Communication Systems

Step 1: Understanding Vidicon. Vidicon is a type of video camera tube that works based on photoconductivity. It converts light into an electrical signal by changing the conductivity of a semiconductor in response to light.
Step 2: Conclusion. Thus, Vidicon operates on the principle of photoconductivity.

Step 1: Understanding radar range equation. The range of a radar is related to the power transmitted by the radar, and the maximum range is proportional to the fourth root of the transmitted power.
Step 2: Conclusion. Thus, the maximum range of radar is proportional to the fourth root of the peak transmitted power.

Step 1: Understanding feedback networks. An amplifier with proper negative feedback or positive feedback will enhance or stabilize the amplifier's gain and improve its performance. Positive feedback amplifies the signal further.
Step 2: Conclusion. Thus, the amplifier with proper positive feedback network circuits is the correct answer.

Step 1: XOR gate behavior. The XOR (exclusive OR) gate is used in binary addition to produce the sum output, which is 1 when there is an odd number of 1s in the inputs.
Step 2: Conclusion. Thus, the XOR gate performs perfect binary addition.

Step 1: Understanding FM transmitter operation. An FM transmitter without input is simply not transmitting a signal. The term "frequency modulation" refers to how the carrier frequency is varied according to the input signal.
Step 2: Conclusion. Thus, it is not correct to name the transmitter in this case as any of the listed options.

Step 1: Understanding the FM transmitter operation. An FM transmitter typically operates at a fixed carrier frequency, which is modulated by the input signal. Without an input signal, the carrier frequency remains constant.
Step 2: Conclusion. Thus, the frequency of operation when there is no input signal is the carrier frequency.

Step 1: Coaxial Cable Transmission.
The transmission of high frequencies in coaxial cables is influenced by both the dielectric properties of the materials and the skin effect, which causes higher frequencies to propagate along the surface of the conductor.

Step 2: Conclusion.
Therefore, the correct answer is option (D).

Final Answer:

Step 1: Understanding Amplifier Types.
Television transmitters typically use a grid-modulated class C amplifier in the output stage for efficient power amplification at high frequencies.

Step 2: Conclusion.
Thus, the correct answer is option (B).

Final Answer:

Step 1: Using the modulation index formula.
The modulation index is given by the formula: where is the maximum current (8.93A) and is the carrier current (8A).

Step 2: Calculation.


Step 3: Conclusion.
Therefore, the modulation percentage is . The correct answer is option (B).

Final Answer:

Step 1: Doppler Effect.
When a source of waves (in this case, the airplane) moves towards the observer (the radar), the wavelength of the waves decreases. This is due to the Doppler effect.

Step 2: Conclusion.
Therefore, the correct answer is option (A).

Final Answer:

Because of their inherent distortion class amplifiers are never used as audio amplifiers, but they are primarily employed in the final stage of TV transmitters. Also, owing to low efficiency of class operations, these amplifiers are not employed where large RF (radio frequency) power is involved e.g., to excite transmitting antenna. In such cases class amplifiers are used, since a class amplifier has a very high efficiency, it can deliver more load power. In out put stage of a TV transmitter grid modulated class amplifier is used.

Step 1: Recall basic structure of Colpitts oscillator.
Colpitts oscillator is an LC oscillator where the feedback is obtained using a capacitive voltage divider.
Step 2: Identify components of feedback network.
The feedback network consists of:
Two capacitors and connected in series and one inductor .
Step 3: Why two capacitors are used.
The two capacitors form a voltage divider which provides the correct fraction of output voltage back to the input.
Step 4: Choose correct option.
Hence the correct option is (B).
Final Answer:

Step 1: Recall relation between antenna length and wavelength.
Antenna length is usually proportional to wavelength, typically .
Step 2: Compare wavelengths of AM and FM.

AM frequency is much smaller, so wavelength is very large.
FM frequency is much higher, so wavelength is small.
Step 3: Conclusion based on wavelength.
Longer wavelength longer antenna needed.
So AM requires longer antenna and FM requires shorter antenna.
Final Answer:

Here
Low side-hand frequency


Upper side-band frequency

Step 1: Total power in AM wave.
where is modulation index.
Step 2: For 100% modulation.
Step 3: Solve for carrier power.
Final Answer:

Step 1: Understand why radio waves can pass/reflect.
Radio communication depends on reflection of waves from ionosphere.
Longer wavelengths (lower frequency) undergo reflection more easily by ionosphere layers.
Step 2: Compare radio waves and light waves.
Radio waves have much larger wavelength compared to visible light.
Because of this, they interact differently with atmospheric and ionospheric conditions.
Step 3: Conclude reason.
Thus radio waves can pass/propagate while shorter waves cannot.
Final Answer:

Step 1: Recall AM sideband frequencies.
In amplitude modulation, sideband frequencies are:
Step 2: Substitute given values.
Carrier frequency:
Modulating frequency:
Step 3: Calculate sidebands.
Final Answer:

Step 1: Meaning of attenuation.
Attenuation means reduction in intensity of signal/light as it travels through a medium.
Step 2: Causes of attenuation in optical fibre.
Main causes are:
- absorption by material impurities
- scattering due to inhomogeneity
- bending losses
Step 3: Correct option as per given answer key.
According to answer key, correct is option (D).
Final Answer:

A very small part of light energy is lost from an optical fibre due to absorption or due to light leaving the fibre area resulting scattering of light sideways by impurities in the glass fibre.

Step 1: Understanding sky wave propagation.
Sky wave propagation refers to the transmission of radio waves that are reflected by the ionosphere back to Earth, primarily used in radio communication.
Step 2: Explanation.
It is mainly utilized in radio communication and not in satellite or TV communication, which use different propagation methods.
Final Answer:

Step 1: Definition of FM transmitter frequency.
The frequency of an FM transmitter without a signal input is the carrier frequency, which is the central frequency of the transmission.
Step 2: Explanation.
When there is no input signal, the transmitter operates at the center frequency, and modulation or frequency deviation occurs only when the signal is applied.
Final Answer:

Answer (a) radio communication

Frequency deviation is the frequency of an FM transmitter without signal input.

The mutual conductance of the triode is related to the amplification factor and the plate resistance by the formula . Using the given values, we calculate the mutual conductance.

Step 2: Conclusion.
The mutual conductance is , corresponding to option (a).

Step 1: Understanding the concept of fidelity.
Fidelity in a radio-receiver means that the receiver amplifies the signal without distortion, maintaining the original quality of the signal.

Step 2: Conclusion.
Since the receiver amplifies all signal frequencies equally well, it ensures high fidelity, which is the correct answer.

The ionosphere bands electromagnetic radiation in a range of frequencies, with higher frequencies passing through and not being reflected back. The critical frequency for reflection is around 40 MHz.

For a triode, the plate current is related to the grid and plate potentials. To keep the plate current constant when the grid potential is decreased, the plate potential must be increased by a corresponding amount. The amplification factor of the triode is 50, which determines how much the plate potential must increase.

Modulation index, is carried by the carrier wave, i.e. there is no signal distortion.