1. In FM modulation, when the modulation index increases, the transmitted power is

Increased

Decreased

Unchanged

None of these

Answer.3. Unchanged

Explanation

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:

φ(t) = A_{c}Cos[ω_{c}t + (gK_{k}, m(t))]

m(t) = the modulating signal

ωc = Carrier frequency

k_{k} becomes kc for FM and kp for PM.

The average power (P_{avg}) is given by:

P_{avg} = A^{2}c/2

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

2. Reactance modulator and varactor diode are used in the _____ method.

DSB –SC generation

DSB –SC reception

SSB generation

FM generation

Answer.4. FM generation

Explanation

Reactance modulators are used for the generation of frequency modulation (FM)

These reactance modulators do not have very good frequency stability. But if it is to be used for broadcast applications, then it is must-have high-frequency stability. It can be achieved through a frequency stabilized reactance modulator.

The stability of the whole circuit depends on the stability of the discriminator

As the discriminator is turned to a frequency that is one-twentieth of the drift in the master oscillator frequency. The frequency drift is reduced to a great extent.

3. FM is a part of general class of modulation known as ______

Angle modulation

Phase modulation

Amplitude modulation

Frequency modulation

Answer.1. Angle modulation

Explanation

Angle modulation is a class of carrier modulation that is used in telecommunications transmission systems.

The class comprises frequency modulation (FM) and phase modulation (PM) and is based on altering the frequency or the phase, respectively, of a carrier signal to encode the message signal.

Angle modulation varies a sinusoidal carrier signal in such a way that the angle of the carrier is varied according to the amplitude of the modulating baseband signal.

4. Foster Seeley discriminator uses a

Single tuned circuit

Double tuned circuit with primary and secondary tuned to the different frequency

Double tuned circuit with primary and secondary tuned to the same frequency

None of these

Answer.3. Double tuned circuit with primary and secondary tuned to the same frequency

Explanation

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.

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.

Foster Seeley makes use of two resonant circuits i.e Double tuned circuit one off-tuned to one side of unmodulated RF carrier frequency and the other off-tuned to the other side of it.

5. A 20 MHz carrier is modulated by a sinusoid of 1 kHz to produce a peak frequency deviation of 100 kHz. What is the approximate bandwidth? If the modulating signal is changed to 800 kHz, what is the bandwidth?

2 kHz, 1600 kHz

202 kHz, 1600 kHz

202 kHz, 1800 kHz

2 kHz, 1800 kHz

Answer.2. 202 kHz, 1600 kHz

Explanation

According to Carson rule, BW is given as:

B.W = (β + 1) 2fm

B.W. = 2[Δf + f_{m}]

B.W. = 2[Δf + f_{m}]

For β < 1 (NBFM) case, only the adjacent two sidebands are significant and the bandwidth is similar to an AM case, i.e.

BW = 2 × f_{m}

Given:

f_{c} = 20 MHz,

f_{m} = 1 kHz,

Δf = 100 kHz

β = 100k/1k = 100 > 1

So, from Carson rule:

BW = 2(Δf + f_{m})

BW = 2(100 + 1) = 202 kHz

Now, f_{m} changes to 800 kHz

β = 100k/800k = 0.12 < 1. This is an NBFM signal.

BW = 2 f_{m} = 2 × 800 k = 1600 kHz.

6. FM is called constant envelope because ______ of carrier wave is kept constant.

Frequency

Amplitude

Phase

Angle

Answer.2. Amplitude

Explanation

The constant envelope is achieved when a sinusoidal waveform reaches equilibrium in a specific system. In frequency modulation, the carrier amplitude remains constant, but its frequency is changed in accordance with the modulating signal. Specifically, the higher the amplitude of the information signal, the greater the frequency change.

7. Frequency curve is _______

Asymptotic to y-axis

Non-asymptotic to x-axis

Asymptotic to x-axis

None of these

Answer.3. Asymptotic to x-axis

Explanation

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 frequency curve is Asymptotic to the x-axis. Asymptotic to the x-axis” means that a function or curve is approaching the x-axis but never crosses it.

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.

8. To produce frequency modulation using a phase modulator

The message signal must be integrated and then used for modulation.

The message signal must be differentiated and then used for modulation.

The phase-modulated signal must be integrated

The phase-modulated signal must be differentiated

Answer.1. The message signal must be integrated and then used for modulation.

Explanation

A standard frequency-modulated wave is written in the form:

${S_{FM}}\left( t \right) = A\:cos\left( {2\pi {f_c}t + 2\pi {k_f}\mathop \smallint \limits_0^t m\left( t \right).dt} \right)$

A standard phase-modulated wave is written in the form:

SP_{m}(t) = A_{c}Cos[ω_{c}t + (K_{p}, m(t))]

m(t) = the modulating signal

ωc = Carrier frequency

kp is in rad/V

Suppose, x(t) = $\mathop \smallint \limits_0^t m\left( t \right).dt$

When x(t) is applied at the phase modulator we will get the following signal at the output:

$S\left( t \right) = A\:cos\left( {2\pi {f_c}t + {k_p}x(t)} \right)$

putting the value of x(t), we get

$S\left( t \right) = A\:cos\left( {2\pi {f_c}t + {k_p}(\mathop \smallint \limits_0^t m\left( t \right).dt)} \right)$

This signal represents a standard FM signal with

2πk_{f} = k_{p}

9. Which of the following are the two most important classes of angle modulation?

Amplitude modulation, frequency modulation

Amplitude modulation, phase modulation

Frequency modulation, phase modulation

Single sideband amplitude modulation, phase modulation

Answer.3. Frequency modulation, phase modulation

Explanation

Angle modulation is a class of carrier modulation that is used in telecommunications transmission systems. The class comprises frequency modulation (FM) and phase modulation (PM) and is based on altering the frequency or the phase, respectively, of a carrier signal to encode the message signal. Angle modulation varies a sinusoidal carrier signal in such a way that the angle of the carrier is varied according to the amplitude of the modulating baseband signal.

10. Which is true in a frequency modulated system?

Variation of carrier amplitude does not affect the quality of reception

The amplitude of carrier is varied according to modulating signal

The transmission does not require line of sight between transmitter and receiver

Receiver uses diode detector

Answer.1. Variation of carrier amplitude does not affect the quality of reception

Explanation

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.