Eeng 360 1

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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
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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.

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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 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.

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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.

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Natural Sampling (Gating)
w(t)
s(t)
ws(t) w(t)s(t)
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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.

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Spectrum of Natural Sampling

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

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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
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Demodulation of PAM Signal

• The analog waveform may be recovered from the
  PAM signal by using product detection,

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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.

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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
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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).
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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.