Se elex students

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Presented by

• Se elex students
• Pvppcoe

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TYPES OF PULSE MODULATION
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Pulse Amplitude Modulation

• Pulse-amplitude modulation, acronym PAM, is a
  form of signal modulation where the message
  information is encoded in the amplitude of a
  series of signal pulses.
• Example A two-bit modulator (PAM-4) will take
  two bits at a time and will map the signal
  amplitude to one of four possible levels, for
  example -3 volts, -1 volt, 1 volt, and 3 volts.
• Demodulation is performed by detecting the
  amplitude level of the carrier at every symbol
  period.

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Types Of PAM

• There are two types of pulse amplitude
  modulation
• 1.Single polarity PAM In this a suitable fixed
  dc level is added to the signal to ensure that
  all the pulses are positive going.
• 2.Double polarity PAM In this the pulses are
  both positive and negative going.
• Pulse-amplitude modulation is widely used in
  baseband transmission of digital data, with
  non-baseband applications having been largely
  replaced by pulse-code modulation, and, more
  recently, by pulse-position modulation.

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Use in Ethernet

• Some versions of the Ethernet communication
  standard are an example of PAM usage.
• In particular, the Fast Ethernet 100BASE-T2
  medium (now defunct), running at 100 Mbit/s, uses
  five-level PAM modulation (PAM-5) running at 25
  megapulses/sec over two wire pairs.
• A special technique is used to reduce
  inter-symbol interference between the unshielded
  pairs.citation needed.
• Later, the gigabit Ethernet 1000BASE-T medium
  raised the bar to use four pairs of wire running
  each at 125 megapulses/sec to achieve 1000 Mbit/s
  data rates, still utilizing PAM-5 for each pair.

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Use in photobiology

• The concept is also used for the study of
  photosynthesis using a PAM fluorometer.
• This specialized instrument involves a
  spectrofluorometric measurement of the kinetics
  of fluorescence rise and decay in the
  light-harvesting antenna of thylakoid membranes,
  thus querying various aspects of the state of the
  photosystems under different environmental
  conditions.

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Use in electronic drivers for LED lighting

• Pulse-amplitude modulation has also been
  developed for the control of light-emitting
  diodes (LEDs), especially for lighting
  applications.
• LED drivers based on the PAM technique offer
  improved energy efficiency over systems based
  upon other common driver modulation techniques
  such as pulse-width modulation (PWM) as the
  forward current passing through an LED is
  relative to the intensity of the light output and
  the LED efficiency increases as the forward
  current is reduced.
• Pulse-amplitude modulation LED drivers are able
  to synchronize pulses across multiple LED
  channels to enable perfect colour matching. Due
  to the inherent nature of PAM in conjunction with
  the rapid switching speed of LEDs it is possible
  to use LED lighting as a means of wireless data
  transmission at high speed.

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Pulse Width Modulation

• Pulse-width modulation (PWM), or pulse-duration
  modulation (PDM), is a commonly used technique
  for controlling power to inertial electrical
  devices, made practical by modern electronic
  power switches.
• The PWM switching frequency has to be much faster
  than what would affect the load, which is to say
  the device that uses the power.
• The main advantage of PWM is that power loss in
  the switching devices is very low.
• PWM has also been used in certain communication
  systems where its duty cycle has been used to
  convey information over a communications channel.

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Types of PWM

• Three types of pulse-width modulation (PWM) are
  possible
• The pulse center may be fixed in the center of
  the time window and both edges of the pulse moved
  to compress or expand the width.
• The lead edge can be held at the lead edge of the
  window and the tail edge modulated.
• The tail edge can be fixed and the lead edge
  modulated.

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Principle

• Pulse-width modulation uses a rectangular pulse
  wave whose pulse width is modulated resulting in
  the variation of the average value of the
  waveform.
• The simplest way to generate a PWM signal is the
  intersective method, which requires only a
  sawtooth or a triangle waveform (easily generated
  using a simple oscillator) and a comparator
• When the value of the reference signal (the red
  sine wave in figure 2) is more than the
  modulation waveform (blue), the PWM signal
  (magenta) is in the high state, otherwise it is
  in the low state.

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Applications
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Telecommunications

• In telecommunications, the widths of the pulses
  correspond to specific data values encoded at one
  end and decoded at the other.
• Pulses of various lengths (the information
  itself) will be sent at regular intervals (the
  carrier frequency of the modulation).
• The inclusion of a clock signal is not necessary,
  as the leading edge of the data signal can be
  used as the clock if a small offset is added to
  the data value in order to avoid a data value
  with a zero length pulse.

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Power delivery

• PWM can be used to control the amount of power
  delivered to a load without incurring the losses
  that would result from linear power delivery by
  resistive means.
• Potential drawbacks to this technique are the
  pulsations defined by the duty cycle, switching
  frequency and properties of the load.
• With a sufficiently high switching frequency and,
  when necessary, using additional passive
  electronic filters, the pulse train can be
  smoothed and average analog waveform recovered.

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Voltage regulation

• PWM is also used in efficient voltage regulators.
  
• By switching voltage to the load with the
  appropriate duty cycle, the output will
  approximate a voltage at the desired level. The
  switching noise is usually filtered with an
  inductor and a capacitor. ne method measures the
  output voltage.
• When it is lower than the desired voltage, it
  turns on the switch. When the output voltage is
  above the desired voltage, it turns off the
  switch.

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Audio effects and amplification

• PWM is sometimes used in sound (music) synthesis,
  in particular subtractive synthesis, as it gives
  a sound effect similar to chorus or slightly
  detuned oscillators played together.
• he ratio between the high and low level is
  typically modulated with a low frequency
  oscillator, or LFO.
• In addition, varying the duty cycle of a pulse
  waveform in a subtractive-synthesis instrument
  creates useful timbral variations.

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Applications for RF communications

• Narrowband RF (radio frequency) channels with low
  power and long wavelengths (i.e., low frequency)
  are affected primarily by flat fading, and PPM is
  better suited than M-FSK to be used in these
  scenarios.
• One common application with these channel
  characteristics, first used in the early 1960s,
  is the radio control of model aircraft, boats and
  cars.
• PPM is employed in these systems, with the
  position of each pulse representing the angular
  position of an analogue control on the
  transmitter, or possible states of a binary
  switch.

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• The number of pulses per frame gives the number
  of controllable channels available.
• The advantage of using PPM for this type of
  application is that the electronics required to
  decode the signal are extremely simple, which
  leads to small, light-weight receiver/decoder
  units.
• Servos made for model radio control include some
  of the electronics required to convert the pulse
  to the motor position the receiver is merely
  required to demultiplex the separate channels and
  feed the pulses to each servo.

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• More sophisticated R/C systems are now often
  based on pulse-code modulation, which is more
  complex but offers greater flexibility and
  reliability.
• Pulse position modulation is also used for
  communication to the ISO/IEC 15693 contactless
  smart card as well as the HF implementation of
  the EPC Class 1 protocol for RFID tags.

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Pulse Position Modulation

• Pulse-position modulation (PPM) is a form of
  signal modulation in which M message bits are
  encoded by transmitting a single pulse in one of
  possible time-shifts
• One of the key difficulties of implementing this
  technique is that the receiver must be properly
  synchronized to align the local clock with the
  beginning of each symbol. Therefore, it is often
  implemented differentially as differential
  pulse-position modulation, whereby each pulse
  position is encoded relative to the previous,
  such that the receiver must only measure the
  difference in the arrival time of successive
  pulses.
• It is possible to limit the propagation of errors
  to adjacent symbols, so that an error in
  measuring the differential delay of one pulse
  will affect only two symbols, instead of
  affecting all successive measurements.

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Illustration of PAM, PWM and PPM(a) is input
(information) signal
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Time division multiplexing of two PAM signals
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Effects of noise on pulses(a) effect on PAM only
for ideal pulses(b) effect on PWM PPM for
practical pulses
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Natural sampling in t- and f-domains
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Flat topped sampling (PAM) in t- and f-domains
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A definition of baseband and bandpass signals
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Relationship between PAM, quantised PAM and PCM
signals
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Input/output SNR for PCM
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Delta PCM transmitter and receiver
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Quantisation error interpreted as noise, i.e.
gq(t) g(t) ?q(t)
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