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.

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 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

Digital information

1 0 1 1 0 0 1 0 1 0

Carrier wave

ASK modulated signal

Carrier present Carrier absent

Amplitude varying-frequency constant

ASK DEMODULATOR The basic block diagram of ASK demodulator ( Non-

Coherent Demodulation):

Notes:

- The BPF is used to reduce both ; the noise and theinterference of other signals through selecting the BW of each signal.

- Sometimes a comparator is used instead of the schmitttrigger. ( Threshold = V/2 ) . They are used to convert theanalog signal to a digital one.

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 The detection process is difficult since

theamplitude is not constant.

ASK is usable only for Low Data Rate.

FREQUENCY SHIFT KEYING (FSK)

In a binary FSK, symbols 1 and 0 are distinguishedfrom each others by transmitting one of the two

sinusoidal signals that differ in frequency by a fixed amount.

0≤ t ≤ Tb

FSK MODULATOR The direct methods that are used to general FSK

signalsare:

Two - oscillator method ( synchronization problem)

Voltage control oscillator:

Illustration of FSK

Digital information

1 0 1 1 0 0 1

Carrier 1 (frequency #1)

FSK modulated signal

Carrier 2 (frequency #2)

Frequency varying-amplitude constant

ASK DEMODULATOR- Non-coherent detection of FSK