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 systemcarriersignal carried as hilltop beaconslighton-off (fire or no fire) telegraphelectric currenton-off (Morse code) cable TVinfrared in optical fibresultra-fast (on-off) pulses mob phonemicrowaveultra-fast (on-off) pulses AM radioradio wavechanging amplitude (AM) FM radioradio wavechanging 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 n 1) to another medium (refractive index n 2), (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 ~82 o

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 (2 8 = 256).

20. Resolution too lowSampling 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 systemspectrum widthbandwidth telephone300 - 3400 Hz3100 Hz FM radio station98.2 - 98.6 MHz0.4MHz