Chapter 1 Fundamentals of Signals

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Chapter 1Fundamentals of Signals

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Basic Building Blocks of a Telecommunication
System

• - All telecommunication systems comprise certain
  fundamental blocks-an input transducer,
  transmitter, transmission medium, receiver, and
  output transducer. The interconnection may be
  one-way (e.g. Radio Broadcast, TV, etc.) or
  two-way (e.g. Telephone, Mobile Radio, etc.)
• - The information to be transmitted is first
  converted into electrical form to produce an
  equivalent electrical signal - a voltage or
  current waveform which is the electrical
  equivalent to the original information. (e.g.
  Microphone, TV camera, Computer Terminal, etc.)

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Basic Building Blocks of a Telecommunication
System
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Analogue Signals

• - Telecommunication transducers produce an
  electronic signal that directly follows the
  instantaneous variations of the original
  information energy. Such signals are called
  ANALGOUE signals.
• - (e.g. a microphone produces an electronic
  signal that follows the variations of sound
  energy that actuate the microphone. A loudspeaker
  receives the analogue electronic signal and
  reproduces the original sound energy variations.)

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Digital Signals

• - A digital signal, unlike continuous analogue
  signals, varies abruptly and changes between
  distinct voltage or current levels. (commonly the
  0 or 1 voltage levels of a binary system.)

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Direct Current (d.c.)

• - In certain electrical circuits the current
  flows only in one direction when the energy
  supply is connected, although the amount or
  strength of the current can be controlled. This
  is produced by an energy source such as a dry
  battery, accumulator or rotating generator.
• e.g. Use of Direct Current Signals - Morse Code
  (d.c. signaling)

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Main Disadvantages of d.c. Signals

• - Difficulty in transmission over long line
  circuits due to attenuation and distortion,
  although regeneration (boosting) and
  amplification are possible.
• - Connecting wires are always needed for the
  whole of telecommunication circuits.

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Alternating Current (a.c.) Waveforms

• - Varying or fluctuating d.c. signals have
  characteristics to a.c. signals. Alternating
  currents reverse direction at regular intervals
  with some repeating pattern or waveform. The main
  advantages of a.c. signals are
• 1) The strength or amplitude can easily be
  altered (e.g. by transformer, amplifier, etc.),
  allowing transmission over long lines.
• 2) Connecting wires are not necessarily required
  for the whole of a telecommunication circuit.

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Sinusoidal Signal Waveforms

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Sinusoidal Signal Waveforms

• - To understand the requirements of communication
  system and the factors which influence
  information transmission it is essential to learn
  the terms used to describe signal waveforms. We
  start by defining the important waveform Sine
  Wave.
• - The sine wave is the fundamental building block
  waveform for all communication systems. All
  practical signals can be synthesised from sine
  waves.

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Sinusoidal Signal Waveforms

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Sinusoidal Signal Waveforms

• - A pure sine wave showing the variation of
  signal strength with time can be displayed on an
  oscilloscope (CRO).

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Terms Used to Describe Periodic Waveform

• - Amplitude The instantaneous value of the
  signal strength, i.e. the magnitude of the signal
  at any instant of time.
• - Peak Amplitude (A) The maximum excursion from
  zero to either the positive or negative peaks.
• - Peak-to-peak Amplitude (2xA) The excursion
  from position of maximum to minimum amplitude.
• - Periodic Time (T) The time for one complete
  cycle of the waveform.
• - Frequency (f) The number of signal waveform
  cycles in one second.
• f 1/ T

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Relationship between Frequency, Wavelength and
Velocity

• - For a.c. waveform, the velocity, distance and
  time are related by
• Velocity, v Distance, D / Time
• Velocity, v Wavelength, ? / Periodic Time, T
  (sec)
• Frequency, f (Hz) 1 / Periodic time, T (sec)
• Velocity, v Wavelength, ? x Frequency, f

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Composition of Complex Waveforms

• - It can be shown by Fourier Analysis that any
  complex waveform is made up of a sinusoidal
  waveform having a certain frequency called
  Fundamental Frequency and a number of other
  sinusoidal waveforms having frequencies that are
  direct multiples of the fundamental frequency
  with decreasing peak values. These direct
  multiples are called Harmonics of the fundamental
  frequency. ( f, 2f, 3f, 4f, . , where f
  fundamental frequency)
• - Not all harmonics are necessarily required to
  synthesize a waveform. (e.g. The symmetrical
  square and triangular waves contain only
  odd-order harmonics)

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Composition of Complex Waveforms

• - A sine wave showing the variation of signal
  strength with time can be displayed on an
  Oscilloscope ( Time Domain)
• - The frequency components contained in a signal
  with amplitude variation can be displayed on
  Spectrum Analyzer (Frequency Domain)

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Voice Frequencies

• - The sounds produced in speech contain
  frequencies which lie within they frequency band
  100 - 10,000Hz. The pitch of the voice is
  determined by the fundamental frequency of the
  vocal cords.
• 200 - 1,000 Hz for women
• 100 - 500 Hz for men
• - The power content of speech is small, a good
  average being of the order of 10 - 20 microwatt.
  However, this power is not evenly distributed
  over the speech frequency range, most of the
  power being contained at frequencies in the
  region of 500 Hz for men and 800 for Hz for
  women.

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Voice Frequencies

• - In an ideal telecommunications system, all the
  frequencies present in a speech waveform would
  be transmitted over the communication system.
• - BUT
• 1) For economic reasons, the devices used in
  circuits that carry speech and music signals have
  a limited bandwidth.
• 2) Particularly for the longer-distance routes,
  a number of circuit are often transmitted over a
  single telecommunication system and this practice
  provides a further limitation of bandwidth.

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Voice Frequencies

• - Therefore, by international agreement the
  audio-frequency band for a commercial quality
  speech circuit routed over a multi-channel
  system is restricted to 300 - 3,400 Hz. This
  means that both the lower and upper frequencies
  contained in the average speech waveform are not
  transmitted.
• i.e. suppression of all frequencies above 3,400
  Hz reduces the quality of the sound does not
  affect its intelligibility.

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Noise Sources

• - Electrical noise id defined as any undesirable
  electrical energy.
• - Noise can be divided into two general
  categories uncorrelated and correlated.
• - Uncorrelated noise is present all the time
  whether there is signal or not. Uncorrelated
  noise can be further subdivided into two general
  categories external and internal.
• - Correlation implies a relationship between the
  signal and the noise. Correlated noise exists
  only when a signal is present.

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Uncorrelated Noise - External Noise/ Internal
Noise

• - External noise is noise that is generated
  outside the device or circuit. There are three
  primary sources of external noise atmospheric,
  extraterrestrial, and man-made.
• - Internal noise is electrical interference
  generated within a device or circuit. There are
  three primary kinds of internally generated
  noise thermal, shot and transit time.
• Thermal noise power N KTB
• N noise power (watts)
• K Boltzmanns proportionality constant (1.38 X
  10-23 joules per kelvin)
• T absolute temperature (kelvin)
• B bandwidth (hertz)

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Uncorrelated Noise - External Noise/ Internal
Noise

• - See Example 1.5 and 1.6
• - Because thermal noise is equally distributed
  throughout the frequency spectrum, a thermal
  noise source is sometimes called a white noise
  source, which is analogous to white light, which
  contains all visible-light frequencies.
• - Also, see Example 1.7 and 1.8 for power
  calculation.

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Correlation Noise

• - Correlated Noise is noise that is correlated
  (mutually related) to the signal and cannot be
  present in a circuit unless there is an input
  signal - simply stated, no signal, no noise!
• - Correlated noise is produced by nonlinear
  amplification and includes harmonic and
  intermodulation distortion, which are both forms
  of nonlinear distortion.

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Signal-to-Noise Power Ratio

• - Signal-to-noise power ratio (S/N) is the ratio
  of signal power level to the noise power level.
• Where Ps signal power (watts)
• Pn noise power (watts)
• The signal-to-noise power ratio is often
  expressed as a logarithmic function with decibel
  unit ( See Example 1.11and 1.12)

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Noise Factor and Noise Figure

• - Noise factor (F) and noise figure (NF) are
  figures of merit used to indicate how much the
  signal-to-noise ratio deteriorates as a signal
  passes through a circuit or series of circuits.
• - Noise factor is simply a ratio of input
  signal-to-noise ratio to output signal-to-noise
  ratio.
• - Noise figure is simply the noise factor stated
  in dB and is a parameter commonly used to
  indicate the quality of a receiver.

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Noise Factor and Noise Figure

• - For an ideal noiseless amplifier with a power
  gain (Ap)
• - For a nonideal amplifier that generate an
  internal noise (Nd)
• - See Example 1.13