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# Amplitude Shift Keying (ASK) & Frequency Shift Keying (FSK) - PowerPoint PPT Presentation

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)

• 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.

• 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

• 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

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

• 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.

• 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:

• The detection process is difficult since the

amplitude is not constant.

• ASK is usable only for Low Data Rate.

• 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

• The direct methods that are used to general FSK signals

are:

• Two - oscillator method ( synchronization problem)

• Voltage control oscillator:

1 0 1 1 0 0 1

Digital information

Carrier 1 (frequency #1)

Carrier 2 (frequency #2)

FSK modulated signal

Frequency varying-amplitude constant

• Non-coherent detection of FSK