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