MAYURBHANJ SCHOOL OF ENGINEERING � LAXMIPOSI ,BARIPADA,757107

• DEPARTMENT- E&TC ENGG.

SEMISTAR- 5^TH

• SUBJECT-A & D Communication
• NAME OF TOPIC –Communication System
• PREPARED BY - U S Panda

�Communication systems

Learning outcomes

• describe communication systems in terms of
• signal, carrier, noise, range, data transmission rate and bandwidth
• a source – journey – detector model, with transmitter and receiver
• modulation and demodulation (encoding and decoding)
• calculate the critical angle for total internal reflection using Snell's law
• describe advantages and limitations of optical fibre systems
• identify UK radio wave bands used for wireless communications
• describe amplitude modulation (AM), frequency modulation (FM) and digital signals graphically and in words
• use a variety of appropriate experiments and simulations when teaching about communications

Communication systems

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Comms technologies change

Communications: key terms

transmitter ……………........… receiver

encoding …………………….. decoding

modulation…………….... demodulation

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All communication systems must contend with noise – unwanted interference. Engineers consider signal-to-noise ratio.

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Other parameters: data transmission rate, range, signal encoding.

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Source–journey–detector

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A useful model when describing communication systems based on:

• visible light
• infrared
• microwaves
• radio waves

Fibre optic systems use light

Infrared light is used more commonly than visible light - less attenuation and dispersion.

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A simple transmitter: button cell & LED

A simple detector:

phototransistor + multimeter

Constructing optical fibres

Two kinds of fibre are used.

within a building

long

distances

schematic diagram

SEP Optical transmission set�

Total internal reflection

In general, when passing from one medium (refractive index n₁₎ to another medium (refractive index n₂₎,

(Snell’s law)

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At the critical angle,

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In optical fibres, the cladding material typically has a refractive index ~1% lower than that of the core, so critical angle is ~82^o

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Wireless communication

… use microwaves and radio waves

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Demonstration: Creating a radio wave

A radio frequency (r.f.) carrier wave of fixed amplitude is generated. �Its amplitude varies once an audio frequency (a.f.) signal is added.

Amplitude modulation

Making a simple radio receiver

A: AM modulated radio wave

B: After diode rectification

C: The r.f. wave is filtered out, leaving a.f. signal

Modulating the amplitude of a carrier wave

Amplitude modulation

A radio frequency (r.f.) carrier wave of fixed amplitude is generated. �Its frequency varies once an a.f. signal is added.

Frequency modulation

Modulating the frequency of a carrier wave

Frequency modulation

SKE Physics 17

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(pulse code modulation, PCM)

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Digital encoding of a carrier wave

Digital encoding

close-up of part of the previous image

Digital encoding

Analogue to digital encoding

analogue signal

sampling and encoding the analogue signal.

Digitised values are in binary form, so the resolution is expressed in bits. 8 bits encode an analogue value as one of 256 different levels (2⁸ = 256).

Resolution too low

Sampling rate too low

Encoding requires a sufficiently high sampling rate & resolution.

Digital to analogue decoding

UK frequency allocations

http://sitefinder.ofcom.org.uk/search

Transmitter locations, with this additional information:

• name of operator
• station type
• height of antenna
• frequency range
• transmitter power
• Maximum licensed power
• type of transmission (usually GSM)

Any waveform can be accurately represented as a sum of sine waves, each with its own frequency.

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If some of the frequencies are lost, then so is some of the information in the signal.

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Spectrum analysis

Bandwidth

Each kind of signal contains a range of frequencies.

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The higher the data rate, the larger the bandwidth and the higher the frequency band needed.

Bandwidth costs money: e.g. monthly charges for your mobile phone and Internet services