Communications Systems Key

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1) In a radar, a beam signal is needed in particular direction which is possible if wavelength of signal waves is very small. Since the wavelength of microwave is a few millimeter, hence they are used in radar.

2 The television signals have frequencies in 100-200 MHz range. As ionosphere cannot reflect radio waves of frequency greater than 40 M back to earth, the sky waves cannot be used in the transmission of TV signals.

3) A diode detector should have the following characteristic for proper detection
a) high rectification efficiency.
b) negligible loading effect on previous stage
c) low distortion.

4) Velocity factor (VF) of a cable is the ratio of reduction speed of light in the dielectric of the cable.
Velocity of light in vacuum is 3 x108 m/sec. It reduces when light passes through a medium. Velocity of light in a medium is given by v=c / √ k & where, c-velocity of light in vacuum and k- is the dielectric constant of the medium Vf. =v/c=1/√k
For a line velocity factor is generally of the order of 0.6 to 0.9.

5) Delta modulation involves simple pulse coding and decoding methods. A simple delta modulation uses just one bit per sample i.e. a ‘non-zero’ sample or one per sample. Thus, this method is convenient to use.

6) Two wire transmission line and coaxial cable are employed for AF and UHF region. For optical fiber is employed for optical frequency.

7). Mixing two frequencies across nonlinear impedance.

8). When light strikeds the atoms of the Laser medium, it must stimulate emission rather than be absorbed. This means that more atoms must be in an excited state than in the ground state. This is an unnatural condition and is known as population inversion.

9). Solid Lasers, Semi conductor Laser, Liquid laser, Gas Laser.

10) Laser surgery ,Laser printing, Optical communication .

11) 1)Transmission System, 2) Switching Systems and 3) Signaling Systems.

12) 1 .The length of the antenna required is so large (L = 5000m ) that is practically impossible to set up it.
2 The energy radiated from the antenna in audio frequency range is Practically zero
3 The audio signals transmitted from the different broadcasting stations will get inseparably mixed.

13) Pulse modulation is a system in which continuous wave forms are sampled at regular intervals. Information regarding the signal is transmitted only at the sampling times together with any synchronizing pulses that maybe required. Pulse modulation is the process of transmitting signals in the form of pulses ( dis continuous signals ) by using special techniques.

14) The term channel is commonly used to special the frequency range allotted to a particular transmission from a broadcast station or a transmitter eg a telephone channel is also used for a link in a transmitter and receiver.

15) Atmospheric pressure decreases with in crease in altitude. The high energy particles (ie & rays and cosmic rays) coming from outer space and entering our earths atmosphere cause ionization of the atoms of the gases present there . The ionizing power of these radiation decreases rapidly as they approach the earth. due to decrease in number of collision with the gas atoms . It is due to this reason that the electrical conductivity of earths atmosphere increases with altitude receives the signal

16) Refractive index p of core of core of optical fibre is slightly higher than glass cladding . Light propagates through and along the fibre by the series of bounces caused by internal reflection at the interface of the core and cladding. For total internal reflection the light should enter the fibre at an angle θ in accordance with core of acceptance angle θcNA = sin θc = √(μ12 - μ22 ) Numerical aperature depends upon diameter of the core It decreases as the diameter of core decreases vice versa

17) i) Pulse Amplitude Modulation : Amplitude of the pulse varies in accordance with the modulating signal.

(ii) Pulse Position Modulation. : Pulse position (ie) time of rise or fall of the pulse ) changes with the modulating signal

18)Dielectric loss increase beyond this frequency.

19)How does the effective power radiated by the antenna vary with wavelength? Power is inversely proportional to wave length 20)what should be the length of the dipole antenna for a carrier wave of 5 X 108hz ? L =c/2 ν.

21) four times.

22) d= √2hR
d= √2x 0.1x 6400
= √1280 km
Area covered by broadcaste, A = π d2 = 3.14 x 1280
= 3919.2 km2
population covered = Area x population density
= 3919.2 x 1000 = 3919200

23) (i) 5 MHz <fc sky wave propagation (ionospheric propagation).
(ii) 100 MHz > fc satellite mode of communication.


25) Maximum voltage of AM wave,
Vmax = 16 / 2= 8mV
Minimum voltage of AM wave,
Vmin = 4 / 2 = 2mV
ma = Vmax - Vmin / Vmax + Vmin
= 8-2 / 8 + 2 = 6 / 10 = 0.6

26) The AM wave equation is given by ;
v = 5(1+0.6cos6280t) sin 221 X 104t volts ………….(i)
(i) Maximum amplitude of AM wave
= EC + maEC =5 + 0.6 X 5 = 8V
Minimum amplitude of AM wave
= EC - maEC =5 - 0.6 X 5 = 2V

(ii) The AM wave will contain three frequency viz
fC-fs   ,    fC  ,    fC+fs
336-1     336   336+1
335kHz   336kHz   337kHz

27) (i) The AM wave has sideband frequency of (fc + fs) and (fc - fs).
Sideband frequency = (500+1) kHz  and  (500-1) kHz
501 kHz and 499 kHz
(ii) Bandwidth required = 499 kHz to 501 kHz = 2 kHz

28) PS = ½ ma2PC
1.246 = 1 + ma2 / 2
ma2 / 2 = 0.246
ma = (2 X 0.246)1/2 = 0.701 = 70.1%

29) Modulation index ,
mf = Maximum frequency deviation / Minimum signal frequency

30) d = (2 × 6400 X103 × 160)1/2 = 45255m
Coverage range, d = (2Rh)1/2
h2 = 4h1 = 4 × 160 = 640m
31) Radius of the area covered by TV broadcast is
d = (2Rh)1/2
= 37500m = 37.5 km
= 4.4 × 106

32) Microwave communication channel width =2 / 100 * 10 GHz = 0.2 GHz
band width of channel = 8 KHz
= 2.5 * 104

33) Energy corresponding to λ = 1400nm = 1400 × 10-9 m is
E = hc / λ = 1.42 * 10-19 / 1.6 * 10-19 eV
For detection E must be equal to greater then Eg. Hence only
suitable semiconductor is C.

34) Critical frequency fc and maximum electron density nmax are related as
fc = 9(nmax)1/2
Squaring we get nmax =fc2 / 81
Given fc = 10MHz = 10 * 106 = 107 Hz
ie, nmax = (107)2 / 81
= 1.23 * 1012m- 3


CBSE Physics (Chapter Wise With Hint / Solution) Class XII (By Mr. Sreekumaran Nair)