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

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

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

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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

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

- 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:

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

- Non-coherent detection of FSK