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Amplitude Shift Keying(ASK)

Amplitude Shift Keying (ASK) (or) On-Off Keying (OOK) is the simplest Digital Modulation technique.

In this method, carrier amplitude is switched between two voltages ON and OFF levels depending upon the input binary sequence.

The carrier signal is a continuous wave (or) sinusoidal waveform

.

The normalized power is

.

The carrier signal can be expresses in terms of power as .

if energy per bit is and the bit interval as then the carrier signal is .

Now according to ASK Binary ‘1’ is represented with carrier voltage and Binary ‘0’ is represented with zero voltage.

in terms of Energy and bit duration ASK signal can be written as 

.

ASK Transmitter:-

The figure shows the ASK generator (or) ASK Transmitter

It is a simple product Modulator, which modulates the incoming binary sequence (in the form of a signal) with the carrier signal S(t)

i.e,

b(t) represents the binary sequence in the form of a signal.

when the  input bit (or) symbol is Binary ‘1’ product Modulator passes the carrier signal and for Binary’0′, A zero output is given which blocks the carrier signal.

.

Coherent ASK Detector:-

The figure shows the Block Diagram of coherent ASK/BASK Detector. The ASK signal is applied to the correlator ( The Block product Modulator followed up by the Integrator).

is multiplied by local carrier C(t) this carrier C(t) is phase locked with that of the carrier used in the Transmitter. As this is coherent reception.

The product is applied to the Integrator. The Integrator integrates the input over one bit interval and the output is given to a threshold device. If the threshold voltage is set to 0 V.

the output of threshold device v(t) (or) v is either ‘1’ (or) ‘0’ based on the following condition.

 

Note:- The input to demodulator is not always most of the times it is interfered with noise n(t) in the channel.

in coherent detection input to the demodulator is simply signal where as in Non-coherent detection the input is noisy ASK signal.

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Pulse Code Modulation (PCM)

After getting an idea about the sampling & Quantization process we are now ready to describe pulse code modulation (PCM)

PCM is the most basic form of digital pulse modulation.

PCM:- In PCM a message signal is represented by a sequence of coded pulses. which is accomplished by representing the signal in discrete form in both time and amplitude.

PCM Block Diagram:-

The Block Diagram of a PCM system is given in the following figure. This Block Diagram consists of the Basic elements of a PCM system.

i. Transmitter

ii. Transmission path.

iii. Receiver.

PCM Transmitter:-  The basic operations performed in the transmitter are

  • Sampling
  • Quantization
  • Coding

is known as Analog-to-Digital conversion (ADC).

LPF:- The LPF prior to the sampler is inclined to prevent the aliasing of the message signal.

Sampler:- Converts the analog Band limited Signal to a discrete signal.

ADC:- It’s a device that performs two operations known as Quantization and Encoding.

Regenerative repeater:- PCM encoded output is transmitted through the transmission path to the receiver, during transmission the PCM output get distorted in the channel by noise which can be generated by the device called regenerative repeater.

PCM Receiver:- The essential operations in the receiver are regeneration of impaired PCM output decoding.

The device that performs decoding and demodulation at the Receiver is known as (DAC) Digital-to-Analog Converter.

A few points about PCM:-

  1. we know that PAM, PWM &PPM analog pulse modulation techniques. where as PCM is a pure Digital Modulation scheme.
  2. It should be understood that the PCM is not a modulation technique in the conventional sense.
  3. The output of PCM is in the coded digital form that is in the form of code words.  a PCM system consists of PCM encoder as well as Decoder.

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