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Subject Code: 12118
SUMMER – 12 EXAMINATIONS
Model Answer
Page No
Q1. Value of intermediate frequency IF for.
i) If value for
a) A M radio receiver :For MW.band = 455 kHz
Or
For sw band=1.6 to 2.3 MHz.
b) FM radio receiver = 10.7mhz
ii) Block dia of communication system
(1mark)
(1mark)
(2mark)
iii) Define.
a) Polarization -:
(1mark)
Polarization is defined as the direction of the electric vector in the
electromagnetic wave radiated by the transmitting antenna.
b) Beam width -:
(1mark)
It is define as the angular separation between. the two half power points on the
power density radiation patterns.
iv) Representation of AM wave in time domain -:
Representation of AM in frequency domain -:
(1mark)
(1mark)
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v) Differentiate between simplex & duplex mode.
(2mark)
Sr.No Simplex Communication
Duplex Communication
1
2
It is unidirectional communication
system means we can only receive the
information
It is bidirectional communication
system. Means we receive as well as
transmit the information.
Eg:- TV broadcasting, radio
broadcasting
Eg:- walky-talky, citizen radio,
armature radio, telephone system,
mobile communication.
Sketch-:
Sketch-: half duplex
Transmit only
Receiver
Transmitter
Transmitter
Receiver
Transmit only
Receive only
Full duplex
Transmitter
Receiver
Transmit as well as receive the
information
i-uda antenna :-
vi) Y
a
g
( 1mark)
Radiation patterern
(1mark)
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vii) Bandwith for AM= 2fm
(1mark)
Bandwith for FM=2[  +fm (max)]
(1mark)
OR
FM = 2fm * no. of sidebands
viii)
Define
a) Amplitude modulation -:
(1mark)
It is defined as the technique of modulation in which amplitude of carrier signal is varied in
accordance with the instantaneous amplitude of modulation signal keeping frequency & phase
constant..
b) Frequency modulation -:
(1mark)
It is defined as the technique of modulation in which the frequency of carrier signal is varied
in accordance with the amplitude of modulation signal keeping amplitude phase constant.
Q1 b)
i)
Power relation in AM
(4mark)
Total power in AM (Pt) = (carrier power )+ (power in USB)+(Power in LSB)
i.e.
Pt = pc+PusB+ PlsB
Where;
Pc =
1
E 2 carr
R
( EC / 2 )
Pc =
R
Pc =
Similary, Pusb =
2
EC 2
2R
2
E 2 sb
PLsB =
R
Where peak amplitude of sideband =
PUSB = P2LB=
ME C
2
(mEC / 2 2 ) 2
R
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2
E
m 2 Ec 2
PU5B =PL5B=
But C  Pc
*
2R
4
2R
PUSB=PLSB =
m2
Pc
4
3
From eq 1 & 2; eqn I becomes,
Pt= Pc +
m2
m2
m2
PC 
PC 
PC
4
4
4
Pt= (1 
m2
) PC
2
Q1. b ii) Advantages of digital Communication system
1)
2)
3)
4)
(each point 1 mark)
Immunity to transmission noise & interference
Communication is kept private & secured.
High Speed.
Use for long distance communication such as between earth & Space.
iii) Black dig. Of FM receiver
RF Amplifiers –
(2marks)
(2mark)
RF amplifier used to improve the signal to noise ratio. It is also used to match t he
input impedance with the antenna impedance.
Mixers –
It is used to mix the input signal frequency (fs) & local Oscillator frequency (f0)
signal& convert the received signal to intermediate frequency(IF)
IF amplifiers –
It amplifies the IF signal of mixer output.
Amplitude Limiter –
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It removes the unwanted amplitude that is added in original FM signal while travelling in free
space. It is removed before demodulation otherwise distortion appears at output.
FM Detector –
IT converts the FM Signal into original modulating signal.
De-emphasis –
The artificially boosted high frequencies at transmitter are removed by de- emphasis.
AF & Power Amplifier –
First the modulating signal is voltage. Amplified & its power is increased to drive the
loudspeaker.
AGC:Automatic gain control is used to ensure that the signal fed to the limiter is within it limiting
range & also prevents overloading of last IF amplifier.
Loudspeaker:It converts modulating signal into sound information.
Q2 any four
(16mark)
a) Equivalent Circuit of a transmission line -:
(2mark)
Each Conductor has a certain length & diameter, so it will have a resistance & inductance.
Since there are two wires close to each other, therefore there will be capacitance
The wires are separated by a medium called the dielectric, which cannot perfect in its
insulation, the current leakage. Through it, can be represented by a shunt conductance
RF equivalent Circuit –
(2mark)
At radio frequencies (RF) the inductive reactance is much larger than the resistance
similarly the capacitive susceptance is also much larger than the shunt conductance. So, Both
R & G may be ignored, which result a lossless line.
RF equivalent circuit is given as
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Q 2 b. Explain the following
i)
Critical frequency –
( 4mark)
( 1mark)
The critical frequency of a layer is defined as the maximum frequency that is returned back to
the an angle 900 to it
It is denoted by (FC).
The critical frequency for F2 layer is between 5MHZ
ii)
MUF – Maximum usable frequency
( 1mark)
The maximum possible value of frequency for which reflection takes place for a given usable
frequency. For that distance & for the given ionosphere. MUF can be used for sky-wave
propagation normal value of MUF vary from 8 MHz to 35 MHz
iii)
Virtual height –
( 1mark)
The incident & refracted rays follow path that are exactly the same as they would have been if
reflection had taken place from a surface located at a greater height, called virtual height of
this layer.
iv)
Skip distance –
( 1mark)
The skip distance is defined as the shortest distance from a transmitter, measured along the
surface of earth at which a sky wave of fixed frequency returns back to the earth.
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Q2 c) Mathematical expression for AM wave
(4mark)
The modulating signal is represented by
Em= em sinwmt
I
Where em= Instantaneous amplitude of modulating signal
Em=Peak amplitude
Wm= Angular frequency
=2 fm
Fm= Frequency of modulating signal.
The Carrier signal is represent by
Ec = Ec Sinwc t
II
Where
e c= Instantaneous amplitude of carrier signal
Ec = Peck amplitude of carrier signal.
Wc= Angular frequency of a carrier signal = 2 fc
&
Fc= Frequency of carrier signal.
But the instantaneous value of envelope of AM wave is given by,
A= Ec+ em
A= Ec +emsinwmt ------from eq
I
eAm = A sinwct
eAm = (Ec+Emsinwmt)Sinwct
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Q2 d) Ratio detector
(2mark)
Explanation
(2mark)
Modification of phase discriminator by adding amplitude. Limiting facility is called ratio
detector. Direction of D2 is reverse biased. With diode D2 is reverse, ‘0’ is positive w.r.t
‘B’ so that vab is the sum voltage ‘C5’ is connected to keep this sum voltage constant therefore
output voltage.
e) Relation between refection coefficient (k) & standing wave ratio (SWR)
(4marks)
We know reflection co-efficient (k) is define as the ratio reflected voltage to the
incident
v
K= r
vi
V= K.vi
But SWR is defined as the ratio of maximum to minimum magnitudes of current or voltage on the
line.
V
SWE = max
Vmin
But = Vmax = Vi + Vr
& Vmin = Vi- Vr
V  KVi
SWR = i
Vi  KVi
Vi(1  K )
=
Vi(1  K )
SWR=
(1  K )
(1  k )
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f) Tropospheric scatters
(2mark)
(2mark)
Explanation
The two directional antennas i.e. UHF transmitting & UHF receiving are pointed so that their
beams interest midway between them, above the horizon due to reflection from atmospheric
layers the signals scattered. This phenomenon is a permanent & not a sporadic one. The most
commonly used for 900MHZ, 2000 & MHZ
The energy contents of a forward scatter which is received by the receive is
very small of the incident power. But very high power is required for this.
Q3 Attempt any four
a) Given :
Pc = 10kw
M1 =0.5
M2 = 0.6
i)
Total power Pt for M=0.5
Pt = Pc (1+
(2mark)
m2
)
2
0 .5 2
) = 11.25 kw
2
Total power Pt for m= 0.6
=10 (1+
ii)
(2mark)
m2
)
Pt= Pc ( (1 
2
0.6 2
 11.8.kw
= 10 ( 1 
2
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b) Simple AGC –
(2mark)
 It is a system which will change the overall gain of Rx automatically
 It is used to keep the Rx output const even when the sg strength at the input of Rx is
changing.
Delayed AGC –
(2mark)
As shown in graph , AGC is not applied until the i/p Sg. Strength reaches the level B.
After pt B AGC bias is applied.
c) Need for stub –
(1mark)
The tuning stub in shunt with main line at certain point to give the impedance matching.
Single stub matching –
(1mark)
OR
The principle element of the x’mer is short ckt. Sec. of line whose open end is connected to
main line at particular dist. is connected to main line at particular dist. from load end.
i/p conductance = chara. Conductance and stub length is adjusted to make the i/p susceptance = 0
Advantage of single stub matching
(1mark)
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

Less power radiation
Effective length variation is possible by shorting bar.
Disadvantage


(1mark)
The matching conditions are correct only at 1 particular frequency.
At all the other frequency the SWR & load impedance can be worse.
d) Advantage of modulation
 Reduction in height of antenna.
 Avoids mixing of sgs.
 Increase range of communication
 Multiplexing is possible.
 Improves quality of reception.
e) Compare AM & FM
(4mark) (any 4 pts)
(4mark) (any 4 pts.)
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Sr no
f)
AM
FM
1
Amplitude of AM wave will
change with modulating
voltage.
Amplitude of FM wave is
constant.
2
Transmitted power is
dependent on modulation
index
Transmitted power remains
constant.
3
AM Rx are not immune to
noise
FM receivers are immune to
noise.
4
BW = 2
BW =2[  +FM]
5
BW is less than FM
BW is large
6
AM equipment are less
complex
FM equipment are more
complex.
7
No. of side bands in AM will
be const =2
No of side bands  depends
on fm
8
Information is contained in
Amplitude variation of carrier.
Information is contained in
frequency variation of carrier.
Folded dipole antenna
It is a single antenna but it consist of 2 elements as shown –
(2mark)
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The first element is fed directly but the 2nd is couple inductively at its ends. The input
impedance of ‫ג‬/2 folded dipole is 288Ω
Radiation pattern
Advantages–
 High i/p resistance
 High BW
 Ease of construction
 Cost efficient
 Impedance matching
(1 Mark)
(1mark)(any 2 pts ½ marks)
Q4 Attempt any four
a)
Explain –
(1mark)
i)
Characteristic impedanceThe characteristic impedance of transmission line Zo is define as impedance measured
at input of line when it length is infinite
ii)
Z0= L / C Ω
SWR –
(1mark)
The standing wave ratio (SWR) is the ratio of max voltage to min voltage.
OR
It is the ratio of maximum current to minimum current on a transmission line
SWR=
iii)
VMAX I MAX

.`
VMIN
I MIN
Reflection coefficient
It is define as ratio of reflected voltage to incident voltage.
OR
It is ratio of reflected current to incident current.
(1mark)
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
iv)
b)
ER I r

Ei
Ii
Propagation Constant
It is given by
Γ= α+ jβ
Where
α = attenuation const
β= Phase constant
(1mark)
Space wave propagation
(1mark)




Frequency above 30 MHz, space wave propagation is used.
It takes place by space waves or direct waves.
These waves travel in straight line from Tx antenna to Rx antenna.
Due to their straight line nature they will be blocked due to curvature of earth.
Advantage


(1mark)
Since it is used horizontal polarization the earth’s imperfection are less severe.
Less interface due to manmade noise.
Disadvantage
(1mark)
 The transmission path is limited by line of sight & radio horizon.
 Power loss due to absorption & scattering.
 Service range not more than 100km.
c)
Modulation index in AM –
(1 mark)
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It is defined as ratio of amplitudes of modulating & carrier waves.
M=
EM
EC
EM= Modulating Vol.
EC= Carrier Vol.
AM Wlf for M=0
d)
(1mark)
Am wlf for M= 50%
(1mark)
AM Wlf for M=100%
(1mark)
PLL –
(2mark)
The PLL consists of
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1)
Phase detector
2)
LPP
3) Error Amplifier
4)
Voltage Controlled Oscillator.
Phase detector
 The 2 I/p s are I/P vol. VS at FS & feedback Vol. from VCO at frequency fo
 It campares these 2 sg & produce a dc vol ve which is prop. to phase different
between fs & fo.
 This vol.is applied to LPF.

Low Phase filters –
IT removes high frequency noise present in phase detector & produce ripple free
dc level.

It is then amplified & applied to VCO (vc)
VCO
 Ve is I/P to VCO.
 The VCO frequency fo is compared with fs by phase detector & it is adjusted
continually till fo=fs.
PLL as FM demodulator:



(2mark)
FM sg. Which is to be modulated is applied at I/P of PLL.
As PLL is locked to FM sg the VCO starts tracking the instantaneous frequency in FM
I/P sg.
The error Vol. produced at O/P of amplifier is prop. To deviation of i/P frequency from
center frequency FM. This ac campo. Of error voltage represent modulating sg. Thus at
error ampli. O/P we get demodulated FM O/P.
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e)
Draw radiation pattern for resonant Dipole.
(1mark)

L = ‫\ג‬2

L=‫ג‬
(1mark)

L = 3‫ג‬/2
(1mark)
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
f)
L =3‫ג‬
(1mark)
Super heterodyning Principle
(2mark)
The basic process of Super heterodyning is to cavort all incoming signal to lower frequency
(I F) where fixed tuned amplifiers can be used to provide fix level of sensitivity and
selectivity. Most of the gain and selectivity in this recover is obtain IF amplifiers.
(2mark)
Q5
Attempt any four of the following;
(16mark)
a) Compare ground wave and space wave propagation for 2 points
Ground wave Propagation
1. Used for radio broadcasting (MW
range)
2. Ground waves are vertically
polarized
3. Ground waves are surface which
travel along the surface of the earth
( any 2 -4 Marks)
Space wave Propagation
Used for TV and FM broadcasting
Horizontally polarized
Space wave travel in a straight line from
transmitter to receiver through space
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4. Application in MW band radio
TV transmission, FM transmission, Satellite
communication.
b) Explain microwave antenna along with two example and radiation pattern
The microwave antenna are expected to be highly directional at microwave frequencies the
wavelength is very small and so the physical dimension of microwave antenna are very small
microwave antenna are used for microwave communication which is essentially point to point
type.
There are two types of microwave antenna:i)
ii)
Dish antenna
Horn antenna
2-Marks
2-Marks
i) Dish Antenna :- Dish antenna are the microwave antenna which use a parabolic reflector.
The antenna is actually placed at the focal point of parabolic reflector.
Dish antenna used as transmitting and receiving antenna. It is a transmitting antenna , then all the
waves coming out of the source(which is at the focal point) are reflected equally by the reflector
from every point. It should be noted that all the reflected waves are in phase with each other . We can
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use the dish antenna as a receiving antenna as well. The parabolic reflector will bring only those rays
together which are coming in direction BA. These rays are brought together at the focal point.
ii) Horn Antenna :The most widely used microwave antenna is horn . A horn antenna is
nothing more than a flared waveguide. The horn exhibits gain and directivity . However
its performance is significantly improved when it is used in combination with a parabolic
reflector. The length of a typical horn is usually 2 to 15 wave lengths at the operating
frequency.
c) Explain Dust propagation with the help of diagram ?
Dust propagation is a special type of phenomenon which is also called as the “Super
reflection”. It is observed at very high microwave frequencies.
As the height above the earth increases. The air density decreases and the refractive index
increases. The change in the refractive index is normally linear and gradual.
Under certain special atmospheric condition a layer of warm air may get trapped above the
cooler air. This happens normally over the surface of water.
Due to this the refractive index will decrease more rapidly with height than usual. Due to the
rapid reduction of refractive index. The microwave will completely bend back towards the earth
surface as shown in the diagram. Microwave are thus continuously refracted inside the dust and
reflected back by the conducting ground or water surface . There waves then propagate around the
curvature of the earth over a distance of 1000Km. The region in which the super refraction takes
place is called as dust. The dust can be formed near the earth surface or at some height from it . It is
then called as the elevated duct.
(Description- 3Marks & Waveform – 1 Mark)
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d) Explain Pre-emphasis and De-emphasis with graph?
Pre- emphasis :-
(2-Marks)
It has been observed that in FM ,the noise has a greater effect on the higher
modulating frequencies. This effect can be reduced by increasing the value of
modulation index (mf) for higher modulating frequencies (mf). This can be done by
increasing the deviation “δ” and δ can be increased by increasing the amplitude of
modulating signal at higher modulating frequencies. Thus if we “boost” the amplitude
of higher frequency modulating signal artificially then it will be possible to improve
the noise immunity at higher modulating frequencies. The artificial boosting of higher
modulating frequencies is called as Pre-emphasis. Boosting of higher frequency
modulating signal is achieved by using the pre-emphasis circuit of Fig (a). The
modulating AF signal is passed through a high pass RC filter, before applying it to the
FM modulator. As fm increases , reactance of C decreases and modulating voltage
applied to FM modulator goes on increasing. The frequency response characteristic of
the RC high pass network is shown in Fig (b). The boosting is done according to this
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prearranged curve. The amount of pre-emphasis in US FM transmission and sound
transmission in TV has been standardized at 75µsec . The pre-emphasis circuit is
basically a high pass filter.
De-emphasis :( 2-Marks )
The artificial boosting given to the higher modulating frequencies in the process
of pre-emphasis is nullified or compensated at the receiver by a process called “Deemphasis”. The artificially boosted high frequency signal are brought to their original
amplitude using the de-emphasis circuit. The 75 µsec de-emphasis circuit is standard
and it is as shown in fig (a) .It is shown that it is a low pass filter. 75 µsec de-emphasis
corresponds to a frequency response curve that is 3dB down at a frequency whose RC
time constant is µsec. The demodulated FM is applied to the De-emphasis circuit with
increase in fm the reactance of C goes on decreasing and the output of de-emphasis
circuit will also reduce as shown in Fig (b)
e) Draw the circuit diagram of envelope detector and explain its working?
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Envelope Detector :The envelope detector is a simple and very efficient device which is suitable for
the direction of a narrow band AM signal. A narrowband AM wave is the one in which the
carrier frequency fc is much higher as compared to the bandwidth of the modulating signal.
An envelope detector produces an output signal that follows the envelope of the input AM
signal exactly. The envelope detector is used in all the commercial AM radio receiver.
Circuit Diagram :( 2-Mark )
The circuit diagram of the envelope detector is shown in fig (a). It consist of a diode and RC
filter.
Operation of envelope Detector :( 2-Marks )
The standard AM wave is applied at the input of the detector. In every positive half
cycle of the input the detector diode is forward biased .It will charge the filter capacitor C
connected across the load resistance R to almost the peak value of the input voltage. As soon
as the capacitor charges to the peak value, the diode stops conducting. The capacitor will
discharge through R between the positive peaks as shown in the Fig (b). The discharging
process continues until the next positive half cycle when the input signal becomes greater than
the capacitor voltage, the diode conduct again and the process repeats itself.
Waveforms :The input-output waveforms for the envelope detector are shown in Fig (b). It shows
the charging discharging of the filter capacitor and approximate output voltage. It can be seen
from these waveform that the envelope of the AM wave is being recovered successfully. We
assume that the diode is ideal which presents a zero resistance when it is ON and infinite
resistance when it is OFF. We also assume that the AM wave applied to the input of the
detector is supplied by a source having internal resistance Rs.
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Selection of the RC time constants:
The capacitor charges through D and Rs when the diode is on and it discharges through
R when the diode is off. The charging time constant Rs C should be short as compared to the
carrier period 1/ fc.
Rs C << RC <<1/ fc
On the other hand the discharging time constant RC should be long enough so that the
capacitor discharges slowly through the load resistance R. But this time constant should not be
too long which will not allow the capacitor voltage to discharge at the maximum rate of
change of the envelope.
1/fc<< RC<<1/fm
where fm = Maximum modulating frequency .
f)
Explain the sensitivity and selectivity term used in Am receiver.
Selectivity :( 2-Marks)
Selectivity refers to the ability of a receiver to select a signal of a desired frequency while
rejecting those an closely adjacent frequencies.
A receiver with good selectivity will isolate the desired signal in the RF spectrum
and eliminate all other signals. Selectivity in a receiver is obtained by using tuned circuits.
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Sensitivity :( 2-Marks )
The sensitivity of a communication receiver refers to the receiver’s ability to pick up
weak signals. Sensitivity is primarily a function of the overall receiver gain; gain of course is
the factor by which an input signal is multiplied to produce the output. The sensitivity of a
communication receiver is usually expressed as the minimum amount of signal voltage SIP
that will produce an output signal that is 10dB higher than the receiver back ground noise.
Sensitivity improves with the help of tuned circuit.
Q6) Attempt any four of the following
a)
Solution :
Given: -
mf = 5, δ = 15 KHz, highest side band = 6
Method 1:BW = 2(δ+ fm (max))
mf = δ/ fm
fm = δ/ mf
( 2-Marks )
= 15* 103/5*103 Hz
= 3 KHz
BW = 2(δ+ fm (max))
= 2(15*103+3*103)
= 36 KHz
(2-Marks )
fm
Method 2 :BW = 2fm* no. of sides bands
mf = δ/ fm
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fm = δ/ mf
fm
( 2-Marks )
= 15* 103/5*103 Hz
= 3 KHz
BW = 2 fm* no. of sides bands
= 2 * 3*6 KHz
= 36 KHz
b) Explain the function of the following blocks in AM receiver
RF Amplifier :-
( 2-Marks )
( 1-Marks )
The fig shows the block diagram of AM receiver. In the super heterodyne receiver, the incoming
signal voltage is combined with a signal generated in the receiver. This local oscillator voltage is
normally converted into a signal of a lower fixed frequency. The signal at this intermediate frequency
contains the same modulation as the original carrier and it is now amplified and detected to reproduce
the original information. The superhetrodyne has the same essential components as the TRF receiver,
in addition to the mixer, local oscillator and intermediate frequency.
IF amplifier :( 1-Marks)
A constant frequency difference is maintained between the local oscillator and the RF circuit
normally through capacitance tuning, in which all the capacitor are ganged together and operated in
unison by one control knob. The IF amplifier generally uses two or three transformer, each consisting
of a pair of mutually coupled tuned circuits. With this large number of double-tuned circuits
operating at a constant, specially chosen frequency, the IF amplifier provides most of the gain (and
therefore sensitivity) and bandwidth requirements of the receiver. Since the characteristic s of the IF
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amplifier are independent of the frequency to which the receiver is tuned, the selectivity and
sensitivity of the superhetrodyne are usually fairly uniform throughout its tuning range and not
subject to the variations that affect the TRF receiver. The RF circuits are now used mainly to select
the wanted frequency, to the reject interference such as the image frequency and (especially at high
frequencies) to reduce the noise figure of the receiver.
For further explanation of the super heterodyne receiver, refer to fig. The RF stage is normally a
wide band RF amplifier tunable from approximately 540 kHz to 1650 kHz (standard commercial Am
band). It is mechanically tied to the local oscillator to ensure precise tuning characteristics.
The local oscillator
( 1-Marks )
is a variable oscillator capable of generating a signal from 0.995 MHz to 2.105 MHz .The
incoming signal from the transmitter is selected and amplified by the RF stage .It is then combined
(mixed) with a predetermined local oscillator signal in the mixer stage .(During this stage , a class C
nonlinear device processes the signals, producing the sum , difference and originals.)
The signal from the mixer
( 1-Marks )
is then supplied to the IF (intermediate- frequency) amplifier. This amplifier is a very-narrow
–bandwidth class A device capable of selecting a frequency of 0.455 kHz ± 3kHz and rejecting all
others.
The IF signal output is an amplified composite of the modulated RF from the transmitter in
combination with RF from the local oscillator. Neither of these signals is usable without further
processing. The next process is in the detector stages, which eliminates one of the sidebands still
present and separates the RF from the audio components of the other sideband. The RF is filtered to
ground and audio is supplied or fed to the audio stages for amplification and then to the speakers.
c)
i) Structure of Ferrite loop antenna:
Application of Ferrite loop antenna:-
( 1-Marks )
( 1-Marks )
1) It is used basically in portable radio receivers.
2) It is used for wireless communication purpose.
ii) Structure of Horn antenna :-
( 1-Marks )
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MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION
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Application of Horn antenna :1)
2)
3)
4)
( 1-Marks)
As feed antennas
All highly directional microwave antenna
Satellite antenna
Antennas on the spacecrafts.
d) Explain ionosphere layer and ionospheric propagation
The Ionosphere layer:
(2-Mark)
The ionosphere is the upper portion of the atmosphere, which absorbs large quantities of radiant
energy from the sun becoming heated and ionized . There are variation in the physical property of the
atmosphere, such as temperature, destiny and composition. Because of this and the different types of
radiation received , the ionosphere tends to be stratified rather then regular in its distribution the most
important ionizing agent are ultraviolet and α ,β and γ radiation from the sun, as well as cosmic rays
an meteors . The D layer is the lowest existing at an average height of 17 KM ,with an average
thickness of 10Km . The degree of its ionization depends on the altitude of the sun above the horizon
and thus it disappears at night.
The E layer is next in height existing at about 100Km with a thickness of perhaps 25Km. Like the
D layer it all but disappears at night. The reason of this Disappearance is the recombination of the
ions into modules. The E layer is the thin layer of very high ionization density, sometimes making an
appearance with the e layer.
The F layer is as shown in fig. Exists at height of 180Km in day time and combines with the F2
layer at night, it day time thickness is about 20Km. Although some HF waves are reflected from it ,
most pass through to be reflected from the F2 layer. Thus the main effect of the F1 layer is to be
provided more absorption for HF waves. The F2 layer is by far the most important reflecting medium
for high frequency radio wave. It is approximate thickness can be up to 20Km and its height range
from 250 to 400Km in day time.
Ionospheric Propagation :( 2-Marks )
At night it falls to height of about 3Km where it combines with the F1 layer its height and
ionization density vary tremendously, as fig.10.12 shows. They depends on the time of day, the
average ambient temperature and the sunspot cycle (see as the following sections dealing with the
normal and abnormal ionosphere) It is most noticeable that the F laver persists at night, unlike the
others. This arises from a combination of reasons. the first is that since this is the topmost layer it is
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MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION
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__________________________________________________________________________________________________
the also the most highly ionized and hence there is some chance for the ionization to remain at night
to some extent at least. The other main reason us that although ionization density is high in this layer,
the actual air density is not, and thus most of the molecules in it are ionized. Furthermore this low
actual density gives the molecules a large mean free path (the statically average distance of molecule
travels before colliding with another molecules ) This low molecular collision rate in turns mean that
in this layer ionization does not disappear as soon as sun set Finally it must be mentioned that the
reason for better HF reception at night are the combination of the F1 and F2 layers into one F layer
and the virtual disappearance of the other two layers which where causing noticeable absorption
during the day.
e)
Solution :Given : RL = 200Ω, SWR = 1 , Zo = ?
Step 1
:
Calculate the reflection coefficient P:
( 2mark)
SWR = 1+p/1-p
1 = 1+p/1-p
1 - p = 1+p
P = 0
Step 2 : calculate the characteristic impedance :
(2mark)
P = ZL – Z0/ ZL + Z0
0 =
ZL – Z0/ ZL + Z0
Z0 = Zl = 200 Ω
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