Download Unit 1 - Mahalakshmi Engineering College

MAHALAKSHMI
ENGINEERING COLLEGE
TIRUCHIRAPALLI – 621213
QUESTION BANK
DEPARTMENT: ECE
SEMESTER: IV
SUBJECT CODE / Name: EC2252 – COMMUNICATION THEORY
UNIT-I AMPLITUDE MODULATION SYSTEMS
PART -A (2 Marks)
1. Define Modulation Index for an AM Signal
(AUC MAY 2010)(AUC MAY 2006)
(AUC MAY 2013)
It is defined as the ratio between message amplitude to that of carrier amplitude.
ma=Em/Ec
2. Draw the circuit of an envelope detector.
(AUC NOV 2007)
3. State any important spectral properties of periodic power signals
(AUC
NOV
2007)
A signal px (t)is said to be periodic if
xp(t) =xp(t+T), (1.1)
for all t and some T. �
( denotes the end of definition, example, etc.)
EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE
Page 1
Let T0 be the smallest value of T for which this is possible. We call T0 as
theperiod of xp(t).
Fig. shows a few examples of periodic signals.
4. Define Amplitude Modulation. Give the expression for AM wave
(AUC NOV 2007)
This is defined as the modulation, in which amplitude of carrier is changed in accordance
to the amplitude of the modulating signal.
5. Give the applications of SSBSC-AM.
(AUC
NOV
2007)
Where power saving and low bandwidth requirement are important
Used in land and air mobile communication, navigation and amateur radio
6. Compare low level modulation and high level modulation. (AUC NOV 2007)
7. A transmitter radiates 9 KW without modulation and 10.125 KW after modulation.
Determine depth of modulation.
(AUC NOV 2007)
8. A 2 MHz carrier having amplitude of 5V is modulated by a 4 KHz audio signal
having amplitude of 2V. Determine the modulation index and draw frequency
spectrum of amplitude modulated wave. (AUC MAY 2008)
EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE
Page 2
9. SSB is suitable for speech signals and not for video signals. Why?
(AUC MAY 2008)
Bandwidth of SSB is half that of DSB-SC AM. Thus twice the number of
channels can be accommodated at a given frequency spectrum.
No carrier is transmitted, hence possibility of interference with other channels are
avoided.
It eliminates the possibility of fading. Fading occurs due to multipath propagation
of electro-magnetic waves.
10. A 500 W carrier modulated to a depth of 60 percent. Calculate the total power in
modulated wave.
(AUC NOV 2008)
11. Define vestigial sideband transmission. Mention its application. (AUC NOV 2008)
In VSB the desired sideband is allowed to pass competently. Whereas a small portion of
the undesired sideband is also allowed
Application: Television broadcasting
12. Compute the bandwidth of the Amplitude modulated signal
C(t)=23*cos(230000πt)[1+0.8*cos (310πt)]
(AUC MAY 2009)
Ma=0.8, 2πfm=310
Fm=fm=310
Fm=49.33Hz
BW=2fm=98.66Hz
13. Define AM and its spectrum
(AUC MAY 2009)
Amplitude modulation is defined as the process in which the amplitude of the
carrier signal is varied in accordance with the modulating signal or message
signal.
EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE
Page 3
14. An AM DSB-FC wave with a peak unmodulated carrier voltage AC=10V, a load
resistor RL=10Ώ and the modulation index m=1. Determine the total power of the
modulated wave.
(AUC MAY 2009)
15. How many AM broadcast stations can be accommodated in a 100 kHz bandwidth if
t he highest frequency modulating a carrier is 5 kHz? AUC DEC 2010)
(AUC MAY 2010)
ANS:20AM Broadcast stations
16. What are the advantage of VSB
(AUC MAY 2011)
It has bandwidth greater than SSB but less than DSB system.
Power transmission greater than DSB but less than SSB system.
No low frequency component lost. Hence it avoids phase distortion.
17. Calculate the local oscillator frequency if incoming frequency is f1 and translated
carrier frequency
(AUC MAY 2011)
The requiredloca oscillator frequcny (f1) is
F2= f1+f2
F1=f2-f1
18. How many AM broadcast stations can be accommodated i n a 100 kHz bandwidth
if the highest frequency modulating a carrier i s 5 kHz?
(AUC DEC 2011)
Ans: 20 AM broad cast stations
19. State the applications of FDM.
(AUC DEC 2010)
(AUC MAY 2011)
Telemetry
Commercial broadcast and television
Communication network
EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE
Page 4
20. What is meant by frequency translation?
21. Compute the bandwidth of the amplitude modulated signal given by s(t)=23[1+0.8
cos(310t)]cos(230000πt)
(AUC MAY 2012)
ANS:
ma=0.8 , 2πfm=310
therefroefm=49.33Hz
B.W=2fm=98.66Hz
22. What are the causes of linear distortion (AUC MAY 2010)(AUC MAY 2012)
Amplitude distortion
Phase distortion
Frequency response distortion
EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE
Page 5
Group delay distortion
Harmonic distortion
Pulse distortion
23. Compare the performance of AM and FM systems.
S.No.
1.
Amplitude modulation
Amplitude of the carrier is
varied according to amplitude
of modulating signal
2. Am has poor fidelity due to
narrow band width
3. Most of the power is in carrier
hence less efficient
4. Noise interference is more
5. Adjacent channel interference is
present
6. AM broad cast operates in MF
and HF range.
7. In AM only carrier and two side
bands are present.
8. The transmission equipment is
simple.
9. Transmitted power varies
according to modulation index.
10. Depth of modulation has
limitations. It can be increased
above 1.
Frequency modulation
Frequency of the carrier is varies
according to amplitude of the modulating
signal
Since the band width is large, fidelity is
better
All the transmitted power is useful.
Noise interference is minimum.
Adjacent channel interference is avoided
due to guard bands.
FM broadcast operates in VHF and UHF
range.
Infinite number of sidebands are present.
The transmission equipment is complex.
Transmitted power remains constant
irrespective of modulation index.
Depth of modulation has no limitation. It
can be increased by increasing frequency
deviation.
25.Compare Bandwidth and power requirement in terms of carrier power for AM,DSBSC,
and SSB(AUC MAY 2013)
Description
AM
DSBSC
SSB
Bandwidth
2fm
2fm
fm
66.68%
83%
50%
75%
power requirement for 33.33%
sinusoidal
power requirement for 33.33%
non sinusoidal
EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE
Page 6
PART- B
1. Discuss coherent detection of DSB-SC modulated wave with a block diagram of the
detector. (AUC NOV 2006) (ii) Write about the coherent detection method in detail for
DSB-SC and SSB- SC. What happens when there is phase mismatch? (8)(AUC DEC
2010)
EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE
Page 7
EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE
Page 8
EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE
Page 9
2. Draw the filtering scheme for the generation of VSB modulated wave and Explain.
(AUC NOV 2007)
Generation of VSB
In VSB
1. One sideband is not rejected fully.
EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE
Page 10
2. One sideband is transmitted fully and a small part (vestige)of the
other sideband is transmitted.
The transmission BW is BWv = B + v. where, v is the vestigial frequency
band.
EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE
Page 11
EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE
Page 12
3. Explain the double side band suppressed carrier modulation technique (AUC NOV
2007)Explain in detail about the balanced modulator with block diagram.(AUC NOV
2007) Discuss the method of demodulation of DSBSC signal using costas loop.(AUC
MAY 2008) Explain the operation of a ring modulator. State its advantages. (AUC NOV
2008) (With the help of a neat diagram, explain the generation of DSB-SC using
balanced modulator. (8) (AUC MAY 2010)
Generation of DSB-SC Signals
The circuits for generating modulated signals are known as modulators. We shall discuss
three basic modulators – nonlinear, switching and ring modulators.
Conceptually, the simplest modulator is the product or multiplier modulator, Figure 1(a).
However, it is very difficult (and expensive) in practice to design a product modulator that
maintains amplitude linearity at high carrier frequencies.
EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE
Page 13
EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE
Page 14
EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE
Page 15
EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE
Page 16
EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE
Page 17
4. Explain frequency translation.
(AUC NOV 2007)
EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE
Page 18
5. An SSB signal is generated by modulating an 800 KHz carrier by the signal m(t) =
cos2000πt + 2sin1200πt. The amplitude of the carrier is Ac=10. Obtain the magnitude
spectrum of the lower sideband SSB signal.
(AUC MAY 2008)
6. With a neat block diagram explain the SSB transmission. (AUC NOV 2008) (ii) How SSB
can be generated using Weaver’s method? Illustrate with a neat block diagram. (6)
Generation of Suppressed-Carrier AM
•Full-carrier
AM is simple but not efficient
•Removing the carrier before power amplification allows full
transmitter power to be applied to the sidebands
•Removing the carrier from a fully modulated AM systems results in
a double-sideband suppressed-carrier transmission.
1. Phasing method
2. Filter Method
Phasing method
This method is a special modulation type of IQ canonical form
of Generalized transmitters
Filter Method
The filtering method is a special case in which RF processing (with a
sideband filter) is used to form the equivalent g(t), instead of using base
band processing to generate g(m) directly.
EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE
Page 19
The filter method is the most popular method because excellent sideband
suppression can be obtained when a crystal oscillator is used for the
sideband filter.
Crystal filters are relatively inexpensive when produced in quantity
at standard IF frequencies.
Weaver’s Method for Generating SSB.
Single-Sideband AM
EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE
Page 20
•The
•As
two sidebands of an AM signal are mirror images of one another
a result, one of the sidebands is redundant
•Using
single-sideband suppressed-carrier transmission results in reduced
bandwidth and therefore twice as many signals may be transmitted in the
same spectrum allotment
•Typically,
a 3dB improvement in signal-to-noise ratio is achieved as a
result of SSBSC
7. (i)Draw an envelope detector circuit used for demodulation of AM and explain its
operation. (10)(i)Draw an envelope detector circuit used for demodulation of Am and
explain its operation.With a help of a neat diagram, explain the operation of an envelope
detector. Why does negative peak clipping take place (AUC MAY 2011)(AUC MAY
2011)
EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE
Page 21
The charging time constant RsC is very small when compared to the
carrier period 1/fc i.e.,
RsC<< 1/fc
Where Rs= internal resistance of the voltage source.
C = capacitor
fc= carrier frequency
i.e., the capacitor C charges rapidly to the peak value of the signal.
The discharging time constant RlC is very large when compared to the
charging time constant i.e.,
1/fc <<RlC<< 1/W
Where Rl= load resistance value
W = message signal bandwidth
i.e., the capacitor discharges slowly through the load resistor.
Advantages:
It is very simple to design
It is inexpensive
Efficiency is very high when compared to Square Law detector
Disadvantage:
Due to large time constant, some distortion occurs which is known
as diagonal clipping i.e., selection of time constant is somewhat
difficult
Application:
It is most commonly used in almost all commercial AM Radio
EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE
Page 22
8. Compare Amplitude Modulation and Frequency Modulation. (6)(AUC MAY 2010)
Amplitude Modulation
Frequency Modulation.
Amplitude of the Carrier is varied in
Frequency of the Carrier is varied in
according to amplitude of modulation signal
according to amplitude of modulation signal
AM has poor fidelity due to narrow bandwidth Since the bandwidth is large fidelity is better
Most of the power is in the carrier hence less
All the trnasmitterd power is useful
efficient
Noise interference is more
Noise interference is minimum
Adjacent channel interference is present
Adjacent channel interference avoided due
to guard bands
9. With necessary diagrams and expressions explain the generation and demodulation of
AM.
(AUC DEC 2011)(AUC APR/MAY 2011)
Switching Modulator (1/4)
◊One way to generate an AM wave: Switching Modulator.
◊ Assume carrier wave c(t) is large in amplitude and the diode acts as an ideal switch.
load voltage v2(t) varies periodically between the values v1(t) and zeros at a rate equal to
the carrier frequency fc.
◊ By assuming a modulating wave that is weak compared with the
EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE
Page 23
carrier wave, we have effectively replace the nonlinear behavior
of the diode by an approximately equivalent piecewise-linear time-varying operation
Unwanted component, the spectrum of which contains
◊ Delta function at 0, ±2fc, ±4fc and so on.
◊ Occupy frequency intervals of width 2W centered at 0, ±3fc, ±5fc and so on, where W is
the message bandwidth. Be removed by a band filter with mid frequency f and
◊ using band-pass mid-band fc bandwidth 2W, provide that fc >2W
Generation of AM waves
There are two methods to generate AM waves
Square-law modulator
Switching modulator
EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE
Page 24
Square-law modulator: -
A Square-law modulator requires three features: a means of summing the carrier and
modulating waves, a nonlinear element, and a band pass filter for extracting the
desired modulation products. Semi-conductor diodes and transistors are the most
common nonlinear devices used for implementing square law modulators. The
filtering requirement is usually satisfied by using a single or double tuned filters.
When a nonlinear element such as a diode is suitably biased and operated in a
restricted portion of its characteristic curve, that is ,the signal applied to the diode is
relatively weak, we find that transfer characteristic of diode-load resistor combination
can be represented closely by a square law :
V0 (t) = a1Vi (t) + a2 Vi2(t) ……………….(i)
Where a1, a2 are constants
Now, the input voltage Vi(t) is the sum of both carrier and message signals
i.e., Vi(t) =Accos 2 fct+m (t) ……………. (ii)
Substitute equation (ii) in equation (i) we get
V0 (t) =a1Ac [1+kam (t)] cos2 fct +a1m (t) +a2Ac
2cos22
fct+a2m2 (t) ………..( iii)
Where ka=2a2/a1
Now design the tuned filter /Band pass filter with center frequency fc and pass band
frequency width 2W.We can remove the unwanted terms by passing this output
voltage V0(t) through the band pass filter and finally we will get required AM signal.
V0 (t) =a1Ac [1+2a2/a1 m (t)] cos2 fct
EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE
Page 25
Assume the message signal m (t) is band limited to the interval –W
f
W
Switching Modulator: -
Assume that carrier wave C (t) applied to the diode is large in amplitude, so that it
swings right across the characteristic curve of the diode .we assume that the diode acts
as an ideal switch, that is, it presents zero impedance when it is forward-biased and
infinite impedance when it is reverse-biased. We may thus approximate the transfer
characteristic of the diode-load resistor combination by a piecewise-linear
characteristic.
The input voltage applied Vi(t) applied to the diode is the sum of both carrier and
message signals.
Vi (t) =Accos 2 fct+m (t) …………….(i)
During the positive half cycle of the carrier signal i.e. if C (t)>0, the diode is forward
biased, and then the diode acts as a closed switch. Now the output voltage Vo (t) is
same as the input voltage Vi (t) . During the negative half cycle of the carrier signal
i.e. if C (t) <0, the diode is reverse biased, and then the diode acts as a open switch.
Now the output voltage VO (t) is zero i.e. the output voltage varies periodically
between the values input voltage Vi(t) and zero at a rate equal to the carrier frequency
f c.
i.e., Vo (t) = [Accos 2 fct+m (t)] gP(t)……….(ii)
Where gp(t) is the periodic pulse train with duty cycle one-half and period
Tc=1/fc and which is given by
gP(t)= ½+2/
[(-1)n-1/(2n-1)]cos [2 fct(2n-1)]…………(iii)
EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE
Page 26
Now design the tuned filter /Band pass filter with center frequency fc and pass band
frequency width 2W.We can remove the unwanted terms by passing this output
voltage V0(t) through the band pass filter and finally we will get required AM signal.
V0 (t) =Ac/2[1+kam (t)] cos2 fct
The AM spectrum consists of two impulse functions which are located at fc & -fc and
weighted by Aca1/2 & a2Ac/2, two USBs, band of frequencies from fc to fc +W and
band of frequencies from -fc-W to –fc, and two LSBs, band of frequencies from fc-W
to fc & -fc to -fc+W.
10. (i)Compare the various amplitude modulation schemes.(6)Compare various Amplitude
modulation systems(i)Compare the characteristics of DSBFC, DSBSC, SSBFC, SSBSC,
VSB schemes.
Description
B.W
Power saving
for sinusoidal
Power saving
for non
sinusoidal
(AUC MAY 2012)
DSB-FC
DSB-SC
SSB-SC
VSB
fm<2f1
75%
2fm
2fm
fm
33.33%
66.66%
83%
33.3%
50%
75%
75%
Generation
methods
Not difficult
Not difficult
Difficult
Difficult
Application
AM broad cast
application
Carrier telephony
Police wireless
mobile etc.
TV high
speed tx
11. How SSB can be generating using Weaver’s method? Illustrate with a neat block
diagram
(AUC MAY 2012)
EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE
Page 27
12. (i)What is frequency division multiplexing? Explain.With neat block diagram, explain the
transmitter and receiver section of Frequency Division Multiplexing. (10) (ii)Explain the
concept of FDM with a suitable block diagram. Discuss Frequency Division Multiplexing (i)
Discuss in detail about frequency translation and frequency division multiplexing
technique with diagrams. (10)
(AUC NOV 2007)(AUC DEC 2011)
EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE
Page 28
13. Explain in detail about demodulation of AM waves. (AUC MAY 2013)
EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE
Page 29
EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE
Page 30
EC2252 – COMMUNICATION THEORY IV Sem ECE – R.Vanitha Asst.Prof./ECE
Page 31