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 up on the input binary sequence.

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

S(t)=A \cos 2\pi f_{c}t .

The normalized power is P=\frac{A^{2}}{2}

A=\sqrt{2P_{s}} .

The carrier signal can be expresses in terms of power as S(t)=\sqrt{2P_{s}} \cos 2\pi f_{c}t.

if energy per bit is E_{b} and the bit interval as T_{b} then the carrier signal is S(t)=\sqrt{\frac{2E_{b}}{T_{b}}} \cos 2\pi f_{c}t.

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

\left\{\begin{matrix} S_{ASK}(t)=\sqrt{2P_{s}} \cos 2\pi f_{c}t\rightarrow \ Binary\ '1' \\ =0 \ \rightarrow \ Binary\ '0' \end{matrix}\right.

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

\left\{\begin{matrix} S_{ASK}(t)=\sqrt{\frac{2E_{b}}{T_{b}}} \cos 2\pi f_{c}t\rightarrow \ Binary\ '1' \\ =0 \ \rightarrow \ Binary\ '0' \end{matrix}\right..

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, S_{ASK}(t)=b(t).S(t)

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.

\left\{\begin{matrix} S_{ASK}(t)=\sqrt{2P_{s}} \cos 2\pi f_{c}t\rightarrow \ Binary\ '1' \\ =0 \ \rightarrow \ Binary\ '0' \end{matrix}\right..

Coherent ASK Detector:-

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

S_{ASK}(t) 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 S_{ASK}(t).C(t) is applied to the Integrator. The Integrator integrates the input over one bit interval T_{b} 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.

v> 0\rightarrow \ a \ symbol \ '0' \ is \ detected. 

v\leq 0\rightarrow \ a \ symbol \ '1' \ is \ detected.

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

in coherent detection input to the demodulator is simply S_{ASK}(t) signal where as in Non-coherent detection the input is noisy ASK signal.


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Author: vikramarka

Completed M.Tech in Digital Electronics and Communication Systems and currently working as a faculty.