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) = AcCos[ωct + (gKk, 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:
Pavg = A2c/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 + fm]
B.W. = 2[Δf + fm]
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 × fm
Given:
fc = 20 MHz,
fm = 1 kHz,
Δf = 100 kHz
β = 100k/1k = 100 > 1
So, from Carson rule:
BW = 2(Δf + fm)
BW = 2(100 + 1) = 202 kHz
Now, fm changes to 800 kHz
β = 100k/800k = 0.12 < 1. This is an NBFM signal.
BW = 2 fm = 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:
SPm(t) = AcCos[ωct + (Kp, 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
2πkf = kp
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.