Transcript TYPES OF PULSE MODULATION

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