ANALOG AND DIGITAL COMMUNICATION.pptx

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1. ANALOG AND DIGITAL COMMUNICATION UNIT I ANALOG COMMUNICATION

2. UNIT I ANALOG COMMUNICATION Introduction to Communication Systems Modulation - Types - Need for Modulation. Theory of Amplitude Modulation Evolution and Description of SSB Techniques Theory of Frequency and Phase Modulation Comparison of Analog Communication Systems ( AM – FM – PM ).

3. INTRODUCTION TO COMMUNICATION SYSTEMS COMMUNICATION: Process of establishing connection between two points for exchanging information. Elements of communication system: Transmitter   Channel Receiver

4. BLOCK DIAGRAM

5. Transmitter:   The transmitter consists of a transducer , modulator , amplifier , and transmitting antenna in radio transmission .

6. A transducer converts sound signals into electric signals. Modulator performs mixing of the electric audio signal with the high-frequency radio wave. The amplifier boosts the power of the modulated signal. The signal is radiated in the space with the aid of an antenna .

7. Communication channel:   The duty of the communication channel is to carry the modulated signal from transmitter to receiver. The communication channel is also called a transmission link or medium . Type of communication Channels or links Radio communication (The basic elements of radio communication system are a transmitter and a receiver) Free space Telephony and Telegraphy communication Transmission line Optical communication Optical fibre

8. Receiver:   The receiver consists of Pickup antenna , Demodulator , Amplifier and Transducer.

9. Pickup antenna : To pick the signal Demodulator : To separate the audio signal from the modulated signal Amplifier : To boost up the weak audio signal Transducer : converts back audio signal in the form of electrical pulses into sound waves.

10. Modulation: Digital and analogue signals are of low frequency and hence cannot be transmitted to long distances. These signals require some carrier. These carriers are known as carrier waves or high-frequency signals. The placement of a low frequency (LF) signal over the high frequency (HF) signal is known as modulation . Demodulation : The modulated signal receives by receiver, it separated the message signal from the modulated carrier signal.

11. Amplitude Modulation (AM) Amplitude modulation (AM) is the process in which the amplitude of the modulated wave is varied in accordance with the amplitude of modulating wave. In AM, the frequency of the carrier wave is unchanged .

12. Let us represent the modulating signal by e m and it is given as, em = Em sin wm t Let us represent the modulating signal by e c and it is given as, ec = Ec sin wc t Where, Em = Amplitude of modulating signal Ec = Amplitude of carrier signal Wm = frequency of modulating signal Wc = frequency of carrier signal

13. By using mathematical expression for above equation, it is given by, E AM = E c + e m E AM = E c + E m sin w m t using instantaneous value of the amplitude modulated wave can be given as, e AM = E AM sin θ e AM = E AM sin w c t e AM = ( E c + E m sin w m t) sin w c t

14. Modulation Index: It is defined as the ratio of maximum amplitude of modulating signal to maximum amplitude of carrier signal is called modulation index. m = Em / Ec When the modulation index is expressed in percentage is called percentage modulation. m = ( Em / Ec ) * 100

15. Frequency spectrum & Bandwidth: The modulated carrier has new signals at different frequencies called sidebands. Upper sideband frequencies f USB = f c + f m Lower sideband frequencies f USB = f c - f m

16. Consider the expression for AM wave equation is given by, e AM = ( E c + E m sin w m t) sin w c t We know that, m = Em / Ec Em = m Ec e AM = ( E c + mE c sin w m t) sin w c t e AM = E c sin w c t + mE c sin w m t . sin w c t

17. We know that , Sin (A) . Sin (B) = 1 / 2 C os (A-B) – 1 / 2 C os (A+B) Applying to the e AM equation, e AM = E c sin w c t + mE c /2 {Cos( Wc –Wm)t } - mE c /2 {Cos( Wc+Wm )t } Bandwidth: It is defined as the difference between upper frequency and lower frequency. B/W = f USB – f LSB = ( f c + f m ) – ( f c + f m ) B/W = 2 f m