Ques. Three waves A, B and C of frequencies 1600 kHz, 5 MHz and 60 MHz, respectively are to be transmitted from one place to another. Which of the following is the most appropriate mode of communication?
(a) A is transmitted via space wave while B and C are transmitted via sky wave
(b) A is transmitted via ground wave, B via sky wave and C via space wave
(c) B and C are transmitted via ground wave while A is transmitted via sky wave
(d) B is transmitted via ground wave while A and C are transmitted via space wave
COMMUNICATION SYSTEM
MCQ (Single correct optioon)
Thinking Process
Mode of communication depend on the frequencies of a wave.
Ans. (b)
Mode of communication frequency range
Ground wave propagation –530 kHz to 1710 kHz
Sky wave propagation –1710 kHz to 40 MHz
Space wave propagation –54 MHz to 4.2 GHz
COMMUNICATION SYSTEM
MCQ (Single correct optioon)
Ques. A 100 m long antenna is mounted on a 500 m tall building. The complex can become a transmission tower for waves with λ
(a) ~ 400 m
(b) ~ 25 m
(c) ~ 150 m
(d) ~ 2400 m
COMMUNICATION SYSTEM
MCQ (Single correct optioon)
Ans. (a)
Given, length of the building (l) is given by
l=500m
we know that, wavelength of the wave which can be transmitted by
λ∼4l=4×100=400m
COMMUNICATION SYSTEM
MCQ (Single correct optioon)
Ques. A 1 kW signal is transmitted using a communication channel which provides attenuation at the rate of −2 dB per km. If the communication channel has a total length of 5 km, the power of the signal received is
[gain in dB=10log(PjP0)]
(a) 900 W
(b) 100 W
(c) 990 W
(d) 1010 W
COMMUNICATION SYSTEM
MCQ (Single correct optioon)
Ans. (b)
Given, power of signal transmitted is given Pi = 1 kW = 1000 W
Rate of attenuation of signal = –2 dB/km
Length of total path = 5 km
Thus,
gain in dB = 5×(−2)=−10 dB
Sol. to be continued …
COMMUNICATION SYSTEM
MCQ (Single correct optioon)
Also,
gain in dB = 10logPiP0
Here P0 is the power of the received signal.
Putting the given values in Eq. (i),
−10=10logPiP0=−10logP0Pi
⇒logP0Pi=1⇒logP0Pi=log10
⇒P0Pi=10⇒1000W=10P0
⇒P0=100W
COMMUNICATION SYSTEM
MCQ (Single correct optioon)
Ques. A speech signal of 3 kHz is used to modulate a carrier signal of frequency 1 MHz, using amplitude modulation. The frequencies of the side bands will be
(a) 1.003 MHz and 0.997 MHz
(b) 3001 kHz and 2997 kHz
(c) 1003 kHz and 1000 kHz
(d) 1 MHz and 0.997 MHz
COMMUNICATION SYSTEM
MCQ (Single correct optioon)
Thinking Process
The amplitude modulated signal consists of the carrier wave of frequency ωc with two additional sinusoidal waves, one of frequency (ωc−ωm) and other of frequency (ωc+ωm). These two waves are called side bands and their frequencies are called side band frequency.
Ans.
(a) Given, frequency of carrier signal is ωc=1 MHz and frequency of speech signal = 3 kHz
Sol. to be continued …
COMMUNICATION SYSTEM
MCQ (Single correct optioon)
=3×10−3 MHz
= 0.003 MHz
Now, we know that,
Frequencies of side bands =(ωc±ωm)
=(1±0.003)
= 1.003 MHz and 0.997 MHz
COMMUNICATION SYSTEM
MCQ (Single correct optioon)
Ques. A mesage signal of frequency ωm is superposed on a carrier wave of frequency ωc to get an Amplitude Modulated Wave (AM). The frequency of the AM wave will be
(a) ωm
(b) ωc
(c) 2ωc+ωm
(d) 2ωc−ωm
COMMUNICATION SYSTEM
MCQ (Single correct optioon)
Thinking Process
In amplitude modulation, the frequency of modulated wave is equal to the frequency of carrier wave.
Ans.
(b) Here, according to the question, frequency of carrier wave is ωc.
Thus the amplitude modulated wave also has frequency ωc.
COMMUNICATION SYSTEM
MCQ (Single correct optioon)
Ques. I-V characteristics of four devices are shown in figure.
Identify devices that can be used for modulation
Ques. to be continued …
COMMUNICATION SYSTEM
MCQ (Single correct optioon)
(a) (i) and (iii)
(b) only (iii)
(c) (ii) and some regions of (iv)
(d) All the devices can be used
COMMUNICATION SYSTEM
MCQ (Single correct optioon)
Thinking Process
A square law device is something where either current or voltage depends on the square of the other.
Ans.
(c) The device which follows square law is used for modulation purpose. Characteristics shown by (i) and (iii) corresponds to linear devices.
Characteristics shown by (ii) corresponds to square law device. Some part of (i) also follow square law.
Hence, (ii) and (iv) can be used for modulation.
COMMUNICATION SYSTEM
MCQ (Single correct optioon)
Ques. A mae voice after modulation-transmission sounds like that of a female to the receiver. The problem is due to
(a) poor selection of modulation index (selected 0 < m < 1)
(b) poor bandwidth selection of amplifiers
(c) poor selection of carrier frequency
(d) loss of energy in transmission.
COMMUNICATION SYSTEM
MCQ (Single correct optioon)
Thinking Process
The frequency of male voice less than that of a female voice.
Ans.
(b) Here, in this question, the frequency of modulated signal received becomes more, which is possible with the poor bandwidth selection of amplifiers.
This happens because bandwidth in amplitude modulation is equal to twice the frequency of modulating signal.
But, the frequency of male voice is less than that of a female.
COMMUNICATION SYSTEM
MCQ (Single correct optioon)
Ques. A basic communication system consists of
Choose the correct sequence in which these are arranged in a basic communication system:
(a) ABCDE
(b) BADEC
(c) BDACE
(d) BEADC
COMMUNICATION SYSTEM
MCQ (Single correct optioon)
Ans.
(b) A communication system is the set-up used in the transmission and reception of information from one place to another.
The whole system consist of several elements in a sequence. It can be represented as the diagram given below
COMMUNICATION SYSTEM
MCQ (Single correct optioon)
Ques. Identify the mathematical expression for amplitude modulated wave
(a) Acsin[ωc+k1Vm(t)t+ϕ]
(b) Acsinωct+ϕ+k2Vm(t)
(c) Ac+k2Vm(t)sin(ωct+ϕ)
(d) AcVm(t)sin(ωct+ϕ)
COMMUNICATION SYSTEM
MCQ (Single correct optioon)
Thinking Process
An arbitrary change in phase angle of the modulating signal is given by ϕ.
Ans.
(c) Consider a sinusoidal modulating signal represented by
m(t)=Amsinωmt
where, Am= Amplitude of modulating signal ωm= Angular frequency =2πVm=ϕVm
Sol. to be continued …
COMMUNICATION SYSTEM
MCQ (Single correct optioon)
Also consider a sinusoidal carrier wave represented by C(t)=Acsinωct
Thus, modulated wave is given by
Here,
Cm(t)=(Ac+Amsinωmt)sinωct
=Ac[1+AcAmsinωmt)sinωct
⇒AcAm=M
Sol. to be continued …
COMMUNICATION SYSTEM
MCQ (Single correct optioon)
⇒Cm(t)=(Ac+Ac×μsinωmt)sinωct
Now, we know that Ac×μ=K [wave constant] and sinωmt=Vm [wave velocity]
Thus, Eq. (iii) becomes
Cm(t)=(Ac+K×Vm)sinωct
Now, consider a change in phase angle by ϕ then sinωct→sin(ωct+ϕ)
Thus,
Cm(t)=(Ac+KVm)(sinωc+ϕ)
COMMUNICATION SYSTEM
MCQ (More than one correct option)
Ques. An audio signal of 15 kHz frequency cannot be transmitted over long distances without modulation, because
(a) the size of the required antenna would be at least 5 km which is not convenient
(b) the audio signal can not be transmitted through sky waves
(c) the size of the required antenna would be at least 20 km, which is not convenient
(d) effective power transmitted would be very low, if the size of the antenna is less than 5 km
COMMUNICATION SYSTEM
MCQ (More than one correct option)
Thinking Process
Transmission of a signal depends on three factors. These are size of antenna, medium of transmission and power of transmitted wave.
Ans. (a, b, d)
Given, frequency of the wave to be transmitted is
vm=15kHz=15×103 Hz
Wavelength λm=vmc=15×1033×108=51×105m
Sol. to be continued …
COMMUNICATION SYSTEM
MCQ (More than one correct option)
Size of the antenna required, l=4λ=41×51×105
=5×103m = 5 km
The audio signals are of low frequency waves. Thus, they cannot be transmitted through sky waves as they are absorbed by atmosphere.
If the size of the antenna is less than 5 km, the effective power transmission would be very low because of deviation from resonance wavelength of wave and antenna length.
COMMUNICATION SYSTEM
MCQ (More than one correct option)
Ques. Audio sine waves of 3 kHz frequency are used to amplitude modulate a carrier signal of 1.5 MHz. Which of the following statements are true?
(a) The side band frequencies are 1506 kHz and 1494 kHz
(b) The bandwidth required for amplitude modulation is 6 kHz
(c) The bandwidth required for amplitude modulation is 3 MHz
(d) The side band frequencies are 1503 kHz and 1497 kHz
COMMUNICATION SYSTEM
MCQ (More than one correct option)
Thinking Process
Here, in this question, options are giving the value of side band frequencies and band width of amplitude modulation. So, first of all find this quantities.
Ans. (b, d)
Given,
ωm=3kHz
ωc=1.5MHz=1500kHz
Sol. to be continued …
COMMUNICATION SYSTEM
MCQ (More than one correct option)
Now, side band frequencies
ωc±ωm=(1500±3)
= 1503 kHz and 1497 kHz
Also, bandwidth =2ωm=2×3=6kHz
COMMUNICATION SYSTEM
MCQ (More than one correct option)
Ques. A TV transmission tower has a height of 240 m. Signals broadcast from this tower will be received by LOS communication at a distance of (assume the radius of earth to be (6.4×106m)
(a) 100 km
(b) 24 km
(c) 55 km
(d) 50 km
COMMUNICATION SYSTEM
MCQ (More than one correct option)
Thinking Process
Range dT=2RhT
Ans. (b, c, d)
Given, height of tower h = 240 m
For LOS (line of sight) communication.
The maximum distance on earth from the transmitter upto which a signal can be received is given by
Sol. to be continued …
COMMUNICATION SYSTEM
MCQ (More than one correct option)
d=2Rh
Here R is the radius of the earth i.e., R=6.4×106m
Putting all these values in Eq. (i), we get
d=2Rh=2×6.4×106×240
=55.4×103m=55.4km
Thus, the range of 55.4 km covers the distance 24 km, 55 km and 50 km.
COMMUNICATION SYSTEM
MCQ (More than one correct option)
Ques. The frequency response curve (figure) for the filter circuit used for production of AM wave should be
(a) (i) followed by (ii) (b) (ii) followed by (i) (c) (iii) (d) (iv)
COMMUNICATION SYSTEM
MCQ (More than one correct option)
Ans. (a, b, c)
Here, for the production of amplitude modulated wave, bandwidth is given by = frequency of upper side band - frequency of lower side band
=ωUSB−ωLSB=(ωc+ωm)−(ωc−ωm)
COMMUNICATION SYSTEM
MCQ (More than one correct option)
Ques. In amplitude modulation, the modulation index m, is kept less than or equal to 1 because
(a) m>1, will result in interference between carrier frequency and message frequency, resulting into distortion
(b) m>1, will result in overlapping of both side bands resulting into loss of information
(c) m>1, will result in change in phase between carrier signal and message signal
(d) m>1, indicates amplitude of message signal greater than amplitude of carrier signal resulting into distortion
COMMUNICATION SYSTEM
MCQ (More than one correct option)
Ans. (b, d)
The modulation index (m) of amplitude modulated wave is
m= amplitude of carrier signal (Ac) amplitude of message signal (Am)
If m>1, then Am>Ac.
In this situation, there will be distortion of the resulting signal of amplitude modulated wave. Maximum modulation frequency (mf) of Am wave is
Sol. to be continued …
COMMUNICATION SYSTEM
MCQ (More than one correct option)
mf=vm(max)Δvmax
= maximum frequency value of modulating wave frequency deviation
If mt>1, then Δvmax>vm. It means, there will be overlapping of both side bands of modulated wave resulting into loss of information.
COMMUNICATION SYSTEM
Very short answer type questions
Ques. 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
COMMUNICATION SYSTEM
Very short answer type questions
Ans.
Analog and digital signals are used to transmit information, usually through electric signals. In both these technologies, the information such as any audio or video is transformed into electric signals.
The difference between analog and digital technologies is that in analog technology, information is translated into electric pulses of varying amplitude. In digital technology, translation of information is into binary formal (zero or one) where each bit is representative of two distinct amplitudes.
Thus, (i) and
(ii) would produce analog signal and
(iii) and
(iv) would produce digital signals.
COMMUNICATION SYSTEM
Very short answer type questions
Ques. Would sky waves be suitable for transmission of TV signals of 60 MHz frequency?
COMMUNICATION SYSTEM
Very short answer type questions
Ans.
A signal to be transmitted through sky waves must have a frequency range of 1710 kHz to 40 MHz.
But, here the frequency of TV signals are 60 MHz which is beyond the required range.
So, sky waves will not be suitable for transmission of TV signals of 60 MHz frequency.
COMMUNICATION SYSTEM
Very short answer type questions
Ques. 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?
COMMUNICATION SYSTEM
Very short answer type questions
Ans.
As the frequency of wave B is more than wave A, it means the refractive index of wave B is more than refractive index of wave A (as refractive index increases with frequency increases).
For higher frequency wave (i.e., higher refractive index) the angle of refraction is less i.e., bending is less. So, wave B travel longer distance in the ionosphere before suffering total internal reflection.
COMMUNICATION SYSTEM
Very short answer type questions
Ques. 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?
COMMUNICATION SYSTEM
Very short answer type questions
Ans.
Let Ac and Am be the amplitudes of carrier wave and modulating wave respectively. So,
Maximum amplitude →Amax=Ac+Am=15V
Minimum amplitude →Amin=Ac−Am=3V
Adding Eqs. (i) and (ii), we get
2Ac=18
Sol. to be continued …
COMMUNICATION SYSTEM
Very short answer type questions
Ac=9V
Am=15−9=6V
Modulating index of wave μ=AcAm=96=32
COMMUNICATION SYSTEM
Very short answer type questions
Ques. Compute the LC product of a tuned amplifier circuit required to generate a carrier wave of 1 MHz for amplitude modulation.
COMMUNICATION SYSTEM
Very short answer type questions
Thinking Process
For tuned amplifier f=2πLC1
Ans.
Given, the frequency of carrier wave is 1 MHz.
Formula for the frequency of tuned amplifier,
2πLC1=1MHz
Sol. to be continued …
COMMUNICATION SYSTEM
Very short answer type questions
LC=2π×1061
LC=(2π×106)21=2.54×10−14s
Thus, the product of LC is 2.54×10−14s
COMMUNICATION SYSTEM
Very short answer type questions
Ques. Why is a AM signal likely to be more noisy than a FM signal upon transmission through a channel?
COMMUNICATION SYSTEM
Very short answer type questions
Ans.
In case of AM, the instantaneous voltage of carrier waves is varied by the modulating wave voltage. So, during the transmission, nosie signals can also be added and receiver assumes noise a part of the modulating signal.
In case of FM, the frequency of carrier waves is changed as the change in the instantaneous voltage of modulating waves. This can be done by mixing and not while the signal is transmitting in channel. So, noise does not affect FM signal.
COMMUNICATION SYSTEM
Short answer type questions
Ques. Figure shows a communication system. What is the output power when input signal is of 1.01 mW ?
[gain in dB = 10 log10(P0/Pi)]
COMMUNICATION SYSTEM
Short answer type questions
Ans.
The distance travelled by the signal is 5 km Loss suffered in path of transmission = 2 dB/km
So, total loss suffered in 5 km = −2×5=−10 dB
Total amplifier gain =10dB+20dB=30dB
Overall gain in signal =30−10=20dB
According to the question, gain in dB=10log10PjP0
Sol. to be continued …
COMMUNICATION SYSTEM
Short answer type questions
∴20=10log10PiP0
orlog10PiP0=2
Here, Pi=1.01mW and P0 is the output power.
∴PiP0=102=100
⇒P0=Pi×100=1.01×100
or P0=101mW
Thus, the output power is 101 mW.
COMMUNICATION SYSTEM
Short answer type questions
Ques. 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).
COMMUNICATION SYSTEM
Short answer type questions
Ans.
Given, height of antenna h = 20 m
Radius of earth = 6.4×106m
At the ground level,
(i) Range =2hR=2×20×6.4×106
= 16000 m = 16 km
Sol. to be continued …
COMMUNICATION SYSTEM
Short answer type questions
Area covered A =π( range )2
=3.14×16×16=803.84km2
(ii) At a height of H = 25 m from ground level
Range =2hR+2HR
=2×20×6.4×106+2×25×6.4×106
=16×103+17.9×103
Sol. to be continued …
COMMUNICATION SYSTEM
Short answer type questions
=33.9×103m
=33.9km
Area covered =π( Range )2
=3.14×33.9×33.9
=3608.52km2
Percentage increase in area = Initial area Difference in area ×100
Sol. to be continued …
COMMUNICATION SYSTEM
Short answer type questions
=803.84(3608.52−803.84)×100
=348.9%
Thus, the percentage increase in area covered is 348.9%
COMMUNICATION SYSTEM
Short answer type questions
Ques. 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 earth’s radius.
COMMUNICATION SYSTEM
Short answer type questions
Thinking Process
Range dT=2Rht
Ans.
Consider the figure given below to solve this question
COMMUNICATION SYSTEM
Short answer type questions
Suppose the height of transmitting antenna or receiving antenna in order to cover the entire surface of earth through communication is ht and radius of earth is R
Then, maximum distance
dm2=(R+ht)2+(R+ht)2
=2(R+ht)2
∴dm=2htR+2htR=22htR
Sol. to be continued …
COMMUNICATION SYSTEM
Short answer type questions
⇒8htR=2(R+ht)2
⇒4htR=R2+2Rht+ht2
⇒R2−2htR+ht2=0
⇒(R−ht)2=0
⇒R=ht
Since, space wave frequency is used so λ«ht, hence only tower height is to be taken into consideration. In three dimensions of earth, 6 antenna towers of each of height ht=R would be used to cover the entire surface of earth with communication programme.
COMMUNICATION SYSTEM
Short answer type questions
Ques. 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 5 MHz are not received by reflection from the F1 layer of the ionosphere while signals of frequencies higher than 8 MHz are not received by reflection from the F2 layer of the ionosphere. Estimate the maximum electron densities of the F1 and F2 layers on that day.
COMMUNICATION SYSTEM
Short answer type questions
Ans.
The maximum frequency for reflection of sky waves
fmax=9(Nmax)1/2
where, Nmax is a maximum electron density.
For F1 layer,
fmax = 5 MHz
Sol. to be continued …
COMMUNICATION SYSTEM
Short answer type questions
5×106=9(Nmax)1/2
Maximum electron density
Nmax=95×1062=3.086×1011/m3
fmax=8MHz
8×106=9(Nmax)1/2
Maximum electron density
Nmax=98×106=7.9×1011/m3
COMMUNICATION SYSTEM
Short answer type questions
Ques. On radiating (sending out) and AM modulated signal, the total radiated power is due to energy carried by ωc,ωc−ωm and ωc+ωm. Suggest ways to minimise cost of radiation without compromising on information.
COMMUNICATION SYSTEM
Short answer type questions
Ans.
In amplitude modulated signal, only side band frequencies contain information. Thus only (ωc+ωm) and (ωc−ωm) contain information.
Now, according to question, the total radiated power is due to energy carried by
ωc,(ωc−ωm) and (ωc+ωm).
Thus to minimise the cost of radiation without compromising on information ωc can be left and transmitting. (ωc+ωm),(ωc−ωm) or both (ωc+ωm) and (ωc−ωm).
COMMUNICATION SYSTEM
Long answer type questions
Ques. (i) The intensity of a light pulse travelling along a communication channel decreases exponentially with distance x according to the relation I=I0e−αx, where I0 is the intensity at x=0 and α is the attenuation constant.
Show that the intensity reduces by 75 after a distance of αln4.
(ii) Attenuation of a signal can be expressed in decibel (dB) according to the relation dB=10log10I0I. What is the attenuation in dB/km for an optical fibre in which the intensity falls by 50% over a distance of 50 km ?
COMMUNICATION SYSTEM
Long answer type questions
Ans.
(i) Given, the intensity of a light pulse I=I0e−αx
where, I0 is the intensity at x=0 and α is constant.
According to the question, I=25% of I0=10025.I0=4I0
Using the formula mentioned in the question,
or
Sol. to be continued …
COMMUNICATION SYSTEM
Long answer type questions
I=I0e−αx
4I0=I0e−αx
41=e−αx
Taking log on both sides, we get
ln1−ln4=−αxlne(∵lne=1)
−ln4=−αx
Sol. to be continued …
COMMUNICATION SYSTEM
Long answer type questions
x=αln4
Therefore, at distance x=αln4, the intensity is reduced to 75% of initial intensity.
(ii) Let α be the attenuation in dB/km. If x is the distance travelled by signal, then
10log10I0I=−αx
where, I0 is the intensity initially.
According to the question,
Sol. to be continued …
COMMUNICATION SYSTEM
Long answer type questions
I=50% of I0=2I0 and x=50km
Putting the value of x in Eq. (i), we get
10log102I0I0=−α×50
10[log1−log2]=−50α
5010×0.3010=α
∴ The attenuation for an optical fibre
α=0.0602 dB/km
COMMUNICATION SYSTEM
Long answer type questions
Ques. 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?
COMMUNICATION SYSTEM
Long answer type questions
Ans.
Let the receiver is at point A and source is at B.
Sol. to be continued …
COMMUNICATION SYSTEM
Long answer type questions
Velocity of waves =3×108 m/s
Time to reach a receiver = 4.04 ms = 4.04×10−3s
Let the height of satellite is
hs=600km
Radius of earth = 6400 km
Size of transmitting antenna =hT
Sol. to be continued …
COMMUNICATION SYSTEM
Long answer type questions
We know that Time Distance travelled by wave = Velocity of waves
4.04×10−32x=3×108
x=23×108×4.04×10−3
=6.06×105=606km
or
Using Phythagoras theorem,
Sol. to be continued …
COMMUNICATION SYSTEM
Long answer type questions
or
d2=x2−hs2=(606)2−(600)2=7236
So, the distance between source and receiver =2d
=2×85.06=170km
The maximum distance covered on ground from the transmitter by emitted EM waves
or
Sol. to be continued …
COMMUNICATION SYSTEM
Long answer type questions
d=2RhT
2Rd2=hT
or
size of antenna hT=2×64007236
= 0.565 km = 565 m
COMMUNICATION SYSTEM
Long answer type questions
Ques. An amplitude modulated wave is as shown in figure. Calculate
(ii) Peak carrier voltage, Vc=2Vmax+Vmin=250+10=30V
(iii) Peak value of information voltage,
Vm=μVc=10066.67×30=20V
COMMUNICATION SYSTEM
Long answer type questions
Ques. (i) Draw the plot of amplitude versus ω for an amplitude modulated were whose carrier wave (ωc) is carrying two modulating signals, ω1 and ω2(ω2>ω1).
(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?
COMMUNICATION SYSTEM
Long answer type questions
Ans.
(i) The plot of amplitude versus ω can be shown in the figure below
Sol. to be continued …
COMMUNICATION SYSTEM
Long answer type questions
(ii) From figure, we note that frequency spectrum is not symmetrical about ωc. Crowding of spectrum is present for ω<ωt.
(iii) If more waves are to be modulated then there will be more crowding in the modulating signal in the region ω<ωc. That will result more chances of mixing of signals.
(iv) To accommodate more signals, we should increase bandwidth and frequency carrier waves ωt. This shows that large carrier frequency enables to carry more information (i.e., more ωm ) and the same will in turn increase bandwidth.
COMMUNICATION SYSTEM
Long answer type questions
Ques. An audio signal is modulated by a carrier wave of 20 MHz such that the bandwidth required for modulation is 3 kHz. Could this wave be demodulated by a diode detector which has the values of R and C as
(i) R=1kΩ,C=0.01μF
(ii) R=10kΩ,C=0.01μF
(iii) R=10kΩ,C=0.1μF
COMMUNICATION SYSTEM
Long answer type questions
Ans.
Given, carrier wave frequency fc=20MHz
=20×106Hz
Bandwidth required for modulation is
2fm=3kHz=3×103Hz
⇒fm=23×103=1.5×103Hz
Sol. to be continued …
COMMUNICATION SYSTEM
Long answer type questions
Demodulation by a diode is possible if the condition fc1≪RC<fm1 is satisfied
Thus,
fc1=20×1061=0.5×10−7
fm1=1.5×1031Hz=0.7×10−3s
and
Now, gain through all the options of R and C one by one, we get
Sol. to be continued …
COMMUNICATION SYSTEM
Long answer type questions
(i) RC=1kΩ×0.01μF=103Ω×(0.01×10−6F)=10−5S
Here, condition fc1≪RC<fm1 is satisfied.
Hence it can be demodulated.
(ii) RC=10KΩ×0.01μF=104Ω×10−8F=10−4S
Here condition fc1≪RC<fm1 is satisfied.
Hence, it can be demodulated.
Sol. to be continued …
COMMUNICATION SYSTEM
Long answer type questions
(iii) RC=10kΩ×1μμF=104Ω×10−12F=10−8S
Here, condition fc1>RC, so this cannot be demodulated.
The emf induced across PQ due to its motion or change in magnetic flux linked with the loop change due to change of enclosed area.
For nth minima to be formed on the screen path difference between the rays coming from S1 and S2 must be (2n−1)2λ.