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In ASK, the two binary values are represented by to different amplitudes of the carrier frequencyThe resulting modulated signal for one bit time is
⎩⎨⎧
=0,01),2cos(
)(binarybinarytfA
ts cπ
Amplitude Shift KeyingDigital information
1 0 1 1 0 0 1 0 1 0
Carrier wave
ASK modulated signal
Carrier present Carrier absentAmplitude varying-frequency constant
Demodulation
• ASK signal has a well defined envelope ‐demodulation by an envelope detector.
(Band‐limited version)
• Some sort of decision‐making circuitry to get the bit stream back.
∫T
dt0∫
T
dt0
ADecision Device
THRESHOLD
Demodulated OutputIncoming ASK
Waveform1,if
threshold exceeded
0, otherwise
)2cos( tf cΠ
BANDWIDTH ISSUES
• The spectrum of the ASK signal shows that it has an infinite bandwidth.
• For practical purpose BW is defined as BW of an ideal band pass filter centered at fc whose output contains about 95% of the total average power content of the ASK signal .
• BW of ASK can be reduced by using smoothed version of the pulse waveform instead of rectangular pulse waveforms.
Pros & Cons
• One of the disadvantages of ASK, compared with FSK and PSK, for example, is that it does not have a constant envelope.
• Phase synchronization:‐ the carrier wave generated locally in the Rx is locked in phase with respect to one which is employed in the Tx
• Timing synchronization:‐ timing of the decision making operation in the receiver with respect to switching instants in the original binary data