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PULSE AMPLITUDE MODULATION Pulse Amplitude Modulation (PAM) is used to describe the conversion of the analog signal to a pulse-type signal in which the amplitude of ... – PowerPoint PPT presentation

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Title: Eeng 360 1


1
Chapter 3 BASEBAND PULSE AND DIGITAL SIGNALING
  • Chapter Objectives
  • Analog-to-digital signaling (pulse code
    modulation ) Binary and multilevel digitals
    signals
  • Spectra and bandwidths of digital signals
  • Prevention of intersymbol interference
  • Time division multiplexing
  • Packet transmission

Huseyin Bilgekul Eeng360 Communication Systems
I Department of Electrical and Electronic
Engineering Eastern Mediterranean University
2
INTRODUCTION
  • This chapter we study how to encode analog
    waveforms into base band digital signals. Digital
    signal is popular because of the low cost and
    flexibility.
  • Main goals
  • To study how analog waveforms can be converted to
    digital waveforms, Pulse Code Modulation.
  • To learn how to compute the spectrum for digital
    signals.
  • Examine how the filtering of pulse signals
    affects our ability to recover the digital
    information. Intersymbol interference (ISI).
  • To study how we can multiplex (combine) data from
    several digital bit streams into one high-speed
    digital stream for transmission over a digital
    system Time-division Multiplexing.

3
PULSE AMPLITUDE MODULATION
  • Pulse Amplitude Modulation (PAM) is used to
    describe the conversion of the analog signal to a
    pulse-type signal in which the amplitude of the
    pulse denotes the analog information.
  • The purpose of PAM signaling is to provide
    another waveform that looks like pulses, yet
    contains the information that was present in the
    analog waveform.
  • There are two classes of PAM signals
  • PAM that uses Natural Sampling (gating)
  • PAM that uses Instantaneous Sampling to produce a
    flat-top pulse.

4
Natural Sampling (Gating)
  • DEFINTION If w(t) is an analog waveform
    bandlimited to B hertz, the PAM signal that uses
    natural sampling (gating) is
  • ws(t) w(t)s(t) Where
  • S(t) is a rectangular wave switching waveform and
    fs 1/Ts 2B.
  • THEORM The spectrum for a naturally sampled PAM
    signal is
  • Where fs 1/Ts, ?s 2p fs,
  • the Duty Cycle of s(t) is d t/Ts ,
  • W(f) Fw(t) is the spectrum of the original
    unsampled waveform,
  • cn represents the Fourier series coefficients of
    the switching waveform.

5
Natural Sampling (Gating)
w(t)
s(t)
ws(t) w(t)s(t)
6
Generating Natural Sampling
  • The PAM wave form with natural sampling can be
    generated using a CMOS circuit consisting of a
    clock and analog switch as shown.

7
Spectrum of Natural Sampling
  • The duty cycle of the switching waveform is d
    t/Ts 1/3.
  • The sampling rate is fs 4B.

8
Recovering Naturally Sampled PAM
  • At the receiver, the original analog waveform,
    w(t), can be recovered from the PAM signal,
    ws(t), by passing the PAM signal through a
    low-pass filter where the cutoff frequency is
    B ltfcutoff lt fs -B
  • If the analog signal is under sampled fs lt 2B,
    the effect of spectral overlapping is called
    Aliasing. This results in a recovered analog
    signal that is distorted compared to the original
    waveform.

LPF Filter B ltfcutoff lt fs -B
9
Demodulation of PAM Signal
  • The analog waveform may be recovered from the
    PAM signal by using product detection,

10
Instantaneous Sampling (Flat-Top PAM)
  • This type of PAM signal consists of
    instantaneous samples.
  • w(t) is sampled at t kTs .
  • The sample values w(kTs ) determine the
    amplitude of the flat-top rectangular pulses.

11
Instantaneous Sampling (Flat-Top PAM)
  • DEFINITION If w(t) is an analog waveform
    bandlimited to B Hertz, the instantaneous sampled
    PAM signal is given by
  • Where h(t) denotes the sampling-pulse shape and,
    for flat-top sampling, the pulse shape is,

THEOREM The spectrum for a flat-top PAM signal
is
12
The spectrum of the flat-top PAM
  • Analog signal maybe recovered from the flat-top
    PAM signal by the use of a LPF.

LPF Response Note that the recovered signal has
some distortions due to the curvature of the
H(f). Distortions can be removed by using a LPF
having a response 1/H(f).
13
Some notes on PAM
  • The flat-top PAM signal could be generated by
    using a sample-and-hold type electronic circuit.
  • There is some high frequency loss in the
    recovered analog waveform due to filtering effect
    H(f) caused by the flat top pulse shape.
  • This can be compensated (Equalized) at the
    receiver by making the transfer function of the
    LPF to 1/H(f)
  • This is a very common practice called
    EQUALIZATION
  • The pulse width t is called the APERTURE since
    t/Ts determines the gain of the recovered analog
    signal
  • Disadvantages of PAM
  • PAM requires a very larger bandwidth than that of
    the original signal
  • The noise performance of the PAM system is not
    satisfying.
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