the same volume of the output when stations of different strengths are received.
• AGC adjusts the gain of RF and IF amplifiers according to need.
• AGC can handle problems like overloading and fading in the receiver.

Explanation:

Foster Seeley discriminator:

• Discriminator circuits can generate electrical output directly proportional to the frequency deviation from the unmodulated RF carrier frequency.
• The simplest circuit could be a balanced slope detector.
• It makes use of two resonant circuits; one off-tuned to one side of unmodulated RF carrier frequency and the other off-tuned to the other side of it.

• Another class of FM detectors known as Quadrature detectors uses a combination of two Quadrature signals.
• The Most commonly used FM detectors namely the foster-Seeley FM discriminator and the ratio detector operates on the principle of quadrature.

Foster Seeley discriminator is used in the demodulation of FM signal and it uses a double-tuned circuit with primary and secondary tuned to the same frequency.