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					                  Chapter 11.
         Sampling and Pulse Modulation


• The Sampling Theorem
• PAM -- Natural and Flat-Top Sampling
      Time-Division Multiplexing (TDM)
      Intersymbol Interference (ISI)
• Pulse Width and Pulse Position Modulation
      Demodulation
• Digital Modulation
      Pulse Code Modulation (PCM)
      Delta Modulation (DM)
• Qualitative Comparisons Of Pulse and Digital
  Modulation Systems
                                                   1
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
               The Sampling Theorem




   Figure 11-1. Impulse sampling of an analog voltage.
                                                         2
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
               The Sampling Theorem


• A sampler is a mixer with a train of very narrow
  pulses as the local oscillator input.
• If the analog input is sampled instantaneously at
  regular intervals at a rate that is at least twice the
    highest analog frequency
                  fs > 2fa(max)

• then the samples contain all of the information of
  the original signal.
                                                           3
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
               The Sampling Theorem

• The analog signal v(t) has a signal spectrum
  represented by the Fourier transform V(f),
• and the sampling signal
                              
                 s t       t  nT    s
                            n  

  consists of instantaneous impulses every nTs sec,
  where n = 0, +1, +2, …
• The Fourier transform of s(t) is
                                     
                 S f              f  nf 
                          1
                                                s
                          Ts      n  

                                                      4
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
               The Sampling Theorem

• The time-domain product performed by the
  sampler produces a sampled output
  spectrum given by
                                  
                Vs  f         V  f  nf 
                           1
                                                   s
                           Ts   n  

• where this spectrum consists of replicas of
  the analog signal spectrum V(f), translated
  in frequency by each of the sampling
  frequency harmonics.
                                                       5
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
               The Sampling Theorem

• The sampler is a wideband (harmonic) mixer
  producing upper and lower sidebands at each
  harmonic of the sampling frequency.
• Figure 11-2a illustrates the correct way to sample: if
  sampling is done at fs > 2fA(max) the upper and
  lower sidebands do not overlap each other,
• and the original information can be recovered by
  passing the signal through a low-pass filter (see
  Figure 11-2c and d).


                                                           6
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
               The Sampling Theorem




    Figure 11-2. Sample spectra and their outputs. (a) fs > 2fA(max)
    Nyquist criteria met. (b) fs < 2fA(max) Frequency foldover of
    “aliasing” distortion occurs. (c) fs > 2fA(max) and recovery of
    original information with low-pass filter. (d) The original analog   7
    signal spectrum following recovery as in (c).
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
               The Sampling Theorem


• However, if the sampling rate is less than the
  Nyquist rate, fs < 2fA(max) the sidebands overlap,
  as shown in Figure 11-2b.
• The result is frequency-folding or aliasing distortion,
  which makes it impossible to recover the original
  signal without distortion.




                                                            8
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
       Pulse Amplitude Modulation –
       Natural and Flat-Top Sampling

• The circuit of Figure 11-3 is used to illustrate pulse
  amplitude modulation (PAM). The FET is the
  switch used as a sampling gate.
• When the FET is on, the analog voltage is shorted to
  ground; when off, the FET is essentially open, so
  that the analog signal sample appears at the output.
• Op-amp 1 is a noninverting amplifier that isolates
  the analog input channel from the switching
  function.
                                                           9
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
       Pulse Amplitude Modulation –
       Natural and Flat-Top Sampling




    Figure 11-3. Pulse amplitude modulator,
    natural sampling.
                                                   10
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
       Pulse Amplitude Modulation –
       Natural and Flat-Top Sampling

• Op-amp 2 is a high input-impedance voltage
  follower capable of driving low-impedance loads
  (high “fanout”).
• The resistor R is used to limit the output current of
  op-amp 1 when the FET is “on” and provides a
  voltage division with rd of the FET. (rd, the drain-to-
  source resistance, is low but not zero)



                                                            11
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
       Pulse Amplitude Modulation –
       Natural and Flat-Top Sampling

• The most common technique for sampling voice in
  PCM systems is to a sample-and-hold circuit.
• As seen in Figure 11-4, the instantaneous amplitude
  of the analog (voice) signal is held as a constant
  charge on a capacitor for the duration of the
  sampling period Ts.
• This technique is useful for holding the sample
  constant while other processing is taking place, but
  it alters the frequency spectrum and introduces an
  error, called aperture error, resulting in an inability
  to recover exactly the original analog signal.
                                                            12
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
       Pulse Amplitude Modulation –
       Natural and Flat-Top Sampling

• The amount of error depends on how mach the
  analog changes during the holding time, called
  aperture time.

• To estimate the maximum voltage error possible,
  determine the maximum slope of the analog signal
  and multiply it by the aperture time DT in Figure
  11-4.



                                                      13
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
       Pulse Amplitude Modulation –
       Natural and Flat-Top Sampling




    Figure 11-4. Sample-and-hold circuit and
                 flat-top sampling.
                                                   14
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
           Time-Division Multiplexing


• In the three-channel multiplexed PAM system of
  Figure 11-6, each channel is filtered and sampled
  once per revolution (cycle) of the commutator.
• Notice that the commutator is performing both the
  sampling and the multiplexing.

• The commutator must operate at a rate that satisfies
  the sampling theorem for each channel.
• Consequently, the channel of highest cutoff
  frequency determines the commutation rate for the
  system of Figure 11-6.
                                                         15
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
           Time-Division Multiplexing




        Figure 11-6. Time-division multiplex of three
                      information sources.
                                                        16
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
           Time-Division Multiplexing


• As an example, suppose the maximum signal
  frequency for the three input channels are
      fA1(max) = 4 kHz, fA2(max) = 20 kHz,
  and fA3(max) = 4 kHz.
• For the TDM system of Figure 11-6,
  the multiplexing must proceed at
       f > 2fA(max) = 40 kHz
  to satisfy the worst-case condition.


                                                   17
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
           Time-Division Multiplexing

• We can calculate the transmission line pulse rate as
  follows:
  The commutator completes one cycle, called a frame,
  every 1/40 kHz = 25 ms.
• Each time around, the commutator picks up a pulse
  from each of the three channels. Hence, there are
     3 pulses/frame x 40k frames/s = 120k pulses/s.



                                                         18
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
           Time-Division Multiplexing

• The 4 kHz channel is being sampled at five times
  the rate required by the sampling theorem. But if
  we slow down the commutator, the 20-kHz channel
  will be inadequately sampled.
• One the thought might be to multiplex at 8 k-
  frames/sec and sample the 20-kHz channel 5 times
  per frame.
• If you sketch this, as is done in Figure 11-7,
  you discover that there are
     7 pulses/frame x 8k frames/s = 56k pulses/s,
  which looks good.                                   19
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
             Time-Division Multiplexing




            Figure 11-7. Possible TDM solution.
                                                   20
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
           Time-Division Multiplexing

• The two missing samples stolen from the 20-kHz
  channel results in inadequate sampling and periodic
  aliasing distortion.
• For no errors, the commutation rate must be 17.14 kHz,
  producing 120k samples/s on the transmission line.
• A better scheme is shown in Figure 11-8 with insertion
  of channel 1 and 3 between two samples of channel 2.
• With 12.5 ms/pulse and 7 pulses/frame, the multiplexing
  rate can be
     (2 pulses/25ms)/(7 pulses/frame) = 11.428k frames/s
  and
     (11.428k frames/s) x (7 pulses/frame) = 80k pulses/s
  with no errors.                                           21
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
             Time-Division Multiplexing




           Figure 11-8. TDM solution for minimum
                  transmission line pulse rate.
                                                   22
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
          Pulse Width and Pulse Position
                   Modulation

• In pulse width modulation (PWM), the
  width of each pulse is made directly
  proportional to the amplitude of the
  information signal.
• In pulse position modulation, constant-width
  pulses are used, and the position or time of
  occurrence of each pulse from some
  reference time is made directly proportional
  to the amplitude of the information signal.
• PWM and PPM are compared and
  contrasted to PAM in Figure 11-11.
                                                   23
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
          Pulse Width and Pulse Position
                   Modulation




     Figure 11-11. Analog/pulse modulation signals.
                                                      24
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
          Pulse Width and Pulse Position
                   Modulation

• Figure 11-12 shows a PWM modulator. This circuit
  is simply a high-gain comparator that is switched
  on and off by the sawtooth waveform derived from
  a very stable-frequency oscillator.
• Notice that the output will go to +Vcc the instant
  the analog signal exceeds the sawtooth voltage.
• The output will go to -Vcc the instant the analog
  signal is less than the sawtooth voltage. With this
  circuit the average value of both inputs should be
  nearly the same.
• This is easily achieved with equal value resistors to
  ground. Also the +V and –V values should not
  exceed Vcc.                                             25
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
          Pulse Width and Pulse Position
                   Modulation




              Figure 11-12. Pulse width modulator.
                                                     26
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
          Pulse Width and Pulse Position
                   Modulation

• A 710-type IC comparator can be used for positive-
  only output pulses that are also TTL compatible.
  PWM can also be produced by modulation of
  various voltage-controllable multivibrators.
• One example is the popular 555 timer IC. Other
  (pulse output) VCOs, like the 566 and that of the
  565 phase-locked loop IC, will produce PWM.
• This points out the similarity of PWM to continuous
  analog FM. Indeed, PWM has the advantages of
  FM---constant amplitude and good noise immunity-
  --and also its disadvantage---large bandwidth.
                                                        27
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
                         Demodulation

• Since the width of each pulse in the PWM signal
  shown in Figure 11-13 is directly proportional to the
  amplitude of the modulating voltage.
• The signal can be differentiated as shown in Figure
  11-13 (to PPM in part a), then the positive pulses are
  used to start a ramp, and the negative clock pulses
  stop and reset the ramp.
• This produces frequency-to-amplitude conversion (or
  equivalently, pulse width-to-amplitude conversion).
• The variable-amplitude ramp pulses are then time-
  averaged (integrated) to recover the analog signal.      28
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
          Pulse Width and Pulse Position
                   Modulation




     Figure 11-13. Pulse position modulator.
                                                   29
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
                         Demodulation

• As illustrated in Figure 11-14, a narrow clock pulse
  sets an RS flip-flop output high, and the next PPM
  pulses resets the output to zero.
• The resulting signal, PWM, has an average voltage
  proportional to the time difference between the
  PPM pulses and the reference clock pulses.
• Time-averaging (integration) of the output
  produces the analog variations.
• PPM has the same disadvantage as continuous
  analog phase modulation: a coherent clock
  reference signal is necessary for demodulation.
• The reference pulses can be transmitted along with
  the PPM signal.                                        30
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
                         Demodulation


• This is achieved by full-wave rectifying the PPM
  pulses of Figure 11-13a, which has the effect of
  reversing the polarity of the negative (clock-rate)
  pulses.
• Then an edge-triggered flipflop (J-K or D-type) can
  be used to accomplish the same function as the RS
  flip-flop of Figure 11-14, using the clock input.
• The penalty is: more pulses/second will require
  greater bandwidth, and the pulse width limit the
  pulse deviations for a given pulse period.
                                                        31
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
                         Demodulation




                Figure 11-14. PPM demodulator.
                                                   32
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
         Pulse Code Modulation (PCM)


• Pulse code modulation (PCM) is produced by
  analog-to-digital conversion process.
• As in the case of other pulse modulation techniques,
  the rate at which samples are taken and encoded
  must conform to the Nyquist sampling rate.
• The sampling rate must be greater than, or equal to,
  twice the highest frequency in the analog signal,
                 fs > 2fA(max)

                                                         33
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
         Pulse Code Modulation (PCM)


• A simple example to illustrate the pulse code
  modulation of an analog signal is shown in Figure
  11-15.
• Here, an analog input sample becomes three binary
  digits (bits) in a sequence which represents the
  amplitude of the analog sample.
• At time t = 1, the analog signal is 3 V. This voltage
  is applied to the encoder for a time long enough that
  the three-bit digital "word", 011, is produced.

                                                          34
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
         Pulse Code Modulation (PCM)


• The second sample at t = 2 has an amplitude of 6 V,
  which is encoded as 110.
• This particular example system is conveniently set
  up so that the analog value (decimal) is encoded
  with its binary equivalent.




                                                        35
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
         Pulse Code Modulation (PCM)




         Figure 11-15. A 3-bit PCM system showing
                      A/D conversion.
                                                    36
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
                Delta Modulation (DM)


• Like PCM, a delta modulation system consists of an
  encoder and a decoder;
• unlike PCM, however, a delta modulator generates
  single-bit words that represent the difference (delta)
  between the actual input signal and a quantized
  approximation of the preceding input signal sample.

• This is represented in Figure 11-19 with a sample-
  and-hold, comparator, up-down counter staircase
  generator, and a D-type flip-flop (D-FF) to derive
  the digital pulse stream.
                                                           37
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
                Delta Modulation (DM)


• The continuous analog signal is band-limited in the
  low-pass filter (LPF) to prevent aliasing distortion,
  as in any sampling system.
• The analog signal VA is then compared to its
  discrete approximation VB.




                                                          38
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
                Delta Modulation (DM)




     Figure 11-19. Possible delta modulation encoder.
                                                        39
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
                Delta Modulation (DM)

• If the amplitude of VA is greater than VB , the
  comparator goes high, calling for positive going
  steps from the staircase generator.
• if, however, VB exceeds VA, the comparator goes low,
  calling for negative-going from the staircase
  generator.
• The comparator also sets the D flip-flop (D-FF) and
  the output will be properly clocked because the
  edge-triggered D-FF can change state only at rising
  edges of the input clock.
                                                         40
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
                Delta Modulation (DM)

• Decoding of the delta modulation (DM) signal
  can be accomplished with an up-down staircase
  generator and a smoothing filter
• or simply by integrating the DM pulses as shown
  in Figure 11-20. The resulting demodulated signal
  is illustrated as curve B.
• A practical implementation of a delta modulator is
  shown in Figure 11-21, where the up-down counter
  and digital-to-analog converter (DAC) comprise the
  staircase generator of Figure 11-19.
• The delta modulator of Figure 11-21 is usually
  referred to as a tracking or servo analog-to-digital
  converter.                                             41
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
                Delta Modulation (DM)




                 Figure 11-20. DM demodulator.     42
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
                Delta Modulation (DM)




      Fig. 11-21. Up-down staircase generator
                  for delta modulator.
                                                   43
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
                Delta Modulation (DM)

• As seen in Figure 11-22, the critical parameters
  determining the quality of a system using a constant
  step size are the designer’s choice of step size and
  sampling period length
• With too small a step size, the analog signal changes
  cannot be followed closely enough; this is called
  slope overload (Figure 11-22a).
• With too large a step size, two problems arise: poor
  signal approximation (resolution) and large
  quantization noise (Figure 11-22b). This condition is
  called granular noise.
                                                          44
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
                Delta Modulation (DM)

   Too long a period has the same problem as too small
    a step size and poor resolution (Figure 11-22c).
   When the period is too short, too much transmission
    bandwidth is required.




            Fig. 11-22. Critical design parameters in constant
                                                                 45
                        step-size linear delta modulation.
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
                Delta Modulation (DM)


Example:
   A 5-V pk, 4-kHz sinusoid is to be converted to
    a digital signal by delta modulation.
    The step size must be 10 mV.
   Determine the minimum clock rate that will
    allow the DM system to follow exactly the
    fastest input analog signal change,
    that is, to avoid slope overload.
                                                     46
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
                Delta Modulation (DM)

Solution:
   The fastest rate of change of a sinewave, v(t) =
    Vsinwt is the slope at the zero crossover points
    (t = 0 in Figure 11-23).
       slope = dv(t)/dt
              = (d/dt).(Vsinwt) = wVcoswt.
   At t = 0, the slope is wV.cos0 = wV or Dv/Dt =
    2pfV, where f is the frequency of the analog
    sinusoid.
                    Dv         10 mV
           Dt                           0.079 ms / cycle
                   2pfV   2p (4000 Hz )5V                      47
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
                Delta Modulation (DM)

 Dt is half the clock period because steps occur only at
  positive transitions of the clock in a practical system.
• Thus, Tclock = 2Dt = 1/fclock, so that fclock = 1/2Dt =
  1/(2x0.079x10-6 s/cycle) = 6.3 MHz.




                Figure 11-23. Example problem.               48
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
                        Practical DM

• A circuit configuration in present use for telecom-
  munication applications involving filters, speech
  scramblers, instrumentation, and remote motor
  control is shown in Figure 11-24.
• To demodulate the digital signal simple integrate
  the pulses. In fact, the integrator has the same RC
  time constant as the modulator above.
• The integrated demodulator output is the same as
  the B curve of Fig. 11-24. The integrator output is
  then put through a sharp cutoff LPF to smooth out
  the final gain amplifier (VGA) and decision logic as
  indicated in Fig. 11-25 for adaptive DM.               49
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
                        Practical DM




       Fig. 11-24. Integrating linear delta modulator
                   block diagram and signals.           50
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
               Adaptive DM (ADM)

• A solution to the tradeoffs and compromises
  of the simple delta modulation system above
  is to have a variable step size system.
• This could be accomplished as in Fig. 11-25
  with a variable-gain amplifier at the output
  of the D-type flip-flop.
• A decision circuit that counts the number of
  + and – steps taken over a given period of
  time and decides whether the step size
  should be increased and by how much.
                                                   51
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
               Adaptive DM (ADM)




           Figure 11-25. Adaptive delta modulator.   52
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
               Adaptive DM (ADM)

• As an example of how this circuit might operate,
  suppose that the adaptive algorithm (decision
  criteria) will be as follows: The preceding four bits
  of ADM output are counted.
• If an equal number of 1s and 0s occur in this
  interval (the last four bits), then the VGA gain will
  be (the FET switch will be a short).
• If more 1s than 0s or more 0s than 1s are counted,
  the step size is doubled, the input to the integrator
  will be doubled.
• The results are constructed for and analog input
  and compared to the results for the linear DM in
  Figs. 11-26 and 11-27.
                                                          53
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
               Adaptive DM (ADM)




    Figure 11-26. ADM with step or double step sizes.
      Arrows are shown along the horizontal axis to indicate
      where the step size changes. These changes are based
      on the number of 1s and 0s of the previous 4 bits.
                                                               54
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
               Adaptive DM (ADM)




       Fig. 11-27. Different DM results depending on
                        step size used.                55
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
         Qualitative Comparisons of
         Pulse Modulation Systems

• Like PAM, PCM can be time-division multiplexed
  because the modulated samples maintain a fixed
  position (slot) and duration in time.
• However, PCM is less noise-sensitive than PAM,
  and PCM can use digital constant-amplitude
  circuitry, unlike PAM, which requires linear
  circuits.
• A disadvantage of PCM is its greater bandwidth
  requirement. For example, in a simple 3-bit PCM
  system, three pulses must be transmitted, whereas
  only one is transmitted for the PAM sample (see
  Figure 11-30).                                      56
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
               Adaptive DM (ADM)




 Figure 11-30. Pulse and digital modulation waveforms.
                                                         57
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
         Qualitative Comparisons of
         Pulse Modulation Systems

• PWM and PPM are rarely used in
  multiplexed communication systems
  because of the large bandwidths required;
  FDM, which requires a more complex
  system than TDM, must also be used.
• While PWM and PPM have better noise
  performance than PAM (like FM and PM
  over AM), PWM and PPM are not easily
  regenerated, and therefore noise
  accumulates over long haul networks.
                                                   58
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
         Qualitative Comparisons of
         Pulse Modulation Systems

• An additional disadvantage for PPM over all the
  other techniques is that PPM, like continuous-phase
  modulation, requires coherent demodulation.
• This usually means that a phase-locked loop and its
  acquisition circuitry are required.
• In addition to these advantages for PCM over other
  pulse modulation techniques, the use of digital
  terminal equipment makes PCM more desirable in
  today’s communications marketplace.

                                                        59
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan