Amplitude Shift Keying

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Amplitude Shift Keying

• (ASK)

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Introduction

• Amplitude-shift keying (ASK) is a form of
  modulation that represents digital data as
  variations in the amplitude of a carrier wave.
• The amplitude of an analog carrier signal varies
  in accordance with the bit stream (modulating
  signal), keeping frequency and phase constant.
• The level of amplitude can be used to represent
  binary logic 0s and 1s.
• We can think of a carrier signal as an ON or OFF
  switch. In the modulated signal, logic 0 is
  represented by the absence of a carrier, thus
  giving OFF/ON keying operation and hence the name
  given.

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Binary ASK
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Bandwidth Issues

• There are sharp discontinuities shown at the
  transition points. These result in the signal
  having an unnecessarily wide bandwidth. Band
  limiting is generally introduced before
  transmission, in which case these discontinuities
  would be rounded off. The band limiting may be
  applied to the digital message, or the modulated
  signal itself.

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ASK Generation
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Demodulation

• ASK signal has a well defined envelope -
  demodulation by an envelope detector.
• (Band-limited version)
• Some sort of decision-making circuitry to get the
  bit stream back.

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Pros Cons

• One of the disadvantages of ASK, compared with
  FSK and PSK, for example, is that it does not
  have a constant envelope. This makes its
  processing (eg, power amplification) more
  difficult, since linearity becomes an important
  factor. However, it makes demodulation easy with
  an envelope detector.

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Frequency Shift Keying

• (FSK)

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Introduction

• A frequency shift keyed transmitter has its
  frequency shifted by the message.
• The transmitter could consist of two oscillators
  (on frequencies f1 and f2), with only one being
  connected to the output at any one time.

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Binary FSK
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FSK Generation
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Demodulation

• Two local carriers, on each of the two
  frequencies of the binary FSK signal, can be used
  in two synchronous demodulators. A decision
  circuit examines the two outputs, and decides
  which is the most likely.

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(No Transcript)
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Phase Shift Keying

• (PSK)

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Quiz

• The following figure shows a fiber optic
  operating procedure
• Please explain what you observe in this figure.

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Quiz
Discuss, in detail, the impact of noise on both
AM FM. Which of them is more immune to noise?
Y?
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Quiz

• Please discuss the concept of frequency hopping
  in bluetooth technology.