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Amplitude Shift Keying

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Amplitude Shift Keying (ASK) Introduction Amplitude-shift keying (ASK) is a form of modulation that represents digital data as variations in the amplitude of a ... – PowerPoint PPT presentation

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Title: Amplitude Shift Keying


1
Amplitude Shift Keying
  • (ASK)

2
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.

3
Binary ASK
4
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.

5
ASK Generation
6
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.

7
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.

8
Frequency Shift Keying
  • (FSK)

9
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.

10
Binary FSK
11
FSK Generation
12
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.

13
(No Transcript)
14
Phase Shift Keying
  • (PSK)

15
Quiz
  • The following figure shows a fiber optic
    operating procedure
  • Please explain what you observe in this figure.

16
Quiz
Discuss, in detail, the impact of noise on both
AM FM. Which of them is more immune to noise?
Y?
17
Quiz
  • Please discuss the concept of frequency hopping
    in bluetooth technology.
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