1. COMMUNICATION SYSTEMS (5marks)
The setup used to transmit information from one point to another is called as communication system

2. PARTS OF COMMUNICATION SYSTEM
Transmitter
#Transmits the message through the communication channel.
#converts the message from the source into a suitable form.
Communication channel (Transmission medium)
#Medium or the physical path that connects a transmitter and the receiver.
#Carries the modulated signal from transmitter to the receiver.
Receiver
#Receives the signal and converts back into its original form.

3. TRANSMITTER
A setup which transmits the message from source to the receiving end through the communication channel.
MAIN FUNCTION :-
# Modify the signal into a suitable form.
# Transmit the signal through the channel.
BASIC COMPONENTS:-
#Message signal generator
# Modulator
#Antenna

4. SCHAMATIC DIAGRAMS TRANSMITTER Antenna
Transmits modulated & amplified signal into the medium
MODULATOR
AMPLIFIER
Microphone
Converts the signal into required form
Increases the strength of the signal
Transducer

5. RECEIVER
A set up that receives the transmitted signals and converts them back into their original form.
MAIN FUNCTION :-
#Receives the selected signal through antenna.
#Amplifies it
# Converts it into required form.
BASIC COMPONENTS:-
# Antenna
#Tuner circuit
#Demodulator
# Audio amplifier

6. Schematic diagram RECEIVER Antenna
Receives the modulated & amplified signal from the medium
Microphone
TUNABLE AMPLIFIER
DEMODULATOR
AUDIO AMPLIFIER
Tunes the receiver to selected station &boosts up the signal from desired station
Retrieves back the original signal from the modulated signal
Amplifies the required audio signal
Transducer

7. BLOCK DIAGRAM OF COMMUNICATION SYSTEM
Information source
Message signal
Channel
Received signal
Receiver
Transmitter
Transmitted signal
Noise
Message signal
User of information

8. Terms to remember:- Transducer Signal Noise Transmitter Receiver
Attenuation
Amplification
Range
Bandwidth
Baseband
Modulation
Demodulation
Repeater

9. TYPES OF COMMUNICATION SYSTEMS
According to the nature of information source
According to the mode of Transmission
According to the method of transmission
According to the type of modulation
1.Analog communication
2. Digital communication
1.Line communication
a)Two wire line communication
b)Coaxial cable
c) Optical fiber cable
2. Space communication
a) Ground wave communication
b)Space or tropospheric wave communication
c)Sky wave or ionospheric wave communication
Speech transmission
Audio visual transmission
Facsimile (FAX)
Sinusoidal carrier waves
1)Amplitude modulation
2)Frequency modulation
3)Phase modulation
Pulsed carrier waves
1)Pulse amplitude modulation
2)Pulse time modulation
3)Pulse code modulation

10. MODULATION
The process by which some characteristic, usually an amplitude, Frequency or phase angle of high frequency carrier wave is varied in accordance with the instantaneous value of low frequency audio signal , called the modulating signal
Need of modulation
To have a practical length of antenna
length of antenna ≈ ʎ/4
To have effective power radiation by antenna
Power radiated α (1/ʎ)²
For frequency multiplexing
Allows all signals to be received by the antenna
To convert a wide band signal into narrow band
Allows more stations to be placed in the same band

11. CONTINIOUS WAVE MODULATION
Carrier wave is a sine wave.
Mainly used in analog transmission.
Types are:-
a)Amplitude modulation (Amplitude of carrier wave is a function of audio or base signal)
b) Frequency modulation (Frequency of carrier wave is a function of audio or base signal)
c) Phase modulation (Phase of carrier wave is a function of audio or base signal)

12. Amplitude modulation = Am /Ac
Amplitude of the carrier wave changes as per the frequency of modulating signal.
Frequency of AM wave is equal to the frequency of unmodulated carrier wave.
Amplitude of carrier wave is a function of audio or base signal.
Bandwidth = twice the frequency of modulating signal
Modulation factor =
= Am /Ac
Percentage modulation = µ
µ = (Amax -Amin)/ (Amax + Amin) x 100%
Change in amplitude of carrier wave
Amplitude of original carrier wave

13. AMPLITUDE MODULATION DISADVANTAGES ADVANTAGES
1)Suffers from large amount of noise.
2)Quality of audio signal is poor.
3)Low efficiency as the information is contained only in the side bands.
ADVANTAGES
1) Easier method for transmitting and receiving audio signals
2)Requires low carrier frequencies (.5MHz – 20 MHz)
3)More area can be covered by transmitter.
4)Requires simple and cheaper transmitters and receivers.

14. FREQUENCY MODULATION
The frequency of modulated signal is a function of base (audio) signal.
Amplitude of modulated signal remains same as the carrier wave
The amount of frequency deviation is proportional to the instantaneous value of amplitude of the modulating signal.
The rate of frequency deviation is equal to the modulating frequency.

15. FREQUENCY MODULATION ADVANTAGES DISADVANTAGES
Highly efficient as all of the transmitted power is useful.
Less prone to noise
No limit for depth of modulation
Gives high fidelity (hi-fi) reception as more number of useful sidebands are present
BW = 2 ( no of significant sidebands ) x modulating frequency. Hence more channel width is needed.
FM transmitters and receivers are more complex and costly.
FM reception is limited to line of sight. Hence less area is covered.

16. PROPAGATION OF ELECTROMAGNETIC WAVES
GROUND WAVE PROPAGATION
SPACE WAVE PROPAGATION
Radio wave travels directly from transmitter to the receiver following the line of sight communication
Transmitting and receiving antennas are placed as high as possible from the earth’s surface.
Range is limited by earth’s curvature and the distance between transmitting and receiving antenna.
Range 100 MHz -220 MHz
Radio wave travels directly from one point to another following the surface of the earth
Transmitting and receiving antennas are close to the earth’s surface.
Energy is partially absorbed by the earth’s surface hence more attenuation.
Not suitable for long distance communication & higher frequencies.
Range 500kHz to 1500kHz
Less affected by atmospheric conditions

17. SKY WAVE OR IONOSPERIC PROPAGATION
A radio wave directed towards the sky and reflected by the ionosphere towards the desired location of the earth is called as sky wave propagation.
Due to change (gradual decrease) of refractive index of earth’s atmosphere, the EM wave bends at critical angle and sent back to the ground station.
Range 2 MHz to 20 MHz.
Highly affected by atmospheric conditions.