Digital modulation techniques

DIGITAL MODULATION TECHNIQUES

(PAM,PPM,PWM,PCM)

Presented By :-

AKASH KUMAR MODIE.No.- 10115009

What is Modulation ?Modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted.

Modulation is of two types :

Analog modulation:the modulation is applied continuously in response to the analog information signal.Digital modulation: In digital modulation, an analog carrier signal is modulated by a discrete signal.The system represents a small set of abstract symbols, e.g., 0 and 1 for a binary transmission system.

Pulse: A pulse is an abruptly changing voltage or current wavewhich may or may not repeat itself. The simplest non repetitive pulse is a stepped up voltage or current which can be obtained by connecting a voltmeter across a battery through a switch and then suddenly closing the switch. The voltmeter will read Zero upto a time when the switch is closed, where upon the voltage will suddenly rise to its maximum value and will stay there.

Digital Modulation:Pulse Modulation: A periodic pulse train is used as a carrier. The following parameters of the pulse are modified in accordance with the message signal. Signal is transmitted at discrete intervals of time. Pulse amplitude modulation Pulse width modulation Pulse position modulation Message signal represented in a form that is discrete in both amplitude and time.The signal is transmitted as a sequence of coded pulses. No continuous wave in this form of transmission.This is called Pulse code modulation.

WHY DIGITAL MODULATION??In digital system, message security can be improved.

The digital communication system have more noise immunity and external interference rejection capabilities.

In digital communication system, the information (or intelligence) which is in digital form can be easily stored where as in analog signal cannot.

Digital signals having two values 0 and 1 and are easy to measure and evaluate. Error detection and correction is very easy in digital communication system as compared to the analog communication system.

Digital communication systems are more flexible in operation than analog communication system.

Disadvantages of digital modulation

The transmission (channel) band width required by the digital communication system is much more than the analog communication system.

Signal must be converted into digital. The cost of this conversion is high.

The digital communication systems are more complex than the analog comunnication systems.

A precise time synchronization is required between transmitter and receiver in Digital system transmission.

PULSE AMPLITUDE MODULATION(PAM)

Amplitudes of regularly spaced pulses varied in proportion to the corresponding sampled values of a continuous message signal.

Pulses can be of a rectangular form or some other appropriate shape.

Pulse-amplitude modulation is similar to natural sampling,where the message signal is multiplied by a periodic train of rectangular pulses.

In natural sampling the top of each modulated rectangular pulse varies with the message signal, whereas in PAM it is maintained flat.

For minimum distortion,the sampling rate should be more than twice the signal frequency.

There are two operations involved in the generation of PAM signal:

1. Instataneous sampling of the message signal m(t) every Ts seconds, where the sampling rate fs = 1/Ts is chosen.

2. Lengthening the duration of each sample so obtained to some constant value T.

Natural Sampling

PAM Signal

Where s(t) denote the sequence of the flat-top pulses generated and m(t) is the message signal recovered from s(t).

PAM Modulators:The PAM modulator is a simple “Emitter Follower” circuit. The modulating signal is applied at the input. At the base, a CLOCK signal is applied. The frequency of the clock signal is made equal to the frequency of carrier pulse train.

When the CLOCK signal is “high”, the circuit behaves as “Emitter follower” and the output follows the input (modulating) signal, when the CLOCK is “low”, the transistor is “cut off” and the output is zero. In this way, at the output we get PAM signal

Advantages of PAM :

It has simple transmitter and receiver designs.

It is used to carry information as well as to generate other pulse modulations.

Disadvantages of PAM :

Amplitude keeps varying so there is noise associated with it.

It requires high bandwidth.

Due to amplitude variation peak power of receiver also varies with it.

PULSE WIDTH MODULATION(PWM)

In PWM, the width of the carrier pulse is varied according to the instantaneous value of the modulating signal, while the amplitude remains constant. This system is also called “Pulse duration modulation” (PDM) or “Pulse length modulation” (PLM).

It can be classified as :

Trailing edge PWM :In this type, the trailing edge of the pulses in varied according to the amplitude of the modulating signal.The leading edges of the pulses, however remains at a fixed rate w.r.t. each other, the timing between each leading pulse edge therefore is constant.

Leading edge PWM :In this type of PWM, the leading edge of each pulse changes according to the amplitude of the modulating signal, the trailing edge of each pulse however is fixed and timing in between the trailing edges is constant.

Symmetrical PWM :In this type, the trailing as well as leading edges of the pulses are varied in accordance with the amplitude of the modulating signal. When the modulating signal is zero,the pulse is at reference width. So long the signal is positive,the pulse width increases. When the signal is negative the pulse width decreases, however the spacing between the pulses remains constant.

(i) Trailing edge (ii) Leading edge (iii) Symmetrical edge

PWM Modulators :

A saw tooth waveform whose period is equal to the largest pulse width to be produced is added to the information signal. A level detector and a.c. pulse shaper are used to produce PWM wave.

Another method to generate PWM is by IC 555. It is basically a monostable multivibrator. The information (modulating signal)is given at pin 5. The carrier pulse is given at pin 2 and output is obtained at pin 3. The modulating signal changes the control voltage and hence the threshold voltage level. As a result, the time period required to charge the capacitor C to the thresholdvoltage changes that gives the PWM output. 

Advantages of PWM :

Noise is less in PWM as the amplitude is kept constant.

The signal and noise separation is easy.

The PWM does not require synchronization between transmitter and receiver.

Disadvantages of PWM :

Large bandwidth is required for PWM communication as compared to PAM.

The transmitter should be able to handle more power (equal to the power of the maximum width pulse).

PULSE PHASE MODULATION (PPM)

In this type, the sampled waveform has fixed amplitude and width whereas the position of each pulse is varied as per instantaneous value of the analog signal.

PPM signal is further modification of a PWM signal. It has positive thin pulses (zero time or width) corresponding to the starting edge of a PWM pulse and negative thin pulses corresponding to the ending edge of a pulse.

PPM Modulator

For generating a PPM signal (Figure) the modulating signal is first converted into the pulse width modulation (PWM) signal by sending it through a PWM converter. The PWM signal is then fed to a differentiator. At the output, positive and negative spikes are obtained. These spikes are passed through a positive clipper which removes the positive spikes and gives negative spikes at the output. The negative spikes are now made to trigger a monostable multivibrator and finally the PPM signal is obtained.

Advantages of PPM

As the amplitude and width is kept constant, the transmitter handles constant power.

As amplitude is constant, It is less noisy.

The signal and noise separation is easy.

Due to constant pulse width and amplitude the transmitted power for each pulse is same.

DisadvantagesIt needs synchronization between transmitter and receiver.

Large bandwidth is required as compared to PAM.

PULSE CODE MODULATION(PCM)

A PCM stream is a digital representation of an analog signal, in which the magnitude of the analog signal is sampled regularly at uniform intervals, with each sample being quantized to the nearest value within a range of digital steps.

Properties determining fidelity of PCM :

The sampling rate which is the number of times per second that samples are taken. According to sampling theorem, number of Pulses per second should be twice of signal frequency.

The bit depth which determines the number of possible digital values that each sample can take.

Analog signal is converted into digital signal by using a digital code.

Analog to digital converter employs three techniques:

1. Sampling: The process of generating pulses of zero width and of amplitude equal to the instantaneous amplitude of the

analog signal. The no. of pulses per second is called “sampling rate”.

2. Quantization: The process of dividing the maximum value of the analog

signal into a fixed no. of levels in order to convert the PAM into a Binary Code. The levels obtained are called

“quanization levels”.

3. Encoding : Encoding maps the quantized values to digital words that are bits long. The mapping is one-to-one so there is no distortion introduced by encoding.

PCM Sampling :

The function of a sampling circuit in a PCM transmitter is to periodically sample the continually changing analog input voltage and convert those samples to a series of constant- amplitude pulses that can more easily be converted to binary PCM code. There are two basic techniques used to perform the sampling function :Natural sampling : when tops of the sample pulses retain their natural shape during the sample interval.

Flat top sampling

For a sample to be reproduced accurately in a PCM receiver, each cycle of the analog input signal (fa) must be sampled at least twice.

The minimum sampling rate is equal to twice the highest input frequency,

fs ≥ 2fa

where fs is minimum sampling rate.

Sampling :

Quantization

Quantization is the process of “rounding off” a sample according to some rule.

E.g. suppose we must round to the nearest tenth, then:

3.752 --> 3.8 0.001 --> 0Quantizing makes the signal discrete in amplitude

and round off the Value to one of q discrete levels.

The error from the true value to the quantum value is called quantization distortion.

Uniform and non-uniform quantization

When the steps have uniform size the quantization called as uniform quantization.

For uniform quantization, the quantization noise is the same for all signal magnitudes.

Nonuniform quantization can provide fine quantization of the weak signal and coarse quantization of the strong signal

Encoding:

Mapping of quantized values to digital words.

No distortion introduced.

 For the sine wave example at left, we can verify that the quantized values at the sampling moments are 7, 9, 11, 12, 13, 14, 14, 15, 15, 15, 14,etc. Encoding these values as binary numbers would result in the following set of nibbles: 0111 (23×0+22×1+21×1+20×1=0+4+2+1=7), 1001, 1011, 1100,1101, 1110, 1110, 1111, 1111, 1111, 1110, etc.

Noise in PCM system

Channel noise: which is introduced anywhere betweentransmitter output and the receiver input, channel noise is always present, once the equipment is switched on.

Quantization noise: which is introduced in transmitter and is carried all the way along to the receiver output.

Limitations of PCM:Choosing a discrete value near the analog signal for each sample leads to quantization error.

Between samples no measurement of the signal is made; the sampling theorem guarantees non-ambiguous representation and recovery of the signal only if it has no energy at frequency fs/2 or higher (one half the sampling frequency, known as the Nyquist frequency); higher frequencies will generally not be correctly represented or recovered.

As samples are dependent on time, an accurate clock is required for accurate reproduction. If either the encoding or decoding clock is not stable, its frequency drift will directly affect the output quality of the device

Conclusion:

Digital Modulation techniques simplifies storing and transmission of signals.

Mixing the signals is easy. All signals look alike after conversion to digital form independent of the source (or language!). Hence they can easily be multiplexed (and demultiplexed)

Coding the message sequence to take care of the channel noise,encrypting for secure communication can easily be accomplished in the digital domain.

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