Communication Systems

Step 1: Understanding digital signals.
Digital signals represent information using discrete values, usually binary levels such as 0 and 1.

Step 2: Analysis of options.
(A) Correct, digital signals have discrete stepwise values.
(B) Incorrect, sound and picture signals are analog in nature before digitization.
(C) Incorrect, continuous variations correspond to analog signals.
(D) Incorrect, sine waves are continuous analog waves.

Step 3: Conclusion.
Digital signals are characterized by discrete stepwise values only.

Step 1: Understand signal degradation.
As a signal travels over long distances, it gets weakened due to attenuation.

Step 2: Role of a repeater.
A repeater receives the weak signal, amplifies it, and retransmits it.

Step 3: Conclusion.
Thus, a repeater is used to extend the range of communication systems.

Step 1: Apply conservation of energy.
Let velocity at the top be and at the bottom be . The height difference between top and bottom is .
Step 2: Energy relation.

Step 3: Minimum condition at top.
For a taut string at the top:

Step 4: Substitute and solve.

Step 5: Ratio of velocities.

Step 6: Conclusion.
The required ratio is .

Step 1: Understanding phase modulation.
In phase modulation (PM), the phase of the carrier signal is varied in accordance with the information signal. The amplitude and frequency of the carrier wave remain constant, while the phase changes with the information signal. Step 2: Conclusion.
Thus, the parameter that is varied in phase modulation is the phase shift, which corresponds to option (A).

Step 1: Understanding the motion.
The body is moving with uniform speed along the circumference of a circle. Since the speed is constant, there is no change in the magnitude of velocity.
Step 2: Radial acceleration interpretation.
Radial acceleration refers to the component of acceleration along the radius. As per the given options and the context of the question, the radial acceleration is taken as zero.
Step 3: Conclusion.
Hence, the radial acceleration of the body is zero.

Step 1: Understanding the block diagram.
In a generalized communication system, the block diagram typically includes four main components: the information source, transmitter, channel, and receiver. The element labelled 'X' in the diagram is located between the transmitter and the receiver.
Step 2: Identifying the role of 'X'.
The role of 'X' in the communication system is to transmit the signal from the transmitter to the receiver. This function corresponds to the "Channel" of the system, which is responsible for carrying the signal and potentially introducing noise.
Step 3: Conclusion.
Thus, the correct answer is the "Channel", corresponding to option (C).

Step 1: Understanding the relation between range and height.
The range for line of sight propagation depends on the height of the antenna, and it is given by , where is the radius of the Earth. If the height of the antenna is doubled, the new range becomes .
Step 2: Conclusion.
Thus, the new range is , which corresponds to option (D).

Step 1: Understanding the ionosphere.
The ionosphere is a layer of the Earth's atmosphere that contains charged particles, including free electrons and positive ions. It is created by the ionization of gas molecules by solar radiation.
Step 2: Analyzing the options.
- (A) Only positive ions is incorrect because the ionosphere also contains free electrons. - (B) Only neutral particles is incorrect because the ionosphere is characterized by ionized particles, not neutral ones. - (C) Only electrons is incorrect because the ionosphere also contains positive ions. - (D) Electrons and positive ions is correct as the ionosphere contains both free electrons and positive ions.

Step 3: Conclusion.
The correct answer is (D), as the ionosphere consists of both electrons and positive ions.

Step 1: Work-Energy Principle.
The work done by the frictional force is equal to the change in kinetic energy of the block. The initial kinetic energy is , and the final kinetic energy is 0 (since the block stops). The work done by friction is , where is the distance.
Step 2: Applying the Work-Energy Theorem.
According to the work-energy theorem: Thus: Solving for :
Step 3: Conclusion.
The distance covered by the block before it stops is , so the correct answer is (C).

Step 1: Identify carrier and modulating frequencies.


Step 2: Sideband frequencies.
Upper sideband: Lower sideband:

Step 1: Recall radiation from a linear antenna.
For a linear (Hertzian) antenna of fixed length, the power radiated depends on the frequency of the emitted electromagnetic waves.
Step 2: Relation between power and frequency.
The radiated power is proportional to the square of frequency:
Step 3: Use relation between frequency and wavelength.

Step 4: Substitute in power relation.

Step 5: Conclusion.
The power radiated is proportional to .