1. COMMUNICATION SYSTEM COMMUNICATION :
It is a process of transmitting information
or signal from one point known as source
to another point known as destination.
Information can be continuous such as
music, picture etc. or discrete signals like
data from computer etc.

2. BLOCK DIAGRAM OF A COMMUNICATION SYSTEM:
I I
N N
F F
O O
R R
M M
A A
T T
I I
N N TX
Noise
CHANNEL RX
OUTPUT
transducer
INPUT

3. As shown in the figure above, the first block at the source is an input transducer which is used to convert physical quantity (non electrical) to electrical quantity. For example voice is converted to electrical quantity using microphone. Similarly at the destination output transducer is used to convert electrical back to physical quantity. For example a loudspeaker is used to convert voice signal in the form of electrical back to physical quantity.

4. There are basically three essential blocks in a communication system.
Transmitter (TX)
Transmission Channel
Receiver (RX)

5. Transmitter :
The output signal of the transducer is a complex signal. It is restricted to desired range of frequencies. On this signal modulation is performed. Modulation is a process of altering the characteristics of carrier signal in accordance with the information. There are basically three types of modulation technique
Amplitude modulation.
Frequency modulation.
Phase modulation.

6. Transmission channel:
It is a medium over which the electronic signal is transmitted from one point to another. This medium can be either wired or wireless.
An example for wired communication is telephony where a pair of physical wires is running parallel between transmitter and receiver. Now a days optical fibers are used in between transmitter and receiver in which light carries the information. Similarly an example for wireless communication is radio communication where two antennas are employed, one at the transmitter and other at the receiver. The transmitter antenna transmits the modulated information into free space and the receiver antenna picks up the modulated information which is later demodulated to get the information back.
The modulated signal is then transmitted over a transmission channel.

7. Noise:
It is a random, undesirable electrical energy that interferes with the transmitted signal. It can be either natural noise such as noise caused by lightning during rainy season or man made noise produced by ignition system of cars etc. Noise is a serious problem which cannot be eliminated but one can reduce the effect caused by it on the signal.

8. Receiver:
It is a collection of electronic circuits designed to convert the modulated signal back to modulating signal. This process is known as demodulation. Finally an output transducer is employed to convert back the information in electrical form to physical form.

9. MODULATION:
Modulation is a process of altering the characteristics of carrier signal in accordance with the information or modulating signal.
Carrier signal: It is a high frequency signal that carries the information from transmitter to receiver.

10. Need for modulation:
The height of the antenna required to transmit and receive radio waves is a function of wavelength of the frequency used. i.e.λ = c/f. At low frequency, λ is high and hence the height of the antenna should be more to transmit the signal (since ‘λ’ is proportional to ‘h’). Therefore high frequencies are used to transmit the information which requires antenna of lesser height.
At low frequency radiation is poor and signal gets highly attenuated. Therefore signal cannot be transmitted over longer distance. Modulation effectively increases the frequency of the signal to be radiated and thus increases the distance over which signal can be transmitted faithfully.
The modulation permits multiplexing to be used. Multiplexing is method of transmitting two or more informations simultaneously over a single channel. In this method each message signal is modulated using different carrier signal frequencies and then transmitted over a single channel. At the receiver the message signals are extracted individually by tuning to their respective carrier frequencies.

11. AMPLITUDE MODULATION:
It is defined as a process of varying the
amplitude of the carrier wave
proportional to the instantaneous
amplitude of modulating signal.

12. CARRIER AND MODULATING SIGNALS

13. AMPLITUDE MODULATED SIGNAL

14. Time domain analysis
Let the equation of carrier signal be c(t) = Accos(2пfct) where Ac is the peak amplitude of carrier signal and fc is the frequency of the carrier signal.
Let the equation of modulating signal be m(t) = Amcos(2пfmt) where Am is the peak amplitude of modulating signal and fm is the frequency of the modulating signal.
Then by the definition of AM:
VAM (t) = [Ac+Amcos(2пfmt)]cos(2пfct)
= Accos(2пfct) + (Am) cos(2пfmt) cos(2пfct)
= Accos(2пfct) + (mAC/2) {cos[2п(fc + fm)t] + cos[2п(fc - fm)t] }
Where ‘m’ is the modulation index of AM signal which is defined as
ratio of amplitude of modulating signal to that of carrier signal.

15. SIGNIFICANCE OF MODULATION INDEX
The significance of modulation index is, it decides the depth of modulation. If it is less than one, then AM signal is known as under modulated signal. If it is more than one, then AM signal is known as over modulated signal. If it is equal to one , then AM signal is known as perfect modulated signal. To obtain the original information, modulation index should always be less than or equal to one.

16. Spectrum of AM signal: Ac
mAc/ mAc/2
fc -fm fc fc + fm f (Hz)

17. As shown in the figure above, the spectrum consisted of three frequency components, one at fc and other two at fc+ fm , fc- fm. The frequencies fc+ fm and fc- fm are known as sideband frequencies i.e. fc+ fm is known as upper sideband frequency and fc- fmis known as lower sideband frequency . The separation between these two frequencies is defined as bandwidth of AM signal. Therefore the bandwidth of AM signal is 2fm.

18. Total power required to transmit AM signal
The total power required to transmit AM signal ( PT ) is sum of power
required to transmit carrier signal ( PC ) and power required to
transmit side band signals ( PTSB ).
Therefore PT = PC + PTSB
= PC + PLSB + PUSB
= (AC2/2R) + (m2AC2/8R) + (m2AC2/8R)
= PC { 1 +( m2/2) }

19. The above equation gives the total power required to transmit AM signal in terms of carrier power and modulation index.
For 100% modulation : m = 1,
Therefore PT = PC { 1 +( ½) }
PC = PT Or PC = %PT
i.e % of total power is wasted in transmitting carrier signal.

20. Current calculation:
PT = PC { 1 + (m2/2) }
IT2R = IC2R { 1 + (m2/2) }
IT = IC{ 1 + (m2/2) }0.5
where ‘IT’ is the current with modulation , ‘IC’ is the current without modulation and R is the resistance of the antenna.

21. Modulation by several sine waves
In modulation by several sine waves, modulating signal consists of several sine waves i.e.
m(t) = Am1cos(2пfm1t) + Am2cos(2пfm2 t)
For modulation by several sine waves overall modulation index will be
mt = {(Am12 + Am22 + …..)/Ac2}0.5
= (m12 + m22 +…….)0.5

22. Therefore Total power will be:
Similarly current with modulation will
be:
PT = PC { 1 + (mt2/2) }
IT = IC { [ 1 + (mt2/2) ]0.5 }

23. Problems:
1. An audio signal 10sin (2п1000t) amplitude modulates a carrier of 40sin (2п2000t). Find
Modulation index
Sideband frequencies.
Bandwidth.
Total power delivered if RL = 1KΩ.
Amplitude of each side band components

24. Solution:
i) Modulation index: m =0.25.
ii) Sideband frequencies :
Upper side band = fC + fm = 300Hz.
Lower side band = fC - fm = 1000Hz.
iii) Bandwidth = 2fm = 2KHz.
iv) Total power delivered:
PT = { 1 + (0.252)/2} =
v) Amplitude of each sideband = mAc/2=5V

25. 2. The antenna current of an AM transmitter is 8A when only carrier is transmitted, but increases to 8.93A when carrier is modulated by a single sine wave. Find the percentage modulation. Determine the antenna current when the depth of modulation changes to 0.8A.
Solution:
i) Given : IT = 8.93A.
IC = 8A.
IT = IC { 1+ m2/2}
m = =70.1%.

26. ii) IT = ?, when m = 0.8 EXERCISE:
IT = 8 {1+(.7)2/2 } = 9.19A .
EXERCISE:
1. A certain transmitter radiates 9KW with carrier unmodulated and KW when carrier is sinusoidally modulated. Calculate modulation index. If another sine wave corresponding to 40% modulation is transmitted simultaneously, determine the total power radiated.

27. FREQUENCY MODULATION It is defined as a process of altering the
frequency of the carrier signal w.r.t.
instantaneous amplitude of modulating
signal.

28. CARRIER AND MODULATING SIGNALS

29. FREQUENCY MODULATED SIGNAL

30. Time domain analysis:
From the definition:
fFM= fC + Kfm(t)
Where Kf is known as frequency sensitivity.
fFM= fC + Kf Amcos(2Πfmt)
= fC + Δf cos(2Πfmt)
where Δf is known as frequency deviation. Its signifies , by how much amount carrier frequency gets deviated.

31. Multiplying by 2Π on both sides :
2ΠfFM = 2ΠfC + 2ΠΔf cos(2Πfmt)
ωFM = ω C + Δ ω cos(2Πfmt)
ω FM =dθ(t)/dt , integrating both the sides :
ω FMt = ω Ct +(Δ ω / ω m)sin(2Πfmt)
Therefore equation of FM Signal:
VFM(t) = ACcos[θ(t)]
= ACcos[ω Ct +β sin(2Πfmt)]

32. where β = Δf/fm is defined as modulation index of FM
where β = Δf/fm is defined as modulation index of FM. Unlike AM modulation index is not restricted to one. It can be more than unity.
Frequency spectrum:
The frequency spectrum of FM signal consisted of infinite number of sideband components ( using Fourier Transform ). Hence theoretically the bandwidth of FM signal is infinity. But practically, the bandwidth of FM signal is restricted using Carson’s rule. According to Carson’s rule the bandwidth of FM signal is given by 2(Δf + fm).

33. Problems: VFM(t) = 10 cos [2Π108t + 5 sin(2Π15000t)] , Calculate
Given a FM equation
VFM(t) = 10 cos [2Π108t + 5 sin(2Π15000t)] , Calculate
Carrier frequency.
Modulating frequency.
Frequency deviation.
Bandwidth using Carson’s rule.

34. Solution:
Carrier frequency: fC = 108Hz.
Modulating frequency : fm = 15KHz.
Frequency deviation : Δf = β fm = 5 * 15 = 75KHz.
Bandwidth = 2(Δf + fm ) = 2( ) = 180KHz.
In an FM system when the audio frequency is 50Hz , modulating voltage is 2.5V , the deviation produced is 5KHz. If the modulating voltage is now increased to 7.5V , calculate the new value of frequency deviation. If the AF voltage is raised to 10V while the modulating frequency is dropped to 250Hz , what is the frequency deviation produced. Also calculate modulation index in each case.

35. Solution:
Given : fm = 50Hz , Am = 2.5V , Δf = 5KHz.
Modulation index: β = Δf/fm= 5*103/50 = 100
ii) If Am = 7.5V , Δf = ?
Kf = Δf/Am=2KHz/V.
Δf = Kf Am = 2*7.5 = 15KHz.
Modulation index: β = Δf/fm= 300.
iii) Δf = Kf Am = 2*10KHz = 20KHz.
Modulation index: β = Δf/fm= 800.

36. EXERCISE:
A carrier of amplitude 5V and frequency 90MHz is frequency modulated by asinusoidal voltage of amplitude 5V and frequency 15KHz. The frequency sensitivity is 1Hz/V. Calculate the frequency deviation and modulation index.
Compare and contrast AM and FM.

37. END