Abcd

1. Communication
systems

2. 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

3. Communication systems
system carrier signal carried as
hilltop beacons light on-off (fire or no fire)
telegraph electric current on-off (Morse code)
cable TV infrared in optical fibres ultra-fast (on-off) pulses
mob phone microwave ultra-fast (on-off) pulses
AM radio radio wave changing amplitude (AM)
FM radio radio wave changing frequency (FM)

4. Comms technologies change

5. Communications: key terms
transmitter ……………........… receiver
encoding …………………….. decoding
modulation…………….... demodulation
All communication systems must contend with noise – unwanted
interference. Engineers consider signal-to-noise ratio.
Other parameters: data transmission rate, range, signal encoding.

6. Source–journey–detector
A useful model when describing communication systems
based on:
– visible light
– infrared
– microwaves
– radio waves

7. Fibre optic systems use light
Infrared light is used more commonly than visible light - less
attenuation and dispersion.
A simple transmitter:
button cell & LED
A simple detector:
phototransistor + multimeter

8. Constructing optical fibres
Two kinds of fibre are used.
within a
building
long
distances

9. schematic diagram
SEP Optical transmission set

10. Total internal reflection
In general, when passing from one medium (refractive index n1) to
another medium (refractive index n2),
(Snell’s law)
At the critical angle,
In optical fibres, the cladding material typically has a refractive
index ~1% lower than that of the core, so critical angle is ~82o
2
2
1
1
sin
sin 
 n
n 
1
2
2
o
2
1
o
2
1
sin
90
sin
sin
so
,
90
and
n
n
n
n
n
c
c
c











11. Wireless communication
… use microwaves and radio waves
Demonstration: Creating a radio wave

12. 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

13. 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

14. Modulating the amplitude of a carrier wave
Amplitude modulation

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

16. Modulating the frequency of a carrier wave
Frequency modulation
SKE Physics 17

17. (pulse code modulation, PCM)
Digital encoding of a carrier wave
Digital encoding

18. close-up of part of the previous image
Digital encoding

19. 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).

20. Resolution too low
Sampling rate too low
Encoding requires a sufficiently high sampling rate & resolution.
Digital to analogue decoding

21. 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)

22. Any waveform can be accurately represented as a sum of sine
waves, each with its own frequency.
If some of the frequencies are lost, then so is some of the
information in the signal.
Spectrum analysis

23. Bandwidth
Each kind of signal contains a range of frequencies.
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
system spectrum width bandwidth
telephone 300 - 3400 Hz 3100 Hz
FM radio station 98.2 - 98.6 MHz 0.4MHz