Unit1 Update

8/13/2019 Unit1 Update

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Modulation is the initial point of transmitting our message signal.

Fig 1.8 Classification of Modulation

Pulse ModulationIn pulse modulation some parameter of a pulse train is varied in accordance with the

massage signal. Two families of pulse modulation may be distinguished:

1).Analog pulse modulati on

2).Digi tal pulse modulati on.

1).Analog pulse modulationIn analog pulse modulation, a periodic pulse train is used as the carrier wave, and some

characteristics features of each pulse (e.g. Amplitude, Position, and Width) is varied in a

continuous manner in accordance with the corresponding sample value of the message signal.

Thus in analog pulse modulation, information is transmitted basically in analog form, but thetransmission takes place at discrete times.

It includes(1)Pulse Amplitude Modulation (PAM)(2) Pulse Time Modulation (PTM)

a). Pulse width Modulation (PWM)

b). Pulse position Modulation (PPM)

2).Digi tal pulse modulationIn digital pulse modulation, on the other hand, the massage signal is represented in a

form that is discrete in both time and amplitude; thereby permitting its transmission in digital

form as a sequence of coded pulses.It includes

1).Delta modulation

2).Adaptive Delta modulation3).Pulse code modulation.

4).Differential Pulse code modulation.


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1.4.1 Pulse Amplitude ModulationA sampled signal consists of a train of pulses, where each pulse corresponds to the

amplitude of the signal at the corresponding sampling time. The signal sent to line is modulatedin amplitude and hence the name Pulse Amplitude Modulation(PAM).

Fig 1.9.a PAM Generation

Fig 1.9.b Flattop Samples representing an analog signal

Depending upon the shape of pulse of PAM, we may classify the PAM as follows

1). Instantaneously sampled PAM or Ideal.

2).Naturally sampled PAM.3).Flat top sampled PAM.

1).Instantaneous sampling of message signal m(t) is chosen for every Tsseconds, where samplingrate fs=1 /Ts.

Reconstruction of transmitted signalThe received PAM signal is transmitted to reconstruction filter. Reconstruction filter refers toLPF here.

The distortion caused by the use of pulse amplitude modulation to transmit an analoginformationbearing signal is referred to as aperture effect.

The distortion can be corrected by connecting an equalizer with low pass reconstruction

filter as shown in fig 1.11.The equalizer has capability to reduce the aperture effect.If sampling frequency is greater than twice the bandwidth, the spectral bands aresufficiently separated from each other .

Band Limiting

Filter

Sampling

Circuit

Sample

and Hold LPF

Transmission

Link

Sampling Pulses

Input

Signal


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Fig 1.11 Reconstruction of PAM signal

The message signal is limited to bandwidth B and the sampling rate fs

is larger than the

Nyquist rate. By using flat-top samples to generate a PAM signal, amplitude distortion is

introduced.

Merits of PAM.

1.Easy to generate and demodulate it.

2. It carry information as well as to generate other modulations.

Demerits of PAM

1).Amplitude of PAM pulse varies in accordance with modulating signal. so interference of

noise is maximum. i.e., Aperture effect

2).Bandwidth required for transmission is very high.

3).Since the amplitude of PAM signal varies with modulating signal, peak power required for

transmitter and receiver is also varied.

4.)For long distance communication, its not suitable since noise can be easily affected.

Pulse Time Modulation (PTM)

Pulse Time Modulation (PTM) is a class of signaling technique that encodes the sample values

of an analog signal onto the time axis of a digital signal.

The two main types of pulse time modulation are:1. Pulse-duration modulation (PDM)

It can be also referred to as pulse-width modulation (PWM, where samples of

the message signal are used to vary the duration of the individual pulses in the carrier.

2. Pulse-position modulation (PPM)Here the position of a pulse relative to its un-modulated time of occurrence is varied inaccordance with the message signal.

Fig 1.12 Generation of PPM & PWM

Using PWM to generate an analog voltage level: A common use is in power suppl ies. The

PWM resolution is selected to be equal to or greater than the resolution requirements of the

power supply. A 5-Vol t power supply that can be adjusted to +/- 1 mi ll i -Volt shoul d use a


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PWM resolution of 5,000 or greater. The PWM output i s then f il tered to obtain acceptable

r ipple. The fi lter can be a simple low-pass fi lter.

F ig A2 PWM circuit using PIC

The fi g A2 shows a PIC microcontroll er generating a 50% duty cycle PWM signal at 5,000

Hz, a two-section 5,000 Hz lowpass f il ter and a pass-transistor wi th a direct curr ent i nput of

+2.5 Volts. The fi lter fr equency = 1/2 pi RC for each section.

PDM or PWM

In PWM the sample values of the analog waveform are used to determine the width of the

pulse signal. Either instantaneous or natural sampling can be used. The PWM is a largeamplitude digital signal that swings from one voltage extreme to the other.

The width of rectangular pulses are varied in accordance with message signal is called as PWM.

Fig 1.13 PPM and PWM waveform


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PPM

Pulse position modulation (PPM) is a pulse modulation technique that uses pulses that

are of uniform height and width but displaced in time from some base position according to the

amplitude of the signal at the instant of sampling.

Pulse position modulation is also sometimes known aspulse-phase modulation.

Generation of PWM & PPM

Generation of PWM and PPM commonly employs various combinations of a sample and

hold circuit, a precision ramp voltage generator and a comparator. The ramp generator produces

a precision ramp voltage which has peak to peak amplitude slightly larger than the maximumamplitude range of the input signals as shown in fig 1.12.

The comparator is a high gain amplifier intended for two stated operation.

If input signal is higher than a preset reference level, the output is held in onestate (i.e. a given voltage level).

Whenever the input signal level is less than the reference level, the output is heldin the other state.

Output state is depends upon whether the input is above and below the threshold (reference

level) of the comparator.

The voltage reference level of the comparator is adjust so that there is always anintersection with the sum of the sample and hold circuit and ramp voltage. In this system, the

first crossing of the reference level indicates the clock timing and the second crossing generates

the variable trailing edge.

Pulse width modulation is widely used in voltage regulators. I t works by switchi ng the voltage

to the load with the appropriate duty cycle; the output wil l maintain a voltage at the desir ed

level. Pulse width modulation is also exploited in sound synthesis, especially subtractive

synthesis, as the process gives a chorus effect or that of sl igh tly detuned oscil lators played

together. Another appli cation of PWM , is the class D ampli fi ers, known for better audio clari ty

alongside its basic function amplification. The class D amplifier produces a PWM

equivalent of the input analog signal, which is in tur n fed to the loud speaker af ter f il teri ng

out the carr ier wave by sending i t thr ough a sui table fil ter network. Class D ampli f iers are

growing in demand owing to its better eff iciency, lightweight, and cost. Also, due to the ful l

on/of f nature of PWM output, such ampli fi ers produce less heat than their conventi onal

analog counterparts.

Advantages of PWM


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Advantages of PPM

1.Easy to generate and demodulate.

Advantages of PPM

1. Pulse position modulation has the advantage over PAM and PDM in that it has a highernoise .

2. signal and noise separation is very easy3. Requires constant transmitter power since the pulses are of constant amplitude and

duration.

Disadvantages of PPM

1. Depending on transmitter-receiver, synchronization is needed.2. Highly sensitive to multipath way interference3. For transmitting narrow pulse, it requires large bandwidth.

1.4.3 Comparison of PAM, PPM, and PDM

Table 1.1 Comparison of PAM, PPM, and PDM

S.No PAM PWM/PDM PPM

1 The instantaneous power of

the transmitter varies.

The instantaneous power of

the transmitter varies.

The instantaneous power

of the transmitterremains constant.

2 Noise interference is high. Noise interference isminimum.

Noise interference isminimum.

3 The bandwidth of thetransmission channel

depends on width of the

pulse.

Bandwidth of transmissionchannel depends on rise

time of the pulse.

Bandwidth oftransmission channel

depends on rising time

of the pulse.

4 Amplitude of the pulse isproportional to amplitude

of modulating signal.

Width of the pulse isproportional to amplitude

of modulating signal.

The relative position ofthe pulse is proportional

to the amplitude of

modulating signal.5 Its operation is similar to

amplitude modulation.

Its operation is similar to

Frequency modulation.

Its operation is similar

to Phase modulation.

6 System to complex. Easy to implement Easy to implement

7 Waveform Waveform Waveform


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A0

Time

t0

Time

Time

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