CHAPTER -15 COMMUNICATION SYSTEMS

An audio signal is modulated by a carrier wave of 20MHz such
that the bandwidth required for modulation is 3kHz. Could this
wave be demodulated by a diode detector which has the values
of R and C as
(i) R = 1 kΩ, C = 0.01μF.
(ii) R = 10 kΩ, C = 0.01μF.
(iii) R = 10 kΩ, C = 0.1μF.

(i) Draw the plot of amplitude versus ‘ω’ for an amplitude
modulated wave whose carrier wave (ωc) is carrying two
modulating signals, ω1 and ω2 (ω2 > ω1). [Hint: Follow
derivation from Eq 15.6 of NCERT Textbook of XII]
(ii) Is the plot symmetrical about ωc? Comment especially about
plot in region ω < ωc.
(iii) Extrapolate and predict the problems one can expect if more
waves are to be modulated.
(iv) Suggest solutions to the above problem. In the process can
one understand another advantage of modulation in terms
of bandwidth?

A 50 MHz sky wave takes 4.04 ms to reach a receiver via
re-transmission from a satellite 600 km above earth’s surface.
Assuming re-transmission time by satellite negligible, find
the distance between source and receiver. If communication
between the two was to be done by Line of Sight (LOS) method,
what should size and placement of receiving and transmitting
antenna be?

On radiating (sending out) an AM modulated signal, the total
radiated power is due to energy carried by ωc, ωc – ωm & ωc + ωm.
Suggest ways to minimise cost of radiation without
compromising on information.

The maximum frequency for reflection of sky waves from a certain
layer of the ionosphere is found to be fmax = 9(Nmax)1/2, where Nmax
is the maximum electron density at that layer of the ionosphere.
On a certain day it is observed that signals of frequencies higher
than 5MHz are not received by reflection from the F1 layer of the
ionosphere while signals of frequencies higher than 8MHz are
not received by reflection from the F2 layer of the inonosphere.
Estimate the maximum electron densities of the F1 and F2 layers
on that day.

If the whole earth is to be connected by LOS communication
using space waves (no restriction of antenna size or tower height),
what is the minimum number of antennas required? Calculate
the tower height of these antennas in terms of earths radius?

A TV transmission tower antenna is at a height of 20 m. How
much service area can it cover if the receiving antenna is (i) at
ground level, (ii) at a height of 25 m? Calculate the percentage
increase in area covered in case (ii) relative to case (i).

Why is a AM signal likely to be more noisy than a FM signal
upon transmission through a channel?

Compute the LC product of a tuned amplifier circuit required to
generate a carrier wave of 1 MHz for amplitude modulation.

The maximum amplitude of an A.M. wave is found to be 15 V
while its minimum amplitude is found to be 3 V. What is the
modulation index?

Two waves A and B of frequencies 2 MHz and 3 MHz, respectively
are beamed in the same direction for communication via sky wave.
Which one of these is likely to travel longer distance in the
ionosphere before suffering total internal reflection?

Would sky waves be suitable for transmission of TV signals of
60 MHz frequency?

Which of the following would produce analog signals and which
would produce digital signals?
(i) A vibrating tuning fork.
(ii) Musical sound due to a vibrating sitar string.
(iii) Light pulse.
(iv) Output of NAND gate.

Draw a plot of the variation of amplitude versus W for an amplitude modulated wave. Define modulation index, State  its importance for effective amplitude modulation. What is the role of a band pass filter in Amplitude Modulation.

What do you mean by demodulation with the help of block diagram explain detection of AM signal is carried out.

Define modulation index. Give its physical significance.

                What do you mean by amplitude modulation with the help of block diagram explain how AM signal is achieved.

Write any two factors which justify the need for modulating a signal. What do you mean by amplitude modulation? Draw a diagram showing an amplitude modulation wave by superposing a modulating signal over  a sinusoidal carrier wave.

Draw a schematics diagram showing the (i) Ground wave (ii) Sky wave and (iii) Space wave propagation modes for e m waves.

                Write the frequency range for each of the following.

(i)                  Standard AM broadcast

(ii)                Television

(iii)               Satellite Commubication 

What is space wave propagation? Give two examples of communication system which use space wave mode. A TV tower is 80 m tall, calculate the maximum distance upto  which the signal transmitting from the tower can be received.

What is sky wave proportion of waves? Explain why sky wave transmission of em wave can not be used for TV transmission.

What is ground wave? Why is ground wave transmission of signals restricted to a frequency of 1500 KHz?

What do you mean by modulation.

What type of modulation is required for television broadcast of voice signal.

What type of modulation is required for television broadcast?

What is the requirement of transmitting microwaves from one to another on the earth?

Bolck diagram of a receiver is as shown.

What is transducer. 

How does the effective power radiated by an antenna vary with wavelength? 

Name the device which can represent digital data by analog signal and vice-versa.

Name the types of communication systems according to the mode of the transmission.

Name the type of communication in which the signal is a discrete and binary coded version of the message of information.

For an amplitude modulated wave, the maximum amplitude is found to be 10 V while the minimum amplitude is found to be 2 V. Determine the modulation index ÃŽÂ¼. What would be the value of ÃŽÂ¼ if the minimum amplitude is zero volt?

A modulating signal is a square wave, as shown in Fig. 15.14.

The carrier wave is given by 

(i) Sketch the amplitude modulated waveform

(ii) What is the modulation index?

 A carrier wave of peak voltage 12V is used to transmit a message signal. What should be the peak voltage of the modulating signal in order to have a modulation index of 75%?

 Is it necessary for a transmitting antenna to be at the same height that of the receiving antenna for line-of-sight communication? A TV transmitting antenna is 81m tall. How much service area can it cover if the receiving antenna is at the ground level?

Digital signals

(i) Do not provide a continuous set of values,

(ii) Represent values as discrete steps,

(iii) Can utilize binary system, and

(iv) Can utilize decimal as well as binary systems.

Which of the above statements are true?

(a) (i) and (ii) only

(b) (ii) and (iii) only

(c) (i), (ii) and (iii) but not (iv)

(d) All of (i), (ii), (iii) and (iv).

Frequencies in the UHF range normally propagate by means of:

(a) Ground waves.

(b) Sky waves.

(c) Surface waves.

(d) Space waves.

Which of the following frequencies will be suitable for beyond-the-horizon communication using sky waves?

(a) 10 kHz

(b) 10 MHz

(c) 1 GHz

(d) 1000 GHz