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Amplitude Shift Keying (ASK) & Frequency Shift Keying (FSK). Digital Communication Systems. Some of the modulation techniques employed by digital communication systems are explained throughout this Lab. Today, Amplitude Shift Keying and Frequency Shift Keying are to be explained.

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Amplitude Shift Keying (ASK) & Frequency Shift Keying (FSK)

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Amplitude shift keying ask frequency shift keying fsk
Amplitude Shift Keying (ASK)&Frequency Shift Keying (FSK)


Digital communication systems
Digital Communication Systems

  • Some of the modulation techniques employed by digital communication systems are explained throughout this Lab. Today, Amplitude Shift Keying and Frequency Shift Keying are to be explained.


Amplitude shift keying ask
Amplitude Shift Keying (ASK)

  • In ASK, a sinusoidal carrier is gated ON and

    OFF by the binary sequence to be transmitted.

  • The message signal m(t) is a unipolar NRZ such that:

    m(t)=0 for logic 0

    0≤ t ≤ Tb

    m(t)= 1 for logic 1


Ask modulator
ASK Modulator

  • The ASK signal is generated as:

    Note: the carrier frequency is chosen such that

    fc=n*Rb ; where n is an integer

  • The transmitted signals are:

    - S1(t)=0 for logic 0

    - S2(t)=V cos (2*pi*fc*t) for logic 1


Amplitude shift keying ask frequency shift keying fsk

Illustration of ASK

1 0 1 1 0 0 1 0 1 0

Digital information

Carrier wave

ASK modulated signal

Amplitude varying-frequency constant

Carrier present

Carrier absent


Ask demodulator
ASK Demodulator

  • The basic block diagram of ASK demodulator ( Non-Coherent Demodulation):

    Notes:

  • The BPFis used to reduce both ; the noise and the

    interference of other signals through selecting the BW

    of each signal.

  • Sometimes a comparator is used instead of the schmitt

    trigger. ( Threshold = V/2 ) . They are used to convert the

    analog signal to a digital one.


Schmitt trigger
Schmitt Trigger

  • A Schmitt trigger is a circuit with positive feedback and a loop gain greater than 1

  • When the input is higher than a certain chosen threshold, the output is high. When the input is below a different (lower) chosen threshold, the output is low, and when the input is between the two levels, the output retains its value.

  • Comparator Versus Schmitt trigger:


Disadvantage of ask
Disadvantage of ASK

  • The detection process is difficult since the

    amplitude is not constant.

  • ASK is usable only for Low Data Rate.


Frequency shift keying fsk
Frequency Shift Keying (FSK)

  • In a binary FSK, symbols 1 and 0 are distinguished

    from each others by transmitting one of the two sinusoidal

    signals that differ in frequency by a fixed amount.

0≤ t ≤ Tb


Fsk modulator
FSK Modulator

  • The direct methods that are used to general FSK signals

    are:

  • Two - oscillator method ( synchronization problem)

  • Voltage control oscillator:


Amplitude shift keying ask frequency shift keying fsk

Illustration of FSK

1 0 1 1 0 0 1

Digital information

Carrier 1 (frequency #1)

Carrier 2 (frequency #2)

FSK modulated signal

Frequency varying-amplitude constant


Ask demodulator1
ASK Demodulator

  • Non-coherent detection of FSK


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