Principles of Communication Systems

Frequency modulation is widely used on frequencies below 30 MHz.

• True
• False

What is the reference line for the modulating signal?

• Un-modulated peak line
• Carrier peak line
• Zero line
• Modulated peak line

The __ varies according to the changes in the environmental conditions and other situations.

• Tactical communication

On its own(tuner), the antenna captures ______________ of all frequencies and sends them to the RF amplifier, which dutifully amplifies them all.

• radio waves

Calculate the depth of modulation when a transmitter radiates a signal of 9.8KW after modulation and 8KW without modulation of the signal.

• 100%
• 67%
• 80%
• 50%

Like all electromagnetic waves, ____in a vacuum travel at the speed of light, and in the Earth's atmosphere at a close, but slightly lower speed.

Convert decibel power gains of 13 dB, 33 dB, and 103 dB to power gains.

Continuous, unmodulated carrier, formerly common for radio direction finding in marine and aeronautical navigation.

If the value is less than 1, the modulation index is 0.5, then the modulated output would be called as___.

Provides the necessary electrical power to operate the transmitter.

• POWER SUPPLY

Noise figure of merit in SSB modulated signal is

• None of the above
• 1
• Greater than 1
• Less than 1

It is known as the deviation and is normally quoted as the number of ______________ deviation.

• kilohertz

When the _______________ is modulated onto the radio frequency carrier, the new radio frequency signal moves up and down in frequency.

• When the _______________ is modulated onto the radio frequency carrier, the new radio frequency signal moves up and down in frequency.
• Answer:
• audio signal

An amplifier has an output power of 5W. What is this amount of power in dBm?

• Pout,dBm=10log10(5W0.001W)=47dBm
• Pout,dBm=10log10(5W0.001W)=57dBm
• Pout,dBm=10log10(5W0.001W)=37dBm

With____, in which the frequency of the carrier signal is varied.

What is the circuit used for producing AM called?

• Modulator
• Duplexer
• Receiver
• Transmitter

Radio communication system works even without the aid of an antenna.

• True
• False

For low level modulation, amplifier used is

• Class A & C
• Class A
• None of the above
• Class C

Modulation is required for

• Multiplexing the signals
• Reduction of Antenna height
• All of the above
• Reducing noise while transmission
• Reduction in the complexity of circuitry

A power amplifier has an output power of 200 W and an input power of 8W. What is the power gain in decibels?

• Ap=Pout/Pin=200W/8W=25.Ap,dB=10log(25)=24dB
• Ap=Pout/Pin=200W/8W=25.Ap,dB=10log(25)=14dB
• Ap=Pout/Pin=200W/8W=25.Ap,dB=10log(25)=15dB

It is narrow band FM that is typically used for two-way radio _________________.

• communication application

Calculate the total modulation Index when a carrier wave is being modulated by two modulating signals with modulation indices 0.8 and 0.3.

• 0.5545
• 0.8544
• 0.6788
• 0.9999

__ carry the information transmitted by the radio signal.

The AM spectrum consists of

• Upper side band frequency
• Carrier frequency
• Lower side band frequency
• All of the above

The _____________ may be called a variety of names including FM demodulator, FM detector or an FM discriminator.

• FM demodulator

In addition to its widespread use for high quality audio broadcasts, FM is also used for a variety of __ radio communication systems.

• TWO WAY

What is the condition for greatest output power at the transmitter without distortion?

• Carrier voltage
• Modulating signal voltage > Carrier voltage
• Modulating signal voltage -> 0
• Modulating signal voltage
• Modulating signal voltage < Carrier voltage

While changing the amplitude of a radio signal is the most obvious method to modulate it, it is by no means the only way.

• True
• False

What is the SNR?

• 240

The ______________ by which the signal moves up and down is important.

• amount

The process of transforming a carrier signal so that it can carry the information of a message signal.

• The process of transforming a carrier signal so that it can carry the information of a message signal.
• Select one:
• Modulation
• Demodulation

This method simply requires the use of a varactor diode placed within the tuned circuit of an oscillator circuit.

• Varactor diode oscillator

In simple ______________, the tuning is adjusted by varying the number of turns of wire in the coil.

• radio receiver circuits

A sensitive amplifier that amplifies the very weak ________________ signal from the antenna so that the signal can be processed by the tuner.

• radio frequency (RF)

Frequency modulation is particularly well known for its use for VHF FM broadcasting.

• True
• False

In this mode of communication, the information is sent through light.

• Tactical Communications
• Duplex Communications
• Optical Communications

FM speech communication , often used for marine radio and many other VHF communications.

Type of communication system where two equipments can communicate with each other in both the directions simultaneously

• DUPLEX COMMUNICATION SYSTEM

Having a narrower band it is not able to provide the high quality of the _____________, but this is not needed for applications such as mobile radio communication.

• wide-band transmissions

The ratio between the modulating signal voltage and the carrier voltage is called?

• Amplitude modulation
• Modulation index
• Ratio of modulation
• Modulation frequency

An AM broadcast station transmits modulating frequencies up to 6 kHz. If the AM station is transmitting on a frequency of 894 kHz, the values for maximum and minimum upper and lower sidebands and the total bandwidth occupied by the AM station are:

• 900 KHz, 888 KHz, 12 KHz
• 894 KHz, 888 KHz, 6 KHz
• 900 KHz, 888 KHz, 6 KHz
• 894 KHz, 884 KHz, 12 KHz

Analog signal may be converted into digital signal by

• Amplitude modulation
• Filtering
• Mixing
• Sampling

Calculate the frequencies available in the frequency spectrum when a 2MHz carrier is modulated by two sinusoidal signals of 350Hz and 600Hz.

• 1999.35, 1999.65 and 2000.6, 2000.4
• 2000.35, 1999.65 and 2000.6, 1999.4
• 1999.35, 1999.65 and 1999.6, 1999.4
• 2000.35, 2000.65 and 2000.6, 2000.4

Convert power gains of 0.01, 0.001, and 0.0005 to decibel values

• 100×log1(0.01)=−20dB1×log100(0.001)=−30dB10×log10(0.0005)=−33dB
• 10×log10(0.01)=−20dB1×log10(0.01)=−30dB1×log10(0.005)=−33dB
• 10×log10(0.01)=−20dB10×log10(0.001)=−30dB10×log10(0.0005)=−33dB

A ____filter is a filter that passes signals with a frequency lower than a selected cutoff frequency and attenuates signals with frequencies higher than the cutoff frequency.

It uses an antenna to capture radio waves, processes those waves to extract only those waves that are vibrating at the desired frequency, extracts the audio signals that were added to those waves, amplifies the audio signals, and finally plays them on a speaker.

• radio receiver

What is SNRdB?

• 23.8dB

An unmodulated AM signal produces a current of 5.4 A. If the modulation is 100 percent, calculatesada) the carrier power, (b) the total power, (c) the sideband power when it is transmitted through an antenna having an impedance of 50Ω.

• 280 W, 2187.5 W, 750.25 W
• 1458 W, 2187.5 W, 729.25 W
• 1438 W, 2187.5 W, 759.25 W
• 278 W, 2187.5 W, 1917.25 W

What is the carrier frequency in an AM wave when its highest frequency component is 850Hz and the bandwidth of the signal is 50Hz?

• 695 Hz
• 825 Hz
• 625 Hz
• 80 Hz

An AM signal has a total power of 48 Watts with 45% modulation. Calculate the power in the carrier and the sidebands.

• 40.59 W, 4.205 W
• 43.59 W, 2.205 W
• 31.59 W, 8.205 W
• 39.59 W, 4.505 W

One particular advantage of frequency modulation is its resilience to _____________.

• signal level variations

Calculate the power in a DSB SC signal when the modulation is 60% with a carrier power of 600W.

• 300 W
• 108 W
• 600 W
• 540 W

A __ consists of several elements that work together to generate radio waves that contain useful information such as audio, video, or digital data.

• RADIO TRANSMITTER

Baseband transmissions use digital signaling and ____.

______use analog and Frequency Division Multiplexing.

The oscillator usually generates a sine wave, which is referred to as a __.

• CARRIER WAVE

The overall power gain (AT)?

• 20

Frequency modulation is able to provide near interference free reception, and it was for this reason that it was adopted for the VHF sound broadcasts.

• True
• False

____are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared radiation.

The ___ carry the information transmitted by the radio signal.

Whether for __ or mobile radio communication systems, or for use in portable applications, FM is widely used at VHF and above.

• FIXED

_____affects the propagation of waves and signals in electrical circuits, in optical fibers, and in air. Electrical attenuators and optical attenuators are commonly manufactured components in this field.

When this _____________ is exposed to radio waves, the waves induce a very small alternating current in the antenna.

• wire

Optical communication system depends on___ as the medium for communication.

When the __ is modulated onto the radio frequency carrier, the new radio frequency signal moves up and down in frequency.

• AUDIO SIGNAL

TRF receiver and super heterodyne receiver are used for

• Both a and b
• Detection of modulating signal
• Removal of unwanted signal
• None of the above

This is a method of combining more than one signal over a shared medium.

Calculate the power saved in an Amplitude Modulated wave when it is transmitted with 45% modulation – Without carrier, – Without carrier and a sideband

• 82%, 18%
• 90%, 95%
• 82%, 91%
• 68%, 16%

Advantage of using VSB transmission is

• Less power required as compared to DSBSC
• None of the above
• Higher bandwidth than SSB
• Both a and b

For _______________, less bandwidth is used.

• communications purposes

The function of multiplexing is

• To allow multiple data streams over multiple channels in a prescribed format
• To reduce the bandwidth of the signal to be transmitted
• To match the frequencies of the signal at the transmitter as well as the receiver
• To combine multiple data streams over a single data channel

____the corresponding wavelength is 10,000 km.

A range of frequencies within a given band, in particular that used for transmitting a signal.

What’s left after the diode has its way with the alternating current signal is a direct current signal that can be fed to an ______________.

• audio amplifier circuit

A receiver’s sensitivity is the minimum received signal power for the receiver to successfully recover the transmitted signal. If a receiver’s sensitivity is -45 dBm, and the received power is 10 μW, will the receiver be able to recover the transmitted signal?

• Since this value is > receiver sensitivity (-45 dBm), the receiver can recover the signal.
• Since this value is = receiver sensitivity (-45 dBm), the receiver can't recover the signal.
• Since this value is < receiver sensitivity (-45 dBm), the receiver can recover the signal.

For over modulation, the value of modulation index m is

• a. m < 1
• c. m > 1
• b. m -> 1
• d. Not predetermined

Three amplifiers with gains of 12.5, 4, and 20 are cascaded as shown in the following diagram (from left to right). The input power is 120 mW. What is the overall gain and the output powers of each stage?

• P1=Pin×AP1=120mW(12.5)=1500mW=1.5WP2=P1×AP2=1.5W(4)=6WP3=P2×AP3=6W(20)=120WAT=AP1×AP2×AP3=12.5(4)20=1000
• P1=Pout×AP1=120mW(12.5)=150mW=1.5WP2=P1×AP2=1.5W(4)=6WP3=P2×AP3=5W(20)=12.5WAT=AP1×AP2×AP3=12.5(4)20=1000
• P1=Pout×AP1=120mW(125)=150mW=1.5WP2=P1×AP2=150W(4)=6WP3=P2×AP3=6W(20)=1.2WAT=AP1×AP2×AP3=12.5(4)20=1000

If a receiver has poor capacity of blocking adjacent channel interference then the receiver has

• Poor selectivity
• Poor sensitivity
• Poor Signal to noise ratio
• None of the above

Aliasing refers to

• None of the above
• Sampling of signals greater than at Nyquist rate
• Sampling of signals at Nyquist rate
• Sampling of signals less than at Nyquist rate

A 3 GHz carrier is DSB SC modulated by a signal with maximum frequency of 2 MHz. The minimum sampling frequency required for the signal so that the signal is ideally sampled is

• 6 MHz
• 4 MHz
• 6.004 GHz
• 6 GHz

Which is not a kind of Analog modulation ?

• Frequency modulation
• Amplitude modulation
• Band pass modulation

The total power in an Amplitude Modulated signal if the carrier of an AM transmitter is 800 W and it is modulated 50 percent.

• 1000.8 KW
• 750 W
• 850 W
• 900 W

This is a particular frequency at which the attenuation reaches a specified value.

More sophisticated tuners use a variable capacitor (also called a ______________) to vary the frequency.

• tuning capacitor

Responsible for separating the audio information from the carrier wave.

• detector

Calculate the bandwidth occupied by a DSB signal when the modulating frequency lies in the range from 100 Hz to 10KHz.

• 24.5 KHz
• 38.6 KHz
• 28 KHz
• 19.8 KHz

In_____, the amplitude of the carrier wave is varied in proportion to that of the message signal being transmitted.

______system depends on light as the medium for communication.

In_____, carries video using two synchronized signal and ground pairs, termed Y and C.

What happens when the amplitude of the modulating signal is greater than the amplitude of the carrier?

• Amplification
• Attenuation
• Decay
• Distortion

The amount of data transmitted for a given amount of time is called

• Signal power
• Frequency
• Noise
• Bandwidth

What is the overall gain in dB (AT,dB)?

• 13 dB

A radio receiver is the _____________ of a radio transmitter.

• opposite

The ___varies in accordance with the instantaneous amplitude of the modulating signal.

It is possible to apply the modulation to a _____________, and it is not necessary to use a linear form of amplification to increase the power level of the signal to its final value.

• low power stage of the transmitter

Broadcast stations in the VHF portion of the frequency spectrum between _____________ use large values of deviation, typically ±75 kHz.

• 88.5 and 108 MHz

These transmissions could offer high fidelity audio, and for this reason, frequency modulation is far more popular than the older transmissions on the long, medium and __.

• SHORT WAVE BANDS

Calculate the power in one of the side band in SSBSC modulation when the carrier power is 124W and there is 80% modulation depth in the amplitude modulated signal.

• 89.33 W
• 79.36 W
• 102 W
• 64.85 W

Radio waves have frequencies as high as _____to as low as 30 hertz.

To generate a __, the frequency of the radio carrier is changed in line with the amplitude of the incoming audio signal.

• FREQUENCY MODULATED SIGNAL

When AM signal is of 25KHz, calculate the number of channels required in Medium Frequency (MF) band of 300KHz-3000KHz.

• 85
• 69
• 54
• 94

The positive and negative peaks of the carrier wave, are interconnected with an imaginary line. This line helps recreating the exact shape of the modulating signal. This imaginary line on the carrier wave is called as ___.

What is the percentage of modulation if the modulating signal is of 7.5V and carrier is of 9V?

• 91
• 0
• 83.33
• 100

What is the line connecting the positive and negative peaks of the carrier waveform called?

• Modulation index
• Envelope
• Maximum amplitude ceiling
• Peak line

Radio waves have frequencies as high as ____to as low as 30 hertz.

_____is the process of transforming a carrier signal so that it can carry the information of a message signal. It superimposes the contents of the message signal over a high-frequency carrier signal, which is then transmitted over communication channels.

The ability of the receiver to select the wanted signals among the various incoming signals is termed as

• Selectivity
• Sensitivity
• Stability
• None of the above

You can adjust the ______________ by varying the amount of inductance in the coil or the capacitance of the capacitor.

• resonant frequency

As with any form of ______________, it is necessary to be able to successfully demodulate it and recover the original signal.

• modulation

The antenna current of the transmitter is 10A. Find the percentage of modulation when the antenna current increases to 10.4A.

• 28.5%
• 64%
• 40%
• 32%

Many receivers include_____________ to better lock on to the intended frequency — or to produce better-quality audio output — and exclude other signals.

• additional filtering and tuning circuits

______________ FM (NBFM) often uses deviation figures of around ±3 kHz.

• Narrow band

______which is an international, non-profit organization developing voluntary standards for digital television.

Calculate the modulation index when the un modulated carrier power is 15KW, and after modulation, carrier power is 17KW.

• 51.63%
• 82.58%
• 68%
• 34.66%

Convert power gains of 100, 1000 and 2000 to decibel values

• 10×log10(100)=20dB10×log10(1000)=30dB10×log10(2000)=33dB
• 1000×log10(10)=20dB1000×log100(100)=30dB100×log20(2000)=33dB
• 100×log10(100)=20dB100×log10(1000)=30dB100×log10(2000)=33dB

The amplitude of the carrier signal containing ____varies as per the amplitude of the signal containing information, at each instant.

An AM transmitter has an antenna current changing from 5 A un modulated to 5.8 A. What is the percentage of modulation?

• 83.14%
• 25.2%
• 46.8%
• 38.8%

The ____________ is simply a length of wire.

• antenna

In Amplitude Modulation, the instantaneous values of the carrier amplitude changes in accordance with the amplitude and frequency variations of the modulating signal.

• True
• False

It is not possible to change the frequency of a signal to give frequency modulation or FM.

• True
• False

Adds useful information to the carrier wave.

• MODULATOR

A power amplifier has a gain of 55 dB. The input power is 600 mW. What is the output power in W?

Emitter modulator amplifier for Amplitude Modulation

• All of the above
• Has a low efficiency
• Operates in class A mode
• Output power is small

Antenna - RF amplifier - tuner - detector - audio amplifier - _____________

• speaker

The modulation technique that uses the minimum channel bandwidth and transmitted power is

• VSB
• SSB
• DSB-SC
• FM

Function of RF mixer is

• None of the above
• Multiplication of two signals
• Addition of two signals
• Rejection of noise

Synchronous detection means

• Both a and b
• None of the above
• Extracting week signal from noise
• Need a reference signal with predetermined frequency and phase

Demodulation is:

• a. Detection
• b. Recovering information from modulated signal
• c. Both a and b
• d. None of the above

Analog communication indicates:

• All of the above
• No numerical coding
• Continuous signal with varying amplitude or phase
• AM or FM signal

If the input power is 1 dBm, what is the output power in dBm?

• 14 dBm

Types of analog modulation are:

• All of the above
• Phase modulation
• Frequency modulation
• Amplitude modulation

The tuner usually employs the combination of an ____________ (for example, a coil) and a capacitor to form a circuit that resonates at a particular frequency.

• inductor

The more powerful the __, the more powerful the broadcast.

• AMPLIFIER

Advantage of using a high frequency carrier wave is

• All of the above
• Signal can be transmitted over very long distances
• Dissipates very small power
• Antenna height of the transmitter is reduced

AM was the earliest modulation method used for transmitting audio in radio broadcasting. It was developed during the first quarter of the 20th century beginning with ___and Reginald Fessenden's.

In the later part of the 19th century, scientists figured out how to electronically generate __ using electric currents.

• RADIO WAVES

In half Duplex communication system, both the two parties can’t communicate simultaneously.

• True
• False

Standard intermediate frequency used for AM receiver is

• 455 Hz
• None of the above
• 455 KHz
• 455 MHz

It is often necessary to manage the constraints within the loop carefully but once done it provides and excellent solution.

• Phase locked loop

Captures the radio waves.

• antenna

__is the process of converting a digital bit stream into an analog carrier wave for transmission via a communication channel.

• Digital modulation

his component’s job is to amplify the weak signal that comes from the detector so that it can be heard. This can be done using a simple ______________.

• transistor amplifier circuit

Creates alternating current at the frequency on which the transmitter will transmit.

• OSCILLATOR

What is the change in the value of transmitted power when the modulation index changes from 0 to 1?

• Remains unchanged
• 80%
• 50%
• 100%

What is the effect of distortion?

• Total information loss
• Amplified information
• Attenuated information
• Error information

A modulation index of 0.5 would be same as

• A modulation index of 0.5 would be same as
• Select one:
• 0.5 of Modulation Depth
• 5% of Modulation Depth
• 1/2% of Modulation Depth
• 50% of Modulation Depth

Two components are required for radio communication: a transmitter and a __.

• RECEIVER

If an AM signal is represented by v = ( 15 + 3 Sin( 2Π * 5 * 103 t) ) * Sin( 2Π * 0.5 * 106 t) volts. (i) Calculate the values of the frequencies of carrier and modulating signals. (ii) Calculate the value of modulation index. (iii) Calculate the value of bandwidth of this signal.

• 1.6 MHz and 8 KHz, 0.2, 16 KHz
• 1.6 MHz and 8 KHz, 0.6, 16 MHz
• 1.9 MHz and 18 KHz, 0.2, 16 KHz
• 2.4 MHz and 18 KHz, 0.2, 16 KHz

NTSC means ____.

In Amplitude Demodulation, the condition which the load resistor R must satisfy to discharge capacitor C slowly between the positive peaks of the carrier wave so that the capacitor voltage will not discharge at the maximum rate of change of the modulating wave (W is message bandwidth and ω is carrier frequency, in rad/sec ) is

• RC > 1/W
• RC > 1/ω
• RC < 1/W
• RC < 1/ω

For _____________, this can be done with a diode that just rectifies the alternating current signal.

• AM signals

The second, called __ or FM, makes slight increases or decreases the frequency of the carrier wave.

• FREQUENCY MODULATION

Unless you want to listen to every radio channel at the same time, you need a circuit that can pick out just the signals for the channel you want to hear. That’s the role of the ____________.

• tuner

A circuit that can extract signals of a particular frequency from a mix of signals of different frequencies.

• tuner

Amplitude modulation is

• Change in amplitude of the modulating signal according to carrier signal
• Change in amplitude of the carrier according to modulating signal frequency
• Change in amplitude of the carrier according to modulating signal
• Change in frequency of the carrier according to modulating signal

Usually ______________ is allowed for each wide-band FM transmission.

• 200 kHz

In the TV receivers, the device used for tuning the receiver to the incoming signal is

• Low pass filter
• Zener diode
• Varactor diode
• High pass Filter

This frequency, called the _____________, is determined by the values chosen for the coil and the capacitor.

• resonant frequency

___ filter that passes signals with a frequency lower than a selected cutoff frequency and attenuates signals with frequencies higher than the cutoff frequency.

The type of system where the sender has to stop sending the signals to the recipient and then only the recipient can respond

• HALF DUPLEX COMMUNICATION SYSTEM

__is the attenuation of radio energy between the feed points of two antennas that results from the combination of the receiving antenna's capture area plus the obstacle-free, line-of-sight path through free space.

Bandwidth required in SSB-SC signal is (fm is modulating frequency):

• 2fm
• < 2fm
• fm
• > 2fm

Super heterodyne receivers

• All of the above
• Have better sensitivity
• Need extra circuitry for frequency conversion
• Have high selectivity

Calculate the side band power in an SSBSC signal when there is 50% modulation and the carrier power is 50W.

• b. 25 W
• d. 12.5 W
• a. 50 W
• c. 6.25 W

When does over-modulation occur?

• Modulating signal voltage -> Carrier voltage
• Modulating signal voltage < Carrier voltage
• Modulating signal voltage > Carrier voltage
• Modulating signal voltage ->0

What is the effect on the transmitted power of AM signal when the modulation index changes from 0.8 to 1?

• 1.088
• 0.3856
• 0.5
• 0.1364

__is a type of radio receiver used as a component of a radio communication link.

Disadvantage of using a DSB or SSB signal modulation is

• None of the above
• Carrier has to be locally generated at receiver
• Both a and b are correct
• Difficult to recover information at the receiver

The first, called __ or AM, makes slight increases or decreases to the intensity of the carrier wave.

• AMPLIDUTE MODULATION