Communication Systems — Part 1 MCQs with Explanations

Frequency Modulation ( FM ) is a method of transmitting information (such as audio signals) by varying the frequency of the carrier wave in accordance with the modulating signal. In commercial FM radio broadcasting, the audio signal (voice or music) is used to modulate the carrier frequency, and this modulated signal is transmitted over the airwaves to be received by FM radios. The channel bandwidth refers to the range of frequencies that the FM signal occupies in the radio frequency spectrum. In FM broadcasting, the signal is allocated a specific frequency band, and this band needs to be wide enough to accommodate the modulated signal. The channel bandwidth is influenced by various factors, including the audio quality, the data rate required for the transmission, and the interference considerations to ensure proper reception. In commercial FM broadcasting, a bandwidth of approximately 150 kHz is chosen as a trade-off between audio quality and spectrum efficiency. Here's why: 1. Audio Quality: A wider bandwidth allows for higher fidelity audio transmission. The wider the bandwidth, the more accurately the original audio signal can be reconstructed at the receiving end. A 150 kHz bandwidth is considered sufficient to provide good audio quality for commercial FM radio broadcasting. 2. Spectrum Efficiency: On the other hand, allocating too much bandwidth for each FM channel would be inefficient in terms of spectrum usage. It would limit the number of radio stations that could coexist within the available radio frequency spectrum, leading to congestion and reduced choices for listeners. 3. Interference: FM channels are typically spaced apart in the radio frequency spectrum to avoid interference between adjacent channels. Allocating a wider bandwidth for each FM channel would require more significant spacing between channels, further reducing the available spectrum for broadcasting. For these reasons, a practical compromise is made by selecting a channel bandwidth of around 150 kHz in commercial FM systems. This allows for good audio quality while optimizing spectrum utilization and providing enough spacing between channels to minimize interference.

In Frequency Modulation (FM), the bandwidth of the signal is determined by Carson's Rule. Carson's Rule states: Bandwidth (BW) ≈ 2 * (Δf + fm) , where: Δf is the frequency deviation, which is 25 kHz in this case. fm is the highest frequency in the modulating signal, which is 5 kHz in this case. Plugging in the values: BW ≈ 2 * (25 kHz + 5 kHz). BW ≈ 2 * (30 kHz). BW ≈ 60 kHz. Therefore, the bandwidth of the FM signal is approximately 60 kHz.

True is the correct answer to “Skip distance depends on time of day and angle of incidence.”. This question is standard in competitive examination syllabi for Communication Systems.

Frequency diversity is the correct answer to “It is found that a ship to ship communication suffers from fading. This can be avoided by using”. This question is standard in competitive examination syllabi for Communication Systems.

A high gain D.C. amplifier is the correct answer to “A buffer amplifier is”. This question is standard in competitive examination syllabi for Communication Systems.

All of the above is the correct answer to “SSB can be generated by”. This question is standard in competitive examination syllabi for Communication Systems.

To prevent overmodulation is the correct answer to “What is the purpose of peak clipper circuits in radio transmitters”. This question is standard in competitive examination syllabi for Communication Systems.

Anticlockwise is the correct answer to “The direction of rotation of a CD is”. This question is standard in competitive examination syllabi for Communication Systems.

Low pass filter is the correct answer to “A woofer should be fed from the input through a”. This question is standard in competitive examination syllabi for Communication Systems.

300 ohm is the correct answer to “The characteristic impedance of a twin wire feeder used for TV signals is about”. This question is standard in competitive examination syllabi for Communication Systems.

Is due to transverse nature of waves is the correct answer to “In EM waves, polarization”. This question is standard in competitive examination syllabi for Communication Systems.

Medium frequency is the correct answer to “The frequency range of 300 kHz to 3000 kHz is known as”. This question is standard in competitive examination syllabi for Communication Systems.

3 kHz is the correct answer to “A telephone channel requires a bandwidth of about”. This question is standard in competitive examination syllabi for Communication Systems.

A fundamental sine wave and odd harmonics is the correct answer to “Fourier analysis indicate that a square wave can be represented as”. This question is standard in competitive examination syllabi for Communication Systems.

35 kHz is the correct answer to “If sampling is done at the rate of 10 kHz. The bandwidth required is”. This question is standard in competitive examination syllabi for Communication Systems.

Armstrong modulator is the correct answer to “Which of the following is an indirect way of generating FM”. This question is standard in competitive examination syllabi for Communication Systems.

The lower modulating power requirement is the correct answer to “One of the advantages of base modulation over collector modulation of a transistor class C amplifier is”. This question is standard in competitive examination syllabi for Communication Systems.

Prevent excessive grid current is the correct answer to “Leak type bias is used in plate modulated class C amplifier to”. This question is standard in competitive examination syllabi for Communication Systems.

Costly is the correct answer to “FM transmitting and receiving equipment as compared to AM equipment is”. This question is standard in competitive examination syllabi for Communication Systems.

455 kHz is the correct answer to “For AM receivers the standard IF frequency is”. This question is standard in competitive examination syllabi for Communication Systems.

Upto 600 bps is the correct answer to “A modem is classified as low speed if data rate handled is”. This question is standard in competitive examination syllabi for Communication Systems.

Large bandwidth is required is the correct answer to “In Pulse Code Modulation system”. This question is standard in competitive examination syllabi for Communication Systems.

200 and 1.01 is the correct answer to “An audio signal (say from 50 Hz to 10000 Hz) is frequency translated by a carrier having a frequency of 106 Hz. The values of initial (without frequency translation) and final (after frequency translation) fractional change in frequency from one band edge to the other are”. This question is standard in competitive examination syllabi for Communication Systems.

Option "Laser beam for both recording and playback" is the correct answer for this question under Communication Systems.

3 is the correct answer to “If output power of a radio receiver is doubled, its volume is increased by __________ dB.”. This question is standard in competitive examination syllabi for Communication Systems.

Shunt capacitance is the correct answer to “For attenuation of high frequencies we should use”. This question is standard in competitive examination syllabi for Communication Systems.

Large bandwidth is required is the correct answer to “The disadvantage of FM over AM is that”. This question is standard in competitive examination syllabi for Communication Systems.

It reduces the bandwidth requirement to half is the correct answer to “VSB modulation is preferred in TV because”. This question is standard in competitive examination syllabi for Communication Systems.

PWM is the correct answer to “Which one of the following is analog”. This question is standard in competitive examination syllabi for Communication Systems.

Infinite capacity is the correct answer to “As per Shannon-Hartley theorem, a noise less Gaussian channel has”. This question is standard in competitive examination syllabi for Communication Systems.

Is more than that of dipole is the correct answer to “In a TV receiver antenna the length of reflector rod”. This question is standard in competitive examination syllabi for Communication Systems.

Change the antenna orientation of location is the correct answer to “To eliminate ghosts in the picture”. This question is standard in competitive examination syllabi for Communication Systems.

Is useful in reducing depolarization effect on received wave is the correct answer to “Circular polarization”. This question is standard in competitive examination syllabi for Communication Systems.

10 is the correct answer to “If in a broadcasting studio, a 1000 kHz carrier is modulated by an audio signal of frequency range 100-5000 kHz, the width of channel is __________ kHz.”. This question is standard in competitive examination syllabi for Communication Systems.

Voltage for which frequency, phase or amplitude is varied is the correct answer to “In Modulation, “carrier” is”. This question is standard in competitive examination syllabi for Communication Systems.

4E is the correct answer to “For a plate-modulated class C amplifier the plate supply voltage is E. The maximum plate cathode voltage could be almost high as”. This question is standard in competitive examination syllabi for Communication Systems.

Duplex arrangement is the correct answer to “A cordless telephone using separate frequencies for transmission in base and portable units is known as”. This question is standard in competitive examination syllabi for Communication Systems.

Horizontal blanking is the correct answer to “In TV systems, equalising pulses are sent during”. This question is standard in competitive examination syllabi for Communication Systems.

1 is the correct answer to “For a given carrier wave, maximum undistorted power is transmitted when value of modulation is”. This question is standard in competitive examination syllabi for Communication Systems.

Gets increased by 50 percent is the correct answer to “The modulation index of an FM is changed from 0 to 1. How does the transmitted power change”. This question is standard in competitive examination syllabi for Communication Systems.