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## Communications MCQ Set 1

1. Which device is has more chance to be found in SSB transmitter?
a) tuned modulator
b) class A RF amplifier
c) class C audio amplifier
d) class B RF amplifier

Answer: a [Reason:] By tuning the circuit we ensure that the circuit will for particular frequencies only. In SSB tuned modulator is more likely to found.

2. The RF section of radio consists both oscillator and buffer stage.
a) True
b) False

Answer: a [Reason:] RF is electromagnetic wave frequencies in the range 3 KHz to 300 GHz. It generally refers to electrical rather than mechanical oscillations.

3. What is the frequency of audio modulation?
a) 1 KHz
b) 2 KHz
c) 3 KHz
d) 4 KHz

Answer: b [Reason:] Frequency of audio modulation is approximately close to 2KHz. It is also said standard frequency for audio modulation.

4. In telegraphy we generally use __________
a) Amplitude modulation
b) Frequency modulation
c) Pulse modulation
d) Vestigial side band modulation

Answer: c [Reason:] In pulse modulation, width or amplitude of pulses is varied. It is mostly used for telegraphy purpose.

5. In FM both the frequency and amplitude of carrier signal is varied in accordance with the instantaneous value of modulating signal.
a) True
b) False

Answer: b [Reason:] In FM the frequency of carrier signal is varied in accordance with the instantaneous value of modulating signal.

6. In the equation, 20cos⁡(108 t+3 sin⁡109 t) the modulation index is ________
a) 1
b) 2
c) 3
4) 4

Answer: c [Reason:] On comparing with the general equation Ac cos⁡(2πfc t+kf sin⁡2πfm t) where kf is modulation index. We find that the modulation index for this equation is 3.

7. Antenna can also be used as a receiver.
a) True
b) False

Answer: a [Reason:] Antennas can be used as transmitter and receiver both. Antenna transmits or receives electromagnetic waves only.

8. What is the advantage of IF modulation?
a) it uses SSB which further reduces its bandwidth
b) modular section and visual exciter can be of any state
c) it cannot be tuned
d) modular section and visual exciter can be solid state devices

Answer: d [Reason:] Advantages of IF modulation is that the modular section and visual exciter can be solid state devices. Its other advantages are it can use VSB filter that can be introduced at low power level. Moreover it is economical.

9. We can use either cone type or horn type loudspeakers in tape recorders.
a) True
b) False

Answer: b [Reason:] In tape recorders only cone type loudspeakers can be used. It not works well with the horn type loudspeakers.

10. The advantage of using mechanical filter in filter system of sideband suppression is good attenuation characteristics.
a) True
b) False

Answer: a [Reason:] The advantage of using mechanical filter in filter system of sideband suppression is smaller size, good band pass, good attenuation characteristics.

## Communications MCQ Set 2

1. Envelope Detector is a/an ________
a) Coherent detector
b) Asynchronous Detector
c) Synchronous Detector
d) Product Demodulator

Answer: b [Reason:] Envelope detectors are asynchronous in nature. The advantage of asynchronous over synchronous is that it is simple, cheap, setup is faster.

2. Effective radiated power (ERP) is equal to ________
a) Pt×Gt×Pd
b) Pt×Pd
c) Gt×Pd
d) Pt×Gt

Answer: d [Reason:] Effective radiated power is equal to the product of antenna gain (Gt) and power transmitted (Pt).

3. Power density from an isotropic antenna is equal to ________ Answer: a [Reason:] Power of a transmitter that is radiated from an isotropic antenna will have a uniform power density in all direction. The power density at any distance ‘R’ for an isotropic antenna is equal to .

4. Compact discs mainly use ________
a) optical recording
b) magnetic recording
c) magnetic retrieval
d) both optical and magnetic recording

Answer: a [Reason:] Compact disk (CD) is mainly used for storage of data. It was developed only for store and play only audio but was later adapted for storage of data also. It mainly used optical recording and retrieval.

5. If the distance between antenna and source doubles, the received signal power decreases by 14.
a) True
b) False

Answer: a [Reason:] It is due to the R2 term in formula, .

6. Power density follows inverse square law.
a) True
b) False

Answer: a [Reason:] With increase in distance power density decreases. So, we can also say that power density is inversely related to distance.

7. There is no requirement of IF amplifier stages for a video monitor.
a) True
b) False

Answer: a [Reason:] Video amplifier is used to amplify video signals before passing them to a Cathode ray tube. And there is no need of IF amplifier stages for a video monitor.

8. Helical antenna is circularly polarized.
a) True
b) False

Answer: a [Reason:] In helical antenna, polarization is equally divided between vertical and horizontal components and thus it is circularly polarized.

9. The intermediate frequency of a superheterodyne receiver is 500KHz. What is the image frequency at 1200KHz?
a) 600KHz
b) 500KHz
c) 700KHz
d) 200KHz

Answer: c [Reason:] Image frequency is equal to (1200 – 500) which is equal to 700KHz.

10. Which of these amplifiers is used for impedance matching?
a) Common Base
b) Common Emitter
c) Common Collector
d) Any one of above

Answer: c [Reason:] Output impedance of common collector is low and thus it is suitable for impedance matching.

## Communications MCQ Set 3

1. Standard intermediate frequency used for AM receiver is ________
a) 455 MHz
b) 455 KHz
c) 455 Hz
d) 20 KHz

Answer: b [Reason:] Standard intermediate frequency (IF) used for AM receiver is 455KHz. IF is obtained by mixing the incoming siganl frequency with the locally generated signal frequency.

2. Which device is used in TV receivers for tuning the receiver to the incoming signal?
a) Varacter diode
b) High pass filter
c) Low pass filter
d) Zener diode

Answer: a [Reason:] Varacter diodes have variable capacitance as a function of variable input voltage. It works in reverse bias. It is generally used for tuning the receivers according to incoming signal. It is also used in RF filters for tuning purposes such as tuning of TV sets.

3. Power density from an isotropic antenna is equal to ________ Answer: a [Reason:] Power of a transmitter that is radiated from an isotropic antenna will have a uniform power density in all direction. The power density at any distance ‘R’ for an isotropic antenna is equal to Pt/(4πR2 ).

4. What is the bandwidth occupied by a DSB signal when the modulating frequency lies in the range from 100Hz to 10KHz?
a) 19.8 KHz
b) 19.8 Hz
c) 19.8 MHz
d) 19.8 GHz

Answer: a [Reason:] According to given problem, fm = 100Hz to 10KHz = 10000 – 100 = 9900Hz. Therefore bandwidth is equal to 2fm = 2 X 9900 = 19.8 KHz.

5. Which statement is true about multiplexing?
a) it is used to reduce the bandwidth
b) it is used to combine multiple data streams over a single data channel
c) it is used to allow multiple data streams over multiple channels
d) it is used to match the frequencies of the signal

Answer: b [Reason:] Multiplexing is a process where multiple data streams from different sources are combined and transmitted over a single data channel. It is placed at the transmitting end to combine the signals.

6. What is the noise figure of merit for an SSB modulated signal?
a) less than 1
b) greater than 1
c) 1
d) 0.5

Answer: c [Reason:] Figure of merit is the ratio of output signal to noise ratio to input signal to noise ratio of a receiver system, oftenly used to describe the performance of a system. Figure of merit for SSB modulation is always 1.

7. In which of the following modulated systems, quantization noise occurs?
a) TDM
b) FM
c) PWD
d) PCM

Answer: d [Reason:] Pulse code modulation (PCM) is a digital form of communication. For demodulation of PCM, it is necessary to convert it into PAM. Quantization noise occurs in PCM only.

8. Which amplifier is preferably used for low level modulation?
a) Class C
b) Class A
c) Class AB
d) Either of Class C or of Class A

Answer: b [Reason:] We called a modulation as low level, when the modulation takes place prior to the output element of the final stage of the amplifier. Class A amplifiers are used for this purpose.

9. If a receiver has poor capacity of blocking adjacent channel interference then the receiver has ________
a) Poor selectivity
b) Poor Signal to Noise ratio
c) Poor sensitivity
d) Poor reception of signals

Answer: a [Reason:] Selectivity is used to separate out closely lying components in terms of frequency. If the receiver has poor selectivity then it also has poor capacity of blocking adjacent channel interference.

10. What is the advantage of using a VSB transmission?
a) Higher bandwidth than SSB
b) Less power required as compared to DSB-SC
c) Lower bandwidth than SSB
d) Both higher bandwidth than SSB and less power required as compared to DSB-SC

Answer: d [Reason:] Vestigial Side Band is similar to single sideband (SSB) transmission, in which the second sideband is not completely removed, but is filtered to remove the range of frequencies that are not required.

11. What is the value of modulation index m, for overmodulation?
a) m < 1
b) m = 1
c) m > 1
d) m << 1

Answer: c [Reason:] When the modulation index m is greater than 1 i.e. m > 1 it is called overmodulation. It generates modulation with distortions in the envelope of the modulated signal which makes difficult to recover the information.

## Communications MCQ Set 4

1. Notch filter is a band pass filter.
a) True
b) False

Answer: b [Reason:] Notch filter is a band stop filter since it allows most frequencies to pass through it, except a specific range. It is just opposite of a band-pass filter.

2. For what modulation index, we can expect maximum undistorted power output of a transmitter?
a) 50%
b) 100%
c) Between 50% and 100%
d) 30%

Answer: b [Reason:] For getting maximum undistorted power output, value of modulation index should be 1. If the value of modulation index is 1 then we can say that it is 100% modulated.

3. Modulation is used to allow the use of practical antennas.
a) True
b) False

Answer: a [Reason:] Modulation is the process of varying one or more properties of carrier with respect to the wave being propagated. It is used to separate different transmission signals and also to ensure that message is transmitted over a long path. It sometimes used to allow the use of practical antennas also.

4. Which of the following statement is incorrect about modulation?
a) It is used to allow the use of practical antennas
b) It is used to separate different transmissions
c) It is used to reduce the required bandwidth
d) It is used to ensure that message is transmitted over long distances

Answer: c [Reason:] Modulation is the process of varying one or more properties of carrier with respect to the wave being propagated. It is used to separate different transmission signals and also to ensure that message is transmitted over a long path. It sometimes used to allow the use of practical antennas also.

5. Why amplitude modulation is issued for broadcasting?
a) It is more immune to noise
b) It has more fidelity
c) It avoids receiver complexity
d) It can provide the required bandwidth

Answer: c [Reason:] AM broadcasting employs amplitude modulation (AM) transmissions. It is highly used as compared to others for broadcasting purpose since it avoids complexity in receiver.

6. An ergodic process is present in communication, if many random signals have ________
a) identical time averages
b) identical ensemble averages
c) identical time and ensemble averages
d) identical bandwidth

Answer: c [Reason:] A stochastic process is said to be ergodic if its properties can be reduced from a single. A non ergodic process changes erratically at an inconsistent rate. Ergodic process exists if many random signals have identical time and ensemble averages.

7. What is the advantage of superheterodyning?
a) High selectivity and sensitivity
b) Low Bandwidth
c) Low adjacent channel rejection
d) Low fidelity

Answer: a [Reason:] The main advantage of superheterodyning is that it has high selectivity and sensitivity. Its bandwidth remains same. Moreover, it has high adjacent channel rejection.

8. Auto-correlation function is non-unique description of signals.
a) True
b) False

Answer: a [Reason:] Auto-correlation function is also known as serial-correlation. It is the correlation of a function with a delayed copy of itself. So it is non-unique description of signals.

9. Relationship between amplitude and frequency is represented by ________
a) Time-domain plot
b) Phase-domain plot
c) Frequency-domain plot
d) Amplitude-domain plot

Answer: c [Reason:] Relationship between amplitude and frequency is represented by frequency-domain plot. While a time-domain graph shows how a signal changes over time.

10. Which of the following statement is not true about auto-correlation function?
a) it is non-unique description of goals
b) it is measure of both time variation and statistical dependence
c) it is an average function
d) it is defined only for periodic signals

Answer: d [Reason:] Auto-correlation function is also known as serial-correlation. It is the correlation of a function with a delayed copy of itself. So it is non-unique description of signals.

## Communications MCQ Set 5

1. AM waves is represented by which equation?
a) [1 + m(t)].c(t)
b) [1 – m(t)].c(t)
c) [1 + m(t)].2c(t)
d) [1 + 2m(t)].c(t)

Answer: a [Reason:] Amplitude wave is represented by [1 + m(t)].c(t) where c(t) is carrier wave equal to ASin(2 π.fc.t) and m(t) is modulation waveform equal to M.Cos(2 π.fm.t + ∅).

2. For attenuation of high frequencies we can use ________
a) inductance
b) shunt capacitance
c) series capacitance
d) combination on inductor and resistor

Answer: b [Reason:] Capacitive reactance, X_c is low for high frequencies. Due to this the high frequencies are shunted to ground and are not transmitted.

3. If each element of signal occupies 40ms, determine its speed?
a) 20 bauds
b) 25 bauds
c) 40 bauds
d) 0.05 bauds

Answer: b [Reason:] 4. A modem is classified as low if speed of data rate is ________
a) upto 600bps
b) upto 200bps
c) upto 400bps
d) upto 500bps

Answer: a [Reason:] Modem is device that can modulate carrier wave as well as demodulate signal to decode the transmitted information. When data rate in bps is upto 600, modem is considered having low speed.

5. Modulation is also called detection.
a) True
b) False

Answer: b [Reason:] Modulation can be seen as the addition of information to a signal. Demodulation is technique to extract our original message signal back. Demodulation is also called detection.

6. It is suitable to connect woofer from the input through ________
a) band pass filter
b) band stop filter
c) low pass filter
d) high pass filter

Answer: c [Reason:] Woofer is considered as a low frequency loud speaker. It has a range from 10Hz to 500Hz. So it is better to use low pass filter for connection.

7. The radiation at right angles is zero means ________
a) l = ʎ
b) l = ʎ4
c) l = 2 ʎ
d) l = ʎ2
where l = length of antenna and ʎ=wavelength.

Answer: a [Reason:] Antenna is used for converting electromagnetic radiation into electric currents or vice-versa. If the length of antenna is equal to the whole wavelength then the radiation at right angles is zero.

8. Power of carrier wave is 500W and modulation index is 0.25. Find its total power?
a) 500W
b) 415W
c) 375W
d) 516W

Answer: d [Reason:] Total power, Pt = 500(1+(0.25)22) = 516W.

9. Commercial frequency deviation is ________
a) 75 KHz
b) 80 KHz
c) 85 KHz
d) 65 KHz