- Outer conductor shields inner conductor
- Characteristic Impedance
- 50Ω / 75Ω
- Attenuation – 5dB/10km at 1GHz

- Cable resistance
- Leakage current between conductors
- Radiation of signal

- Velocity ratio = 0.66

- Differential signal
- Characteristic Impedance &asymp: 100Ω
- Attenuation – 2.2dB/10m at 100MHz

- Cable resistance
- Leakage current between conductors
- Radiation of signal

- Velocity ratio = 0.6

- Electromagnetic waves
c = 3 × 10⁸m/s
- Attenuation – 92.4dB/km at 1GHz

- Cladding 125µm diameter
- Core 50 - 62.5µm diameter
- Total Internal Reflection
- Attenuation – 3dB/km at 1GHz

• Enables more information to pass along a communications medium
• Frequency Division Multiplexing

- Analogue and Digital Information
- Separate carrier frequency for each signal

Time Division Multiplexing

- Digital Information only
- Separate time slot for each signal

• Attenuation

- Ohmic losses
- Radiation

• Crosstalk

- Interference from adjacent cables

• Dispersion

- Different frequencies travel at different speeds

• Distortion

Signals produced and added to a signal as a signal passes through a communications system

• Noise

Random additional signals added to a signal as it passes through a communications system

• The Decibel – logarithmic ratio

- Add together for each subsystem

• Amplification increases both the signal and the noise

– Frequency constant
- Amplitude varies
- Broadcast bandwidth is 9kHz
- Long and medium wave bands

– Constant amplitude
- Varying frequency
- Broadcast bandwidth is 100kHz
- VHF wave band

• Quality factor – Q
• Resonance when

• Aerial/earth:-

– changes em waves into electrical signal

• rf tuned circuit:-

– filters out required signals

• Demodulator:-

– removes bottom half of the AM signal

• Rf filter:-

– removes the remaining rf signal

• Output:-

– recovered information signal

• Poor selectivity:-

– Only one tuned circuit
- Increase the number of tuned circuits
- Difficult to tune several tuned circuits together

• Poor sensitivity:-

– No amplification
– uses energy received by the aerial
- Add rf amplifier
- Add af amplifier

- The incoming signals are mixed with the local oscillator signal
- One of the resulting frequencies is the Intermediate frequency
- This is filtered and amplified by the if amplifier

•1 rf amplifier

- Initial amplification and selection of signals from the aerial

•2 Mixer

- Combines local oscillator and radio signal to produce the intermediate frequency(if)

•3 Local oscillator

- Produces difference frequency

•4 if amplifier

- Contains many tuned circuits and amplifiers
- Provides most of the selectivity and sensitivity for the receiver

•5 Automatic Gain Control (AGC)

- Maintains a constant information output as the rf signal varies

•6 Demodulator

- Extracts the information from the carrier

•7 AF amplifier

- Amplifies the information

• if frequencies:-

AM receivers – 455kHz
VHF receivers – 10.7MHz
Analogue TVs – 39.5MHz

• f_s ± f_o = if

• f_s = signal frequency
• f_o = local oscillator frequency
• if = Intermediate frequency

• Image response

• Very short communication by radio from mobile handset to base station
• Repeater
• Regenerator
• Frequency reuse
• Cell size depends on the number of channels required
• Separate up link and down link frequencies

- GSM – 900MHz,
- DECT – 1.8GHz,
- 3G – 2GHz

• Analogue voltage measured at regular time interval and the values transmitted
• Sample rate

• Constant amplitude pulses
• Width of pulse indicates the magnitude of the analogue signal

• The pulses are of constant amplitude and duration
• The pulses are delayed at each sample by an amount that is dependent upon the amplitude of the signal

• The amplitude of the analogue signal is sampled
• The sample is then converted into a binary value by an ADC
• The binary value is transmitted in serial form

• Serial v Parallel
• Half Duplex

- One device transmits, the other receives
- Only one transmission medium is needed

• Full Duplex

- Both devices transmit and receive at the same time
- Two transmission media are needed

• Synchronous / Asynchronous • Handshaking

RTS - CTS

65₁₀ (00100001₂)

• Start bit
• Stop bit(s)
• Parity bit
• Baud rate
• Bit rate

• On the rising edge of each clock pulse, the data from a D-type flip-flop is stored in the next D-type flip-flop
• This data transfer occurs all of the way along the shift register
• Data from the output of the last flip-flop is lost
• New data applied to the input of the first flip-flop is taken into the shift register
• Timing Diagrams

– Multiplexer

– Demultiplexer

Both need a clock and a two bit counter

• Laser diode transmitters
  0.9 – 1.6µm
• PIN diode receivers
• Dispersion

• Total Internal Reflection
• Attenuation

- Scattering
- Absorption
- Radiation

• Compare with wired systems