Radio Waves

• Radio systems start with sound passing into a microphone, the sound signal is converted into a radio signal and at the end, it is converted back into a sound signal
• The information signal is transmitted with a carrier wave; higher frequency, so shorted aerial required and different frequencies for different situations
  
• Modulation is the variation of either the amplitude or the frequency of the carrier wave
• Advantages of Modulation over Direct:
  • Shorter aerial required
  • Longer transmission range
  • Less attenuation
  • Allows more than one station in a region
  • Less distortion

Amplitude Modulation (AM)

• For amplitude modulation, the amplitude of the carrier wave is made to vary in synchrony with the displacement of the information signal
• The frequency of the carrier wave does not vary
  
• The amplitude of the signal must be less than half of the amplitude of the carrier wave
• The variation in the amplitude of the carrier wave is a measure of the displacement of the information signal
• The rate at which the carrier amplitude varies is equal to the frequency of the information signal
• An amplitude modulated wave consists of three components
  • Original carrier wave of frequency f_c and amplitude A_c
  • A wave of frequency f_c – f_a and Amplitude ^A_a/₂
  • A wave of frequency f_c + f_a and amplitude ^A_a/₂
    
• The central frequency f_c is that of the high-frequency carrier wave
• The other two are known as sidebands
• The range of frequencies from the min to max in the modulated carrier wave is called its bandwidth
  • (f_c + f_a) – (f_c – f_a) = 2f_a

Frequency Modulation

• For frequency modulation, the frequency of the carrier wave is made to vary in synchrony with the displacement of the information signal
• The amplitude of the carrier wave does not vary
  
• The change in frequency of the carrier wave is a measure of the displacement of the information signal
• The rate at which the carrier wave frequency is made to vary is equal to the frequency of the information signal

Comparison of AM and FM

Analogue and Digital Signals

• Noise: Random, unwanted signal that adds to and distorts a transmitted signal
• Attenuation: Progressive power loss in the signal as it travels along the transmission path
• Analogue Signal: Signal has same variation (with time) as the data and is continuously variable
• If an analogue signal is transmitted over a large distance, it will be attenuated and pick up noise
• For it to continue to travel, the signal is amplified by a repeater amplifier, however, the noise would also be amplified, causing the signal too become very noisy
  
• Digital Signal: Consists of a series of ‘highs’ and ‘lows’ and has no intermediate values
• Digital signals are made up of only highs and lows so even though they get noisy during transmission, regenerator amplifiers reproduce the original digital signal and hence ‘filter out’ the noise
  
• Advantages of Digital Signals:
  • Signal can be regenerated and noise can be eliminated
  • Extra data can be added to check for errors
  • Multiplexing: Digital signals from a large number of different sources can be made to share the same path
  • Digital circuits are more reliable and cheaper to produce
  • Data can be encrypted for security

Analogue-to-Digital Conversion

• In digital transmission, the analogue signal is converted to digital using an analogue-to-digital converter (ADC)
• When received, it is converted back to analogue using a digital-to-analogue converter (DAC)
• To convert an analogue signal into digital, its voltage value is measured at regular intervals (sampling)
• These instantaneous voltage values (samples) are converted into binary numbers representing their value
  
• The binary bit 1 represents a ‘high’ voltage and 0 represents a ‘low’ voltage, hence, a digital signal is made of a series of high and low voltages
• The binary system has base 2 and each digit of a binary number is called a bit
• The bit on the left-hand side of a binary number is the most significant bit (MSB) and has the highest value
• The number of bits per sample limits the number of possible voltage levels (with 4 bits, there are 2⁴ = 16 levels; with 8 bits, there are 2⁸ = 256 levels
• A higher sampling frequency means that more information can be gathered from the analogue signal
• Improving Reproduction of Input Signal:
  • Increase the number of bits in the digital number at each sampling so that the step height is reduced
  • Increase the sampling frequency so the width of the step is reduced
• When transmitting the digital signal, a parallel to serial converter can be used to take all the bits and transmit them one after another, down a single line rather than having, e.g. 8 cables for an 8 bit number
• When received, a serial to parallel converter can convert the signal back to the original form

Channels of Communication

• Wire Pairs:
  • For example, linking a (land) telephone to the (local) exchange
    
  • The potential difference between the two wires is the signal
  • Each wire acts as an aerial and picks up unwanted electron=magnetic waves and distorts signal
  • Attenuation of the signal is high since energy is lost as heat due to resistance of the cable as well as electromagnetic radiation
  • Cross-linking/Cross-talk: Signal in one wire pair is picked by a neighboring wire pair
• Coaxial Cable:
  • For example, connecting an aerial to a television
    
  • Function of copper braid:
    • Acts as a ‘return’ for the signal
    • Shields inner core from noise/interference and cross-talk

  

• Radio and Microwave Link:
  • For example, linking a ground station to a satellite
    
  • Surface waves travel close to the surface of the earth and diffract around it due to long wavelengths
  • Sky waves travel in the atmosphere in straight lines, reflecting back and forth between the ionosphere and earth’s surface, hence, can go a long distance
  • Space waves have a higher frequency, so pass through the ionosphere and transmit in the line-of-sight
  • Disadvantages of Using Ionospheric Reflection:
    • Unreliable because ion layers vary in height/density
    • Cannot carry information required as bandwidth is too narrow
    • Coverage is limited and reception is poor in hilly areas

    

• Microwave Link:
  
  • The transmitter focuses the radio waves towards the receiver by using an aerial shaped as a dish as without it, the strength of the signal would decrease greatly
  • Each dish points towards a dish on another tower and transmit microwaves back and forth in the l ine-of-sight
  • Microwaves are secure and difficult to tap into as the bean travelling is narrow and doesn’t spread
• Optic Fibers:
  • Optic fibers are thin flexible glass rods and are used to carry digital information in the form of pulses of infrared radiation transmitted using total internal reflection
  • They are transmitted with infrared radiation because it has lower attenuation than visible light
  • Advantages:
    • Large bandwidth, so it can carry more information
    • Low attenuation of signal
    • Low cost
    • Smaller diameter, less weight; easier handling/storage
    • High security/no cross-talk
    • Low noise/no electromagnetic interference

Satellite Communication

• Carrier wave transmitted from Earth to satellite
• Satellite receives greatly attenuated signal
• Signal amplified and transmitted back to Earth at a different carrier frequency e.g. 6/4, 14/11, and 30/20
• Different frequencies prevent swamping of up-link signal
• High frequencies in GHz used:
  • no ionospheric reflection
  • large information carrying capacity

  
  

Signal Attenuation

• Attenuation is the gradual decrease in power of a signal the further it travels
• Power ratios are expressed in decibels (dB) because the numbers involved are smaller can cover a wider range
• Attenuation/amplification between two positions can be expressed in dB by:
  • number of decibels = 10 lg(^P_out/_Pin)
  • If the value is positive, there is an increase in power, hence, the signal has been amplified
  • If the value is negative, there is a decrease in power, hence, the signal has been attenuated
• Attenuation of cables is given as attenuation per unit length and is found by:
  • attenuation per unit length (dB km^-1) = ^attenuation/_{length of cable}
• Signal must be distinguishable above the level of noise and this can be measured by the signal-to-noise ratio:
  • signal-to-noise ratio = 10 lg(^{signal power}/_{noise power})
• Repeaters amplify both signal and noise so signal-to-noise ratio remains constant, however regeneration of digital signal removes most noise, therefore, high signal-to-noise ratio